M.A. Thesis - University of Colorado (Boulder), USA. 2002.

1. Diurnal Patterns and Microclimatological Controls
on Stomatal Conductance and Transpiration at
High Creek Fen, Park County, Colorado.
Heide Maria Baden,
Department of Geography,
University of Colorado,
Boulder.

2. This Master Thesis has been defended before the following committee:
ii

3. Acknowledgements
This research was funded in part by The Nature Conservancy. Additional
support was granted by the Germanistic Society of America and the
Graduate School of this University. I thank Terri Schulz of The Nature
Conservancy for her support in the field and on the defense committee. I
especially thank Peter Blanken for outstanding and persistent advice. I
further thank Karen Weingarten, our graduate secretary for immeasurable
patience and support. Last but not least I thank my parents for their
everlasting love.
Fuer die
Regenbogenkinder
iii

4. TABLE OF CONTENTS
SIGNATURE PAGE........................................................................... ii
ACKNOWLEDGEMENTS AND DEDICATION.............................. iii
TABLE OF CONTENTS.................................................................... iv
LIST OF TABLES.............................................................................. vii
LIST OF FIGURES............................................................................ viii
LIST OF PHOTOGRAPHS................................................................ xii
LIST OF SYMBOLS........................................................................... xiii
CHAPTER 1. INTRODUCTION
1.1. OBJECTIVES OF THIS RESEARCH……………......... 1
1.2. THE SCALE OF THE DISCIPLINE………………......... 4
1.3. EVAPORATION AND EVAPOTRANSPIRATION......... 5
CHAPTER 2. LITERATURE REVIEW
2.1. LITERATURE REVIEW OF EARLY WORKS..……...... 9
2.1.1. I.S. BOWEN AND THE BOWEN RATIO…………........ 9
2.1.2. H.L. PENMAN AND POTENTIAL EVAPORATION…. 10
2.1.3. C. WARREN THORNTHWAITE ……………………… 13
2.2. THE FIELDS OF AGRO-AND BIOMETEOROLOGY.. 18
2.3. BIOCLIMATOLOGY AND HUMAN HEALTH……….... 19
iv

5. 2.4. AGROMETEOROLOGY AND CROPS……………...... 21
2.5. RECENT PUBLICATIONS…………………………....... 26
2.5.1. JOHN L. MONTEITH…………………………………..... 26
2.5.2. BIOMETEOROLOGICAL MODELING……………….... 29
2.6. CONCLUSION………………………………..………….. 33
CHAPTER 3. BACKGROUND
3.1. INTRODUCTION…………..…………………………….. 36
3.2. PHOTOSYNTHESIS AND ENERGY BALANCE ..…... 38
3.3. STUDY SITE DESCRIPTION ………………………….. 45
3.3.1. TOPOGRAPHY, HYDROGEOLOGY, AND
HISTORY………………………………………………..... 45
3.3.2. CLIMATE AND ENERGY BALANCE AT
HIGH CREEK FEN……..………………………………... 51
3.3.3. VEGETATION AT HIGH CREEK FEN……………….... 53
3.4. THE FOUR SITES AND THEIR INHABITANTS…….... 55
3.5. STUDY HYPOTHESES……......................................... 60
3.5.1. PROBLEM STATEMENT 1: DOES HEIGHT ABOVE
GROUND INFLUENCE PHYSIOLOGICAL
RESPONSES WITHIN AN INDIVIDUAL SPECIES?.... 61
3.5.2. PROBLEM STATEMENT 2: DOES SOIL MOISTURE
CONTROL RATES OF STOMATAL CONDUCTANCE
AND TRANSPIRATION FROM SAME SPECIES IN
DIFFERING LOCATIONS?........................................... 62
3.5.3. PROBLEM STATEMENT 3: WHEN EXPOSED TO THE
SAME MICROCLIMATE, DO DIFFERENT SPECIES
VARY IN STOMATAL CONDUCTANCE AND
TRANSPIRATION?....................................................... 63
v

6. CHAPTER 4. METHODS
4.1. INTRODUCTION……..........……………………………. 65
4.2. ON-SITE CLIMATE STATION………………………...... 65
4. 3. METHODS OF DATA COLLECTION AT THE FOUR
SITES.......................…………………………………….. 67
4.4. THE DATA SET………………………………………….. 71
4.4.1. DATA SET PREPARATION…………………………….. 74
CHAPTER 5. RESULTS
5.1. INTRODUCTION……………………………………....... 77
5.2. METEOROLOGICAL DATA OBSERVED
BY THE TOWER……………………………………….... 77
5.3. RESULTS FOR PROBLEM STATEMENT 1………….. 78
5.4.1. RESULTS FOR PROBLEM STATEMENT 2.a……….. 90
5.4.2. RESULTS FOR PROBLEM STATEMENT 2.b……….101
5.5. RESULTS FOR PROBLEM STATEMENT 3………....105
CHAPTER 6. DISCUSSION.....................................131
CHAPTER 7. CONCLUSION.....................................137
REFERENCES.................................................................................142
APPENDIX A....................................................................................147
APPENDIX B....................................................................................148
vi

7. LIST OF TABLES
Table 5.1. Minima, maxima, and means of transpiration [E] in mmol
m-2 s–1 and stomatal conductance [g] in mol m-2 s–1 for S. monticola at
z = 40, 70, 100 cm.
Table 5.2. Transpiration [E] measured from three distinct heights of
S. monticola measured on DOY 188 (July 7th), 2001 expressed in
mmol m-2 h–1 and g H2O m-2 h-1.
Table 5.3. Minima, maxima, means, and standard deviations of  in
the wet [ (w)] and dry [ (d)] location. Ranges were 8 and 6% for the
wet and dry location, respectively.
Table 5.4. Comparing the means of transpiration [E] and stomatal
conductance [g] for the two populations (d) and (w) via a paired
samples t-test, results show paired samples correlations for E and g of
S.candida in dry and wet location as highly significant.
Table 5.5. Comparing paired samples differences of transpiration
[E] and stomatal conductance [g] show a higher predictability of the
differences in g (80.2 % confidence) than differences in E (35 %
confidence).
Table 5.6.a. Transpiration [E], expressed in mmol m-2 h-1 and g m-2
h-1, on DOY 174 (June 23rd), 2001, from S. candida (d) in soil moisture
[ ] ~45 % and S. candida (w) in  ~50 %.
Table 5.6.b. Transpiration in the wet location [E (w)] exceeds
transpiration in the dry location [E (d)] by 30.0 %. Hence, S.candida
(w) in  ~50% transpired one third more than S.candida (d) in  ~45%.
Table 5.7 Transpiration [E] from all six species on DOY 191 (July
10th), 2001 expressed in mmol and grams H2O m-2 s-1 as well as h-1.
Fluxes are listed in decreasing order from top to bottom.
Table 5.8. Mean daily stomatal conductance [g] from all six species
on DOY 191 (July 10th), 2001 expressed in mol m-2 s-1 as well as h-1.
vii

8. LIST OF FIGURES
Figure 3.1. Map shows the northwestern part of the Garo
quadrangle topographic map; the study site located near High Creek
is circled; the Colorado index map shows the location of Park County.
Figure 3.2. Soil moisture transect from southeast (0) to northwest
(1000 m) taken across the fen on July 1st, 2001. With distance
increments of 33 m, 31 data points were recorded. Low  values
represent areas outside the fen.
Figure 4.1. Wetting and Drying Curve of 1500 cm3 High Creek Fen
Soil determined in the laboratory. Wetting: 20x75 ml of H2O were
added to the oven-dried soil in increments of 5 minutes; through this
process, actual soil moisture was continuously increased by 5 %, and
HydroSense delay times were recorded. Drying: soil was repeatedly
placed in oven, weighed, and delay times were recorded, until no
further weight was lost. The following fit was created for all data
points:  = - 55.36 + 62.74 ms +13.97 ms2.
Figure 4.2. HydroSense Calibration Curve from both wetting and
drying curve data; to view the fit from this new calibration, this figure
shows how the originally reported delay time increasingly
overestimates increasing actual volumetric water content [ ] by a
factor of up to 2 at saturation.
Figure 5.1. Vapor pressure deficit [VPD] and air temperature [TA] as
observed by the tower for DOY 188 as decimal time, where 188 =
00:00:00 hours on July 7th, and 188.5 = noon. Graph shows that VPD
is a function of TA.
Figure 5.2.a. Stomatal conductance [g] for S. monticola from leaves
at heights of z = 40 cm, z = 70 cm, and z = 100 cm.
Figure 5.2.b. Transpiration [E] and from leaves of S. monticola at
heights of z = 40, z = 70, and z = 100 cm.
Figure 5.3.a. Leaf temperature [TL] of S.monticola and quantum flux
[Q] measured at a leaf at 40 cm height show that the plant’s TL does
not react to Q. Also, compared to the incident radiation at z = 100,
this height of z = 40 catches a larger amount more quickly in the
morning (e.g., from 06:30 until 07:00, the leaf receives 100 to 850
mol m-2 s-1).
viii

9. Figure 5.3.b. Leaf temperature [TL] of S. monticola and quantum
flux [Q] measured at a leaf of 70 cm height.
Figure 5.3.c. Leaf temperature [TL] of S. monticola and quantum
flux [Q] measured at a leaf located at 100 cm tree height. Compared
to the other heights, this part of the plant reacts with TL most
aggressively to a change in Q.
Figure 5.4.a. Regression of transpiration rates (E) of S. candida in
the dry location against E from S. candida in the wet location as mmol
H2O transpired m-2 s-1.
Figure 5.4. b. Regression of stomatal conductances (g) of S.
candida in the dry location against g of S. candida in the wet location
expressed as molar flux through stomatal magnitude m -2 s-1.
Figure 5.5.a. Transpiration [E] for S. candida on DOY 174 in a dry
(d) and wet (w) location show a visible, although not statistically
significant difference in mmol of E released m-2 s-1 throughout the day;
the mid-day data gap is due to temporary system failure.
Figure 5.5.b. Stomatal conductance [g] for S.candida in the dry (d)
and wet (w) location again show a visible, however, not statistically
significant difference in the flux of mol m –2 s-1 of g on DOY 174
(summer solstice).
Figure 5.6. The scatter plot shows mean daily transpiration [E] in
dependence upon soil moisture []. Plant locations 1 – 3 were
grouped as the drier locations, 4 – 6 as the mesic, and 7 – 9 as the
wet, close to saturated locations. E from case 3 with av = 20.8 % did
not differ from the average E values produced by cases 7 and 9.
Figure 5.7. The scatter plot shows mean daily stomatal
conductance [g] in dependence upon soil moisture []. Again, cases 1
– 3 were grouped as the drier locations, 4 – 6 as the mesic, and 7 – 9
as the wet, close to saturated locations.
Figure 5.8. Stomatal conductance [g] plotted against quantum flux
[Q] for all six species investigated at High Creek Fen. Data may be
compared with general statements made about C3 plants in Nobel
(1999).
ix

10. Figure 5.9. Stomatal conductance [g] in dependence upon leaf
temperature [TL] of all six species investigated at High Creek Fen. Data
may be compared with general statements made about C3 plants in Nobel
(1999), where photosynthetic rate doubles between 20 and 30 C, and
maximizes between 30 and 40 C.
Figure 5.10. Stomatal conductance [g] as controlled by vapor
pressure deficit [D] surrounding all six plant species investigated at
High Creek Fen. Usually, g can be expected to decrease
exponentially with increasing D. Since D is highly correlated with TL,
most data points are expected to fall into the same quadrant from both
this, and the previous figure (5.11.).
Figure 5.11. Stomatal conductance[g] regressed with soil moisture
[] measured in the separate locations of the six plants researched in
the fen; this graph should not be interpreted as revealing soil moisture
tolerance ranges – respective plants may grow in areas not
represented here. However, all  spectra of B. glandulosa as well as
most  spectra of S. candida should be found in this graph; the
researcher searched the fen for locations of these species that
encompassed the complete  range in this fen. Generally, all plant
underlying soils were saturated between 50 and 55 %.
Figure 5.12. Transpiration [E] and stomatal conductance [g] from
Betula glandulosa on DOY 191 (July 10th), 2001. This species
reaches gmax around 10:00 a.m., and then gradually decreases g over
the afternoon, when TL and D become limiting. As seen from Table
5.7., B. glandulosa ranks highest in E compared to the other five
species.
Figure 5.13. Transpiration [E] and stomatal conductance [g] from
Carex aquatilis on DOY 191; here, mid-day stomatal depression
effecting necessary reduction of the quantity of water vapor demand
by the atmosphere is evident. Compared to gmax from B. glandulosa
and S. brachycarpa, gmax from C. aquatilis is a third, and half as large
as that of S. monticola. S. candida exceeds it by a factor of 2.5.
Figure 5.14. Transpiration [E] and stomatal conductance [g] from
Salix brachycarpa on DOY 191. Again, mid-day stomatal depression
to reduce water stress is evident. Morning conductance allows this
species to still rank third in E compared to the other five species.
x

11. Figure 5.15. Transpiration [E] and stomatal conductance [g] from
Salix candida on DOY 191; compared to the previously seen (5.12 –
5.14) flux developments over time, the silver willow shows a high
morning, toward evening gradually decreasing g. Nevertheless, mid-
day stomatal depression is visible, as well as a second depression
starting after 14 hours solar time (15:10 MDT), when the tower
showed a solar flux of 1008 W m-2. Stomatal conductance increased
after 15 hours (16:10 MDT), when intensity of radiation dropped again.
Figure 5.16. Transpiration [E] and Stomatal conductance [g] from
Salix monticola on DOY 191. As also seen from Table 5.7., this
species seems best adapted to its environment, since it has the
strongest E of all compared plants. Clouds were over the area when
the steep drop in stomatal conductance occurred around 13:30 hours
solar time. Possible explanation for the drop in g may be a TL of 32.8
C at this time, which may have caused the partial stomatal closure.
Figure 5.17. Transpiration [E] and Stomatal conductance [g] from
Salix planifolia on DOY 191 show the typical behavior of an
unstressed plant with no mid-day stomatal depression. Ranking 5th in
E and g (Tab. 5.7.) might allow a stress-free life in this environment.
Figure 5.18. Stomatal conductance [g] from B. glandulosa, S.
candida, C. aquatilis, S. monticola, S. brachycarpa, and S. planifolia
on DOY 191. On this daily basis, C. aquatilis conducted least, S.
monticola most. See Tables 5.7. and 5.8. for numeric details.
Figure 5.19. Transpiration [E] from B. glandulosa, S. candida, C.
aquatilis, S. monticola, S. brachycarpa, and S. planifolia on DOY 191.
On this daily basis, S. planifolia conducted least, B. glandulosa most
amounts of H2O. S. planifolia was also the least stressed (no mid-day
stomatal depression). See Table 5.7. and 5.8. for numeric details.
xi

12. LIST OF PHOTOGRAPHS
Title page Sunrise over High Creek Fen in Summer 2001.
Photograph 3.1. Cumulus Cloud over High Creek Fen (view to NE)
in Summer 2001.
Photograph 3.2. View across the fen from NW (transect survey
pole) to SE shows approximate transect location; the location of the
meteorological tower is included on transect.
Photograph 3.3. Dense ground-cover of willow, birch, and sedge at
High Creek Fen, Summer 2001. Blue Spruce in the background
greatly influence turbulence at the site.
Photograph 3.4. Betula glandulosa (Swamp Birch) in a drier location
at High Creek Fen. Summer 2001. This species occurs in a range of
locations where 15 % <  < 60 %.
Photograph 3.5. Close view of the thick, dark-green leaves of Salix
candida (Silver Willow). Although not measured, S.candida’s
physiology suggests multi-storied, dense chlorophyll pigmentation.
Photograph 3.6. Salix monticola
Photograph 3.7. Salix brachycarpa
Photograph 4.1. On-site climate station in Summer 2001
Photograph 4.2. Porometer measurements by Researcher; machine
strapped on via belt, storage module attached to belt on the back,
cuvette in right hand.
Photograph 7.1. High Creek Fen looking west toward the Mosquito
Range.
xii

13. LIST OF SYMBOLS
Symbol Definition Units
D Atmospheric Water Vapor Deficit kPa
E Transpiration mmol m-2s-1
g Stomatal conductance mol m-2s-1
gmax Maximum stomatal conductance mol m-2s-1
E Latent heat flux W m-2
K Incoming shortwave radiation W m-2
K Reflected shortwave radiation W m-2
L Incoming longwave radiation W m-2
L Reflected longwave radiation W m-2
 Volumetric soil moisture %
Q Quantum flux mol m-2s-1
RH Relative humidity %
Rn Net radiation W m-2
TA Air temperature C
Tdew Dew point temperature C
TL Leaf temperature C
TS Soil temperature C
xiii

14. CHAPTER 1. INTRODUCTION
1.1. OBJECTIVES OF THIS RESEARCH
While broad-scale climates of the Earth‘s major vegetative
regions have been well studied, a fine-scale investigation of local
environments is required to understand the influence of both
atmosphere and soil on local vegetation dynamics. An area‘s
microclimate often distinguishes itself from the regional climate by
peculiarities such as soil texture, topography, or biomass (Rouse 2000).
As functions of microclimate, water and solar energy are among the
main lifelines for plants, and their abundance and availability are
therefore a question of precise locality. Assessing the sensitivity of
plants from different regions to soil moisture and microclimate allows
researchers to establish a gauge for these plants‘ susceptibility to
disturbances such as drainage and climate change.
Net all-wave radiation and its partitioned sensible and
evaporative heat flux are extremely important components of both the
energy and water balances of an area, especially those of high- latitude
and alpine wetlands, which partition up to 80% of their net radiation into
the evaporative, or latent heat flux [E] (Rouse 2000). Plants that
inhabit these areas therefore constitute a considerable local source of

15. water vapor to the atmosphere. Results from measuring and modeling
the E over such surfaces can aid researchers in improving current
climate models (Beringer et al. 2001).
This research focuses on fine-scale exchange of both water and
energy between the soil, the plant, and the atmosphere in a 750-acre
fen in central Colorado. In particular, the combined effects of the
atmospheric vapor pressure deficit, solar energy flux, leaf temperature,
and soil moisture availability on plant stomatal conductance and
transpiration of water vapor during the photosynthetically active part of
the day were examined. While a complete list of resources controlling
plant physiological responses includes N and CO2 (Kazda 1995), this
research investigates water and energy resources. Understanding their
role, their spatial and temporal distribution at certain locations, and their
availability and use in relationship to particular plant species was the
goal of this research.
Salicaceae (willow), Betulaceae (birch), and Cyperaceae
(sedges) are typical examples of wetland species of the arctic, alpine,
and boreal tundra regions. As meteorological and soil moisture
conditions exert limitations and affect the magnitude of plant
transpiration [E], this research focused on analyzing the effect of
variation in the spatial and temporal magnitudes of these environmental
2

16. variables on stomatal conductance [g]. First, g and E rates from one
individual of Salix monticola at three different heights (40 cm, 70 cm,
and 100 cm above ground) were compared. This was to assess
whether there was a significant difference in the magnitudes in g and
the plant‘s stomatal responses at different heights. Second, g and E of
two specimens of Salix candida situated in a dry (40-45 % volumetric
soil moisture,  ) and a wet (50-55 %) location were compared. Third, g
and E of nine Betula glandulosa situated in dry (with an average  of
18 %), mesic (35 %), and wet (51 %) locations were compared. Part
two and three of the study analyzed this soil moisture variability to
which plants in different microclimatological locations were exposed,
and evaluated intraspecific variation in g and E based upon soil
moisture abundance. Lasty, differences in g and E between six
different species exposed to the same environmental conditions were
examined. Species included were Salix monticola, S. brachycarpa, S.
planifolia, S. candida, Carex aquatilis and Betula glandulosa. This
fourth part of the study determined whether different species have
differing adaptations to the same microclimatological conditions.
Results of all four studies enhanced the understanding of local
vegetation dynamics in this high altitude wetland.
3

17. 1.2. THE SCALE OF THE DISCIPLINE
This research defines the microenvironment as the area that
surrounds an animate object, e.g. a plant, an animal, or a human being.
The scale beyond which neither the object, nor its environment have a
direct or indirect influence on each other shall be the limit to the micro
scale. Micrometeorology concerns itself with the processes that occur
within or closely above the atmospheric boundary layer, beyond which
the Earth‘s surface has little influence on the atmospheric processes.
The height of the boundary layer varies constantly with wind and
temperature. On a calm day with a large sensible heat flux, the height
of the boundary layer reaches its maximum. Correspondingly, ―areas
experiencing greater wind speeds tend to have shorter vegetation, such
as cushion plants in alpine tundra or the procumbent forms on coastal
dunes‖ (Nobel 1999). Inside the atmospheric boundary layer, turbulent
(wind-driven) transport is the predominant motion of the gas molecules
that make up the air. This research investigates the lower boundary of
the atmospheric boundary layer ending where the plant roots do not
reach any further. This soil-plant-atmosphere is the region of direct
hydrogeologic influence on the plant and its atmospheric environment.
However, potential upwelling of water from even deeper regions in the
ground must be considered.
4

18. 1.3. EVAPORATION AND EVAPOTRANSPIRATION
Evaporation and, in the presence of transpiring plants,
evapotranspiration are of the few basic climatic factors that scientists
are neither able to estimate easily, nor extrapolate from remotely
sensed data. They are important variables, because their values are
needed to assess the water, and the energy budget of all organisms.
Measurements taken on the ground are highly dependent on
numerous physical factors that include temperature, radiation, humidity,
soil moisture, and ground heat flux. As a mandatory agent to the
photosynthetic process, water is needed to dissolve carbon and keep
leaf surfaces cool. If a plant‘s water supply is at its end, i.e. the roots
cannot draw up any more water from the ground, the plant will dry up
completely. Plants have evolved physiological features to acclimate
themselves to their microclimate, and the physiology and phenology of
a plant tell a lot about the climate of the area.
As Lieth (1997) mentions in his abstract on phenological
monitoring, the "data on vegetation development provided by the
phenologists during the last two centuries are about the most reliable
information available for the evaluation of global trends of
environmental parameters." As an example of this, Blanken
(pers.comm. 2000) stated that the decrease in stomatal density on the
5

19. leaves of plants over the past 1000 years is evidence that plants are
getting more efficient at photosynthesis as atmospheric CO2
concentrations have increased.
Evaporation has been and still is especially important in arid
climates such as the Southwestern United States, where this study has
been conducted. Here, the water supply for E depends on the relatively
small amount of precipitation that is received (often in the form of snow)
as well as underground aquifers that occasionally allow their water to
surface in streams. In these arid regions, the usually dry air constantly
demands water vapor from the surface of the earth, and its inhabitants.
Stream and ground water flow may be an important contributor
to the water supply of vegetated surfaces. As in the case of High Creek
Fen in Park County, Colorado, evapotranspiration exceeds precipitation
by a factor of 3 (Blanken, pers. comm. 2002). This fact ponders the
question where the additional water may be added to the system. The
hydrogeological processes seem to provide moisture to the
microenvironment through lateral in- and outputs of water from surface
and subsurface flow systems, such as those Rouse (1998) observed in
similar ecosystems. This goes to show that evaporation is not at all
strictly a function of infiltrated water just through precipitation. To
explain the amount of water evaporated by a surface, it is therefore
6

20. necessary to acquire information about the hydrogeological features of
the ground beneath it. The potential amount of water available to the
plants at High Creek Fen is yet to be estimated through local research.
The prime factor that drives evapotranspiration, radiation, must
be investigated. Incident solar radiation is measured at the site by a
permanently installed pyranometer. If a cloud passes over the area, the
incident radiation is diminished, leading to several feedback processes,
which will be discussed in the later sections. The second factor that
accounts for the amount of E, the saturation vapor pressure deficit of
the air, gives an estimate of the evaporative demand at the surface.
The presence of plants on the surface greatly modifies the
energy balance and the partitioning between evaporation and
transpiration. Evaporation from the non-vegetated part of the surface,
as well as the amount of water transpired by the plant [E] yield
evapotranspiration [E]. The plants‘ transpiration rates are influenced
by the same physical factors as the rates of evaporation, however, their
need to conserve water will induce stomatal resistances that lower the
rate of transpiration. Stomata are the physiological means of plants to
regulate water loss and CO2 uptake throughout the photosynthetic
process. Biochemical triggers like hormones regulate stomatal
resistance, i.e. the partial closure of stomata, which, depending on the
7

21. saturation vapor pressure deficit [D], may lower the rate of transpiration.
8

22. CHAPTER 2. LITERATURE
2.1. LITERATURE REVIEW OF EARLY WORKS
Questions that explore the role of evapotranspiration in the water
budget and bioclimate have quite a long history, as well as an extended
field of origin. Scientific articles can be found since before the turn of
the 20th century, many of the early ones published in the U.S.
Department of Agriculture Bulletin; many articles on evaporation and
evapotranspiration came from several different scientific fields,
including physics and meteorology, agro-ecology, as well as hydrology,
soil science, botany and plant physiology. Having mentioned the
interconnectedness of micrometeorology to almost all physical
sciences, the first part of this chapter is focused on several earlier
publications that brought new thoughts and findings into the field.
2.1.1. I. S. BOWEN AND THE BOWEN RATIO
Bowen (1926) experimented with evaporation as a measurement
of latent heat loss in comparison to sensible heat loss. With his paper
on the Bowen Ratio, he introduced the ratio of the sensible heat flux [H]
to E, which typically ranges from 0.1 for an irrigated crop to 5 for
desert environments. The Bowen Ratio when combined with the
energy balance, is used in a great number of papers that concern
9

23. themselves with energy fluxes (e.g., Blanken and Rouse 1994, Burba et
al. 1999, Takagi 1998). Such values tell a knowledgeable climatologist
a lot about the place where it was measured, even if she has not been
there personally – much like the morphology of a plant gives away the
nature of its surrounding microclimate.
2.1.2. H.L. PENMAN AND POTENTIAL EVAPORATION
In 1947, the British meteorologist H.L. Penman modeled
evaporation in his well-known paper titled ―Natural evaporation from
open water, bare soil, and grass‖ published in the Proceedings of the
Royal Society of London, describing pan evaporation experiments, as
well as evaporation from soil and vegetation. His experiments only
looked at potential evapotranspiration, i.e. from water-saturated
surfaces. Although this did not account for stomatal conductance as a
resistance to the magnitude of plant transpiration, he laid the
groundwork for the still widely used Penman-Monteith combination
equation which models E as controlled by plant physiological
parameters.
In his introduction, Penman states, that ―a complete survey of
evaporation from bare soil and transpiration from crops should take into
account all relevant factors [but that his current] account will be largely
10

24. restricted to [considering processes] after thorough wetting of the soil
by rain or irrigation, when soil type, crop type and root range are of little
importance.‖ Penman goes into the physical requirements for the
occurrence of evaporation, which are ―a supply of energy to provide the
latent heat of vaporization [i.e. solar radiation] and some mechanism for
removing the vapor, i.e. there must be a sink for vapor.‖ His arguments
consider the laminar boundary layer in which non-turbulent, but
diffusive movement of air takes place. This is an important concept in
the aerodynamic considerations made when calculating fluxes at the
leaf level.
Penman‘s discussion on the energy balance introduces the
important concept of assumptions. In bioclimatological modeling,
assumptions must be made in order to translate the reality into
mathematical formulae. While the assumption of horizontal
homogeneity, for example, works well for oceans and lakes, it is an
assumption also made in most canopy flux models, so that x and y
coordinates are negligible, and all statistical moments (mean, variance,
skewness, kurtosis) are forced into the vertical z coordinates. Often
unrealistic to natural environments, assumptions allow scientists to rule
out possibilities by making reliable estimates, much like a predator
11

25. circling its prey (i.e. the research question). It is a slow, yet useful way
of approaching the solution to a hypothesis.
The assumption Penman makes is that the factor of heat storage
is negligible, a factor that indeed can be assumed zero for
measurements at the leaf level, however not at the canopy level
(Monson 2000). Penman admits that ―obtaining a reliable daily mean
value of the dew point temperature remains one of the main
experimental problems to be solved‖—data that with nowadays‘
technology is easily obtained (for example a chilled-mirror hygrometer).
Penman gives a detailed description of the instruments used. However,
to be meticulous about the description of the exact type or make of an
instrument, gives experienced micrometeorologists and other scientists
appropriate insight into potential errors of a measurement. It is also
mentioned that the accuracy of the cloudiness factor is a hard one to
obtain. The reason may be that although pyranometers had been
invented, measurements for 24 hours a day were taxing, whereas
nowadays, data loggers take the place of a measurement-reading
scientist (let‘s invent an automatic porometer).
The article is a cornerstone work in micrometeorology. Its
terminology is still used in today‘s lectures. The use of units is
confusing to metric scale users, since they are miles per day for wind
12

26. velocity, and switch between inches per month and mm/day for
evaporation, but fortunately the scientific community today is in the
process of collectively changing to the (more sensible) metric system.
2.1.3. C. WARREN THORNTHWAITE
Thornthwaite incorporated evaporation into his global climate
classification model (1951). His quantitative method distinguished
aridity from humidity in climates of the Low-Latitudes, Mid-Latitudes,
and High-Latitudes as a function of potential evaporation and soil-water
storage capacity reflected in the plants‘ need for water, which generally
increases from the poles toward the equator. His climate classification
is still used in geographic education. However, for the
microclimatologist, this kind of classification is of lesser interest. More
important here were Thornthwaite‘s contributions to bioclimatology on
the micro scale. The following paragraphs will explore some thoughts
of Thornthwaite and his group of scientists at Johns Hopkins University
in New Jersey.
A monograph on bioclimatology, compiled in 1954 by
Thornthwaite, May, and Mather, consists of several articles on the
effects of the physical environment on life, including human issues like
health and housing. In the book‘s preface, May points out that in light
13

27. of its omnipresence on Earth, bioclimatology‘s ―scope is tremendous‖.
The authors see the field in its early stage, where ―various niches of
ignorance will be filled as more […] data becomes available‖
(Thornthwaite et al. 1954). According to May, and not surprisingly, the
first man to concern himself with the field was the Greek Hippocrates.
His work that May refers to is Airs, Waters, and Places, a treatise that
deals with ―the action of climate on living things‖. Another interesting
part in the preface explains May‘s view on the variation of climate.
―Climates vary not only between the poles and the equator, between
the level sea and the tops of the mountains, but between a hollow as
big as the palm of one‘s hand in a field and a similar depression
several feet away. All these variations occur according to natural
laws, some of which man has discovered and learned to understand,
some of which remain mysterious and represent the field of research
for tomorrow.‖
May describes the processes between climate and physical
environment, which are constantly modifying each other, as in ―a race
towards a state of equilibrium that will never be reached‖.
From this same compilation, an article by Thornthwaite and
Mather (1953) titled ―Climate in Relation to Crops‖ gives interesting
historical facts about the first developments of bioclimatology, including
information on the 17th century French scientist Réaumur, who
developed an index in 1735 that attempts to quantify the heat required
14

28. for a plant to reach maturity. The index was acquired by summing the
degrees of mean daily air temperatures during certain stages of
development of a plant. Réaumur called this sum the ―thermal
constant‖ for the particular plant (Thornthwaite et al.1954), based on his
observations. Thornthwaite later explains how Réaumur was wrong
since ―his thermal constants were not constant,‖ but showed that one
plant in higher latitudes yielded a smaller constant than the same plant
in lower latitudes. Thus, ―less heat was required in cold climates than in
warm to bring about a given amount of development‖ and a cold year
had a smaller thermal constant than a warm year. Thornthwaite et al.
conclude their paragraph about Réaumur‘s heat index that ―the many
changes and refinements that have been introduced in recent years
have not removed the basic deficiencies of the heat unit theory.‖
Although this method did not render successful for crop
scheduling, its theory seems quite interesting. Keeping in mind that it
was developed 265 years ago, the ideas show scientific ingenuity and
expertise. Also, its findings harmonize with the zonal idea of climatic
regions, and with some climatic imagination, show May‘s idea that life is
modified by the environment, while at the same time the environment is
modified by life in a ―race for equilibrium‖.
15

29. Thornthwaite and Mather (1953) develop a list of concerns about
the current needs of the field of bioclimatology, and later describe their
method that stems from research with their group of bioclimatologists in
New Jersey. This approach will be outlined later. According to them,
the needs of the discipline in 1953 were a collection of observational
data, since the Federal Weather Service was obviously not able to
deliver anything but regional data, thus giving information on
―observations […] inadequate to the solution of most problems.‖ They
argue ―the climate of a region as determined by means of the
standardized observations is more or less of an abstraction‖ and ―the
region is a composite of innumerable local climates‖ including ravines,
south-facing slopes, hill tops, meadows, corn fields and woods. They
go on to say that ―the climates of areas of very limited extent are called
microclimates. They are clearly the ones that concern the farmer, the
agronomist and the biologist‖ (Thornthwaite et al. 1954).
The authors point out the importance of approaching the
problems, of, e.g., the effects of frost, drought or extremely high
temperatures on plants, from both the climatological as well as the
biological side through the cooperation of scientists from the respective
groups. This call for synthesis has, as far as I am concerned, been
increasingly heard, maybe because most attempts at integration have
16

30. proven very successful. This success could be attributed to the first
ecological principle, that all things are interrelated.
As with synthesis, another suggestion from the authors is the
development of a climatic calendar that organizes the observational
data according to the relationship between climate and plants. The
development of such a device could help ―schedule successive
plantings of vegetable crop to yield uniform harvest.‖ The Laboratory of
Climatology at Seabrook devised a method to control soil moisture,
targeting the ―twin problems of crop and irrigation scheduling.‖ Their
goal was not to just observe peas and corn, but to devise a more
comprehensive method that links the ―water used by plants in
transpiration and growth [to] the rate of plant development.‖
A well-developed discussion on the water budget of plants is
given, that introduces the term evapotranspiration. The ―return flow of
water from the ground to the atmosphere‖ is a ―climatic factor as
important as precipitation‖ that is not only dependent on climate, but
also ―related to certain vegetation and soil factors [such as] type and
stage of development of the vegetation, the method of cultivation, the
soil type, and above all the moisture content of the soil.‖ The
discussion goes on to distinguish the actual from the potential
evapotranspiration; the latter is reached only in a well-hydrated soil. Its
17

31. value is ―independent of soil type, kind of crop, or mode of cultivation
and is, thus, a function of climate alone.‖
The abstract explains further facts about plant processes. The
wording ―green plants manufacture food within their leaves by a
process called photosynthesis, using water from the soil and carbon
dioxide from the air as raw materials‖ may bring a smile to today‘s
reader‘s faces; it seems amazing that this article is not even 50 years
old, yet goes to show that Thornthwaite can truly be counted as one of
the forefathers of bioclimatology.
It should seem viable that young, beginning scientists owe much
gratitude to people like Thornthwaite‘s group, who explain these early
developments of bioclimatology with such patiently detailed vocabulary
and well-chosen examples that make understanding of the subject
easily possible. The words used are free of scientific vanity and their
sole purpose is straightforward communication.
2.2. THE FIELDS OF AGRO--AND BIOMETEOROLOGY
Agro- and biometeorology have made it their goal to elucidate
the relationships between organisms and their physical environment.
Both fields take the science of pure micrometeorology a step further, as
their questions concern themselves with the interactions of life forms
18

32. with their surrounding climatic situations. Incentives to tackle the
complexity of these relationships have been given by the potential
advantages of understanding these interactions, from maximizing the
yield of a crop to healing human diseases.
The first issue of the International Journal of Bioclimatology and
Biometeorology (this name later changed to International Journal of
Bioclimatology) was published in 1957. It featured four parts. One
concerned general bioclimatology, the second dealt with plant –
microclimate interactions. The third and fourth parts explored effects of
climate on animals and humans. The plant-related topics include a
paper on the influence of soil preparation on the microclimate of weedy
clear-cut fields before reforestation. Also, topics discussed guidelines
for bioclimatological measurements and whether microclimate can be
predicted (Pascale 1957).
2.3. BIOCLIMATOLOGY AND HUMAN HEALTH
The fourth section in the first edition of the above journal shows
that early concerns of bioclimatology stemmed not only from agricultural
incentives, but also from questions regarding climate's direct effects on
human beings. Those questions were, for example, acclimation to high
19

33. altitudes, predictability of asthma attacks, and the influence of
meteorological fronts on the general wellness of people.
Just one year later, in 1958, the medicinal journal Fundamenta
balneo-bioklimatologica was established, which deals with the
atmospheric influences on living organisms. According to Jordan
(1981), balneo-bioclimatology is both a subsection of bioclimatology
and balneology, i.e. therapy through baths, and it stands for applied
therapy through climate. I cite Jordan here not to go into detail about
balneo-bioclimatology, but because his thoughts are a valuable
contribution to understanding the development of bioclimatology. He
begins by citing Alexander von Humboldt's definition of climate as "all
changes in the atmosphere that noticeably affect our organs," thereby
speaking of the dialectic system of humans and their physical
surroundings. Jordan goes on to explain the difference between
looking at stimulus and response versus stimulus and responsibility.
'Stimulibility', or the readiness to be stimulated by outside processes,
modifies the reaction, and therefore the responsibility of an organism.
Changes occur along rhythmic or periodic processes. Jordan shares a
further thought by proposing that reactions can initiate either positive or
negative feedback mechanisms, since the stimulus may modify one
20

34. rhythm and that rhythm may then modify the response in either
direction.
This little excursion proves quite interesting, especially when
relating it to the mass and energy balances of vegetated surfaces. On
a sunny day, the balance of energy loss and gain at the surface can be
disturbed by the passage of a thick cloud. This occurs because the
cloud intercepts the path of the radiation, which again results in a net
heat loss at the surface of the earth. The now cooling surface will
diminish the water vapor concentration gradient between the surface
and the air (warmer air can hold more moisture), as well as cause a
lower temperature gradient, the results being less evapotranspiration
and a lower rate of sensible heat transfer. When the new gradients
have caused their respective responses to be adjusted, a new energy
balance has been established (Monson 2000).
2.4. AGROMETEOROLOGY AND CROPS
After this intermezzo of how bioclimatology affects humans
directly, this part of the chapter offers to look at literature that deals with
the climate's effects on human food, i.e. crops as an indirect relation to
humans. As mentioned above, evaporation and evapotranspiration are
very important processes especially in arid regions. Irrigation to
21

35. maximize crop yield has primarily been researched in those areas,
where dry conditions called for water resource management. During
the 1930s (in the late 1940s together with Criddle), Blaney researched
evaporation as well as evapotranspiration especially in the
Southwestern U.S. Their work, published primarily through the U.S.
Soil Conservation Service, developed ways of estimating ―consumptive
use and irrigation water requirements (Blaney and Criddle 1949).‖ A
number of other scientists also explored optimized timing of irrigation
(Van Bavel and Wilson 1952) in the pursuit of water resource
conservation (Veihmeyer 1951).
A study from the College of Agriculture at Berkeley, California
shows approaches taken toward irrigation methods in the late 1920s.
The authors Beckett, Blaney, and Taylor (1930) research the amount of
water required for irrigation to produce a successful crop of Avocado
and Citrus trees in San Diego County. The goal of the study was not
just crop maximization, but finding optimal irrigation efficiency, since
water resources were scarce and expensive even in the 1920s.
"Efficiency of irrigation is defined as the percentage of the water applied
that is shown in soil-moisture increase in the soil mass occupied by the
principal rooting system of the crop." The authors describe the
watersheds, classify soils and climate, and map the rainfall and soil
22

36. moisture patterns down to four feet depth. Detailed observations,
including height and age of trees, root development and the interval
between irrigation lead the authors to an "estimated seasonal
requirement [of water] at maturity." The study finds an average water
resource efficiency of 60% "under good irrigation practice." Finally, the
authors make several predictions about certain crops and their
particular irrigation needs during, e.g. a period of drought of "more than
6 weeks". An important result of the study was that, "as long as the soil
moisture is above the wilting point, the moisture content has no
measurable effect on the rate of moisture extraction," a warning to not
waste water through excessive irrigation.1
From the Commission for Agrometeorology (CAgM) of the World
Meteorological Organization (WMO), four agrometeorologists
(Seemann et al. 1979) chose to compile a book titled
"Agrometeorology," since students of this young discipline had no
complete reference book to study by. In this book, J. Seemann, who is
obviously an advocate of the meso-scale, or topoclimatology, defends
the topic of his choice with this abruptly ending sentence
"macroclimatology is based on a wide network of measurements and
does not register the special features resulting from topographical
1
I just recently visited Riverside County in CA, and was amazed by the amount of
avocado and citrus trees. I am sure that Blaney and his fellow scholars laid the
groundwork for this intensive use of irrigation in agriculture.
23

37. differentiation of the terrain, whereas the microclimate comprises areas
which are far too small.‖ 2 One can only guess, for what purposes his
statement would make sense, but maybe he was talking about a mid- to
large-size farm. And indeed, the microclimate can vary between two
areas just a few meters apart, yielding a problem with the accuracy of
larger scale prediction of e.g., highly accurate crop cycles.
However, Chirkov, the second author of the book
"Agrometeorology" is more precise when giving his ideas about
microclimate. He explains, "microclimate of meadows, fields, forest
fringes, glades, and lakes is produced by the disparity in the radiative
heating of the subjacent surface." Chirkov facilitates the agricultural
point of view toward microclimate by asking where to expect frost, when
to expect frost-free periods, and what the differences are between
south-facing versus north-facing slopes in respect to optimal time of
sowing. He coins the term "phytoclimate" as the "meteorological
conditions produced amongst plants" and therefore as a modified
microclimate that is "controlled by the structure of the plant cover [i.e.
height, density] and the width of inter-row spaces." Chirkov relates
species, habitus, age of plant community, density of stand (plantation),
as well as the sowing or planting method, illumination intensity, air and
2
I did not explore Seemann's article any further, but found his statement rather
funny and therefore worthy of being shared here.
24

38. soil temperature and humidity, and wind intensity values, to come to the
conclusion that the phytoclimate must be considered closely in order to
make predictions of any sort. He gives the example that a vegetated
soil can have a temperature difference of up to 25 C compared to a
soil in an open location.
For accurate information on planting, sowing, or irrigating, he
suggests that vertical measurements must be taken (an approach
fundamental to current-day research) and the fields‘ distances to a
reservoir or a forest strip are to be assessed. The data shall then be
compared to that of the nearest weather station. Maps shall be made
that mirror the practical importance of data for the plant development
and crop formation, an idea that resembles Thornthwaite's crop
calendar.
Finally, Chirkov suggests that for agricultural purposes, the
microclimate can be improved, e.g. in cold or humid climates by ridging
the surface to reduce overhumidification, or in arid regions by thinning
out timber to preserve moisture. Another strategy to reduce wind and
turbulence, and therefore soil erosion, according to Chirkov, is to plant
forest strips in between fields that are 25 times their height apart. If the
trees of the forest strips were 20 meters tall, Chirkov suggests one
25

39. forest strip every 500 meters. However, he does not go into potential
soil water competition between trees and crops.
2.5. RECENT PUBLICATIONS
After the groundwork of biometeorology has been highlighted, it
is worthy to now explore several paragraphs on contemporary work,
especially focusing on John L. Monteith, since he still plays a large role
in today‘s cutting edge of synthesizing science. Several other
researchers and their attempts to model mass and energy balances will
also be outlined. In the conclusion, the researcher‘s own view and
future goals about her place in the discipline will be mentioned.
2.5.1. JOHN L. MONTEITH
In ―Vegetation and the Atmosphere‖ (1975), one of Monteith‘s
many books, he states that ―micrometeorology is the measurement and
analysis of the state of the atmosphere near the surface of the earth
whether life is present or not. His main objective was to ―provide a
quantitative framework‖ for describing processes such as heat and
mass transfer in terms of the prevalent mechanisms that operate
through radiative heat exchange, turbulent diffusion, or conduction of
heat in the soil. Like his fellow Penman, Monteith stresses the
26

40. importance of considering the distribution of sources and sinks of heat,
mass, and momentum in the canopy, mechanisms that are currently still
being explored by biometeorologists, and that are hard to quantify
directly.
Interestingly, Monteith mentions the dialectic that
―micrometeorologists have tended to regard vegetation as a steady
state system [which it is not, whereas] plant physiologists have tended
to overlook the significance of the state of the system [i.e. the
atmosphere].‖ With this comment, he stresses the importance of
sharing insights amongst scientists from seemingly separate fields. He
praises the recent contributions biochemists have made to ―our (i.e. the
meteorologists‘) understanding of physiological mechanisms elucidating
biochemical pathways, interactions, and feedback.‖
Monteith‘s thought on biometeorologic models ― [which] link
adjacent levels of organization from cell to leaf, leaf to plant, plant to
community‖ is that ―the input to such models is a set of equations
(received by assumptions) relating the rates of processes to the states
which govern these rates.‖ An example has been outlined in the last
paragraph of the section on human health. The processes Monteith is
talking about are physical and chemical, and his following elaborations
stress the intricate and complex interrelationships between the ―state of
27

41. the environment, the state of the plant, and the nature of the relevant
physical and physiological mechanisms.‖ Monteith expanded
Penman‘s energy balance equation to the Penman-Monteith
combination equation, in which he considers the effects of physiology
on aerodynamic and stomatal resistances. His modification allows
scientists to predict processes much more accurately.
In a later section, he mentions micrometeorology‘s contributions
to ecology, which include such application of physical principles to the
―relationship of states to processes.‖ Such principles are Newton‘s Law
of Motion explaining the transfer of momentum; the First Law of
Thermodynamics elucidating the radiation balance; the Conservation of
Mass for water balance; Ohm‘s Law for understanding resistance, and
Fick‘s Law to explain diffusion.
Conclusively, Monteith suggests the importance of applying
micrometeorologic knowledge to ameliorate crop successes, to
understand the relationship between weather and disease, or even the
parasite susceptibility of a host, that is often related to ―certain physical
states like temperature and humidity.‖ To achieve this, Monteith calls
for ecological records to be ―interpreted by interdisciplinary teams of
physicists and biologists‖ while keeping in mind that progress in this
field can only be maintained with a ―sensible balance between all these
28

42. essentials: development of instruments and recording systems,
interpretation of measurements, construction of mathematical models,
and most of all, the collaboration of micrometeorologists and ecologists
prepared to learn from each other.‖
Monteith has followed this vision. In 1995's "Accomodation
between Transpiring Vegetation and the Convective Boundary Layer",
outlines the interactions of meteorology and vegetation, giving special
regard to feedback mechanisms in the relationships of soil-plant, plant-
surface layer, and surface layer-planetary boundary layer. These
include the crucial balancing role of stomata in the physical
dependencies of fluxes and resistances to fluxes. Monteith's paper is
an extraordinary example of recent synthesis, as it combines the latest
findings of biochemistry, physiology, and environmental physics.
2.5.2. BIOMETEOROLOGICAL MODELING
Current research on the microclimatological boundary-layer
scale is extremely active. The field has been influenced by many of the
physical sciences, as each field‘s advances of knowledge contribute to
the understanding of the whole complex web of complicated processes.
With technological innovations, intricate measurements of biosphere—
atmosphere interactions have been made possible, e.g. the eddy-
29

43. covariance technique that simultaneously measures large-scale fluxes
of certain entities, e.g. CO2 concentration and vertical wind speed
(Monteith and Unsworth 1990) using highly accurate (and expensive)
sonic anemometers. The Penman-Monteith combination equation is
used in several papers that have been referenced (Blanken and Rouse
1994, Chen et al. 1997, Takagi 1998, Burba et al. 1999) to model
evapotranspiration at the leaf- and the canopy level, taking into account
the boundary layer conductance as meteorological conditions change,
i.e. stormy versus calm weather, or dry versus moist air. Generally,
measurements can be recorded with minimal time constraints, and
computer software allows for statistical modeling and plotting of the
data. Biometeorologic modeling is important in the attempt to make
predictions of future events. In an era where the conservation of
species richness has become a general concern, the modeling of
nutrient and surface water cycles becomes a helpful tool in
understanding multidimensional interactions between the many agents
of a biome.
Rey Benayas et al. (1999) approach the quantification of species
richness by modeling the relationship of "- and -diversity" of species
to "moisture status and environmental variation". In their study,
"environmental status is measured as actual evapotranspiration." This
30

44. approach deems especially interesting, since the loss of wetlands due
to development has been rapid. While many states have a
development prohibition of wetlands intended for their general
protection as densely populated, species rich areas, money still seems
to have the last word too often, and development of wetland areas is
still a possible threat to their inhabitants (refer to MaryPIRGS, 1999,
when The University of Maryland wanted to build a new stadium on a
wetland and succeeded).
A large amount of current research focuses on exploring
biometeorological processes in forests, wetlands, and grassland
vegetation. Some papers are part of a joint effort of exploring major
regions of the earth, and those regions‘ importance on a global level.
An example of such a project is the Boreal Ecosystem-Atmosphere
Study (BOREAS), which according to Chen et al. (1997) "has the goal
of understanding the contribution of boreal ecosystems to the global
carbon budget and their response to global change". He goes on to
explain that "solar energy is the driving force for biological activities
resulting in the observed energy and gas fluxes". He further elaborates
that the canopy structure, i.e. over- and understory features "requires
special attention in the radiation modeling". Overall goals of Chen et
al.‘s study were to compare the radiation balance inside the canopy" at
31

45. different times throughout the growing season and to assess general
patterns of leaf area index (LAI) over a "nearly complete seasonal
cycle." LAI is an important variable that needs to be measured to
model canopy stomatal conductance. Measured in square meters of
leaf area over square meters of ground, this index quantifies the
magnitude of photosynthetic potential, i.e. the leaf area above ground
through which gas exchange can occur, best pictured in the comparison
between a tropical forest (LAI~12) and a desert with sparse vegetation
(LAI~0.2). In his concluding discussion, Chen et al. state that LAI is
important not only because it "defines the photosynthetically active leaf
surface area responsible for plant growth and CO2 uptake", but also
since it delivers an estimate of rainfall that is intercepted by the leaves.
Lastly, he includes how the latest efforts to estimate LAI have improved
the applicability of remotely sensed data on canopy structure.
Rouse (1998) uses a water balance model to generate data for
General Circulation Models (GCM's) that attempt to predict future
climatic scenarios. As Rouse determined in his study on a subarctic
sedge fen, the increase in air temperature over the next decades will
lead to a drier environment of the present day fen, unless precipitation
increases by more than 20%. He goes on to predict several scenarios,
including extremely wet and extremely dry years, and their effects on
32

46. the fen habitat. With such a significant change in the water balance of
fens like this, the decrease in species richness is almost certain. A
critique of GCM's however, was made by Blanken (pers. comm. 2001).
According to him, "GCM's still fall apart today", because the missing
data about soil make-up and moisture is not measurable through
satellite observations.
The application of models contains multiple sources for potential
error, because their derivations rely on assumptions that are only barely
true in certain scenarios. If the research area in question deviates from
the scenario described in the model, e.g. a crop field could qualify for
the assumption of horizontal homogeneity, not though a forest, the
scientist will have to correct for these deviations, or chose a different
model altogether. It is the responsibility of the scientist to use
statistical models in a sensible way, and to refrain from tasks that are
too complex for the human mind to explain.
2.6. CONCLUSION
The field of biometeorology has made invaluable progress over
the last decades, and much of this success stems from the continuing
effort of scientists to synthesize their specialized research. The reader
may ask where the discipline is headed, and where the goals for future
33

47. research should be placed. In 1969's "Geography and Public Policy",
Gilbert White emphasized the importance of "translating findings into
changed public policy". The pursuit of a profession should undoubtedly
be linked with the incentive to make a change for the better. For why
should geography "fabricate a nifty discipline about the world while that
world and the human spirit are degraded?" In tune with Gilbert White's
spirit, one has to ask, what are the "truly urgent questions" of today,
and whether researchers are able to tackle research questions "in the
light of possible social implications?" as there are bountiful problems to
be solved, both on the local and the global scale.
A change for the better to which everyone can contribute through
personal input and research reaches out toward reestablishing
inalienable rights not only for human beings, but also for every species
that inhabits this planet. Also, other geographical fields like urban
geography are developing proposals that increase sustainability in
cities, ideas that may decrease people's needs to migrate further and
further into other species' habitats. Interdisciplinary, physical research
in biometeorology will be a necessary and powerful tool in changing
public policy. Understanding ecosystems and all agents that steer
them, as well as potential changes in biomes through anthropogenic
impact may enable inspired researchers to succeed in reaching their
34

48. goals, engaging all sources of creativity. Here's to Gilbert White: "We
must work with all our heart and mind".
35

49. CHAPTER 3. BACKGROUND
3.1. INTRODUCTION
The role of E in the water and energy balance of high latitude
wetlands is well documented (e.g., Blanken and Rouse 1994, Rouse
2000). Further, studies quantifying this flux have been conducted on
fairly homogenous areas like forest canopies or sedge meadows (e.g.,
Blanken and Rouse 1995), and stomatal conductance has been scaled-
up to the canopy level using a leaf area index (e.g., Chen et al. 1997,
De Pury and Farquhar 1997). Additionally, habitat loss and decreasing
biodiversity have recently found increasing attention in both public and
academic spheres. Whereas Ehrlich (1994), Pimm et al. (1995), and
Myers et al. (2000) focused on biodiversity hotspots and conservation
priorities, Blanken and Rouse (1996) investigated fine-scale processes
in specific habitats and assessed the ecological and meteorological
characteristics that explain the existence of particular plant
communities. Lastly, Rey Benayas et al. (1999) developed an index
that correlates E of an area to its biodiversity.
Wetlands in particular are known for both their exceptional
properties to filter water and to provide habitat for species that depend
on a unique combination of environmental factors, forming an oasis for
example, for waterfowl that often travel several thousands of kilometers
36

50. to satisfy their physiological demands at such sites. Plant diversity of
such areas is often remarkable; therefore, varying spatial and temporal
distributions of limiting or controlling factors deserve special attention.
Recent data indicate a 53% loss of U.S. wetlands between 1780
and 1980 (Moser et al. 1996), and data for Colorado estimate an annual
loss of 60 acres in the state alone (Denver Post, Dec 8, 2000). This
loss is mainly due to Colorado‘s population increase and concurrent
growth of development and water demand. Colorado ranks eighth in
the list of states with the largest net population gains recorded from
1995 to 2000 (U.S. Census Bureau 2000). Working to keep biodiversity
loss minimal, The Nature Conservancy (TNC), a global organization
dedicated to the preservation of endemic species and natural
communities, has purchased over 50,000 acres of land in Colorado with
the objective to preserve and restore native species and biological
communities. Brand and Carpenter (1999) have stated that TNC
strives for ecologically intelligent decisions through collaboration with
scientists to characterize future site management strategies.
High Creek Fen, a 750-acre extreme rich fen 2850 meters above
sea level (a.s.l.) near Fairplay, CO, is part of TNC‘s preserve system.
TNC, as well as the scientific community in general, is lacking accurate
data for this type of ecosystem in the Rockies. This research fills part
37

51. of this knowledge gap, and lays the groundwork for the formation of
successful management strategies to be implemented by TNC over the
next several years.
3.2. PHOTOSYNTHESIS AND ENERGY BALANCE
Through photosynthesis, plants use the sun‘s photosynthetically
active radiation (PAR), referred to in this work by quantum flux [Q], to
produce the energy required for the synthesis of carbohydrates. Q,
which represents the flux of PAR in the visible spectrum, is included in
the sun‘s electromagnetic field between 0.4 and 0.7 m. Cell water
necessary for photosynthesis evaporates through the stomata at rates
that are determined by the magnitude of stomatal conductance in
addition to other factors. Inevitable while stomata are opened, the loss
of water due to a water vapor deficit of the ambient air surrounding the
leaf additionally offers evaporative cooling to the leaf‘s surfaces. Up to
the point where physiological constraints or N availability limit the
turnover rate of the Calvin cycle, Q is a strong driving force in the
photosynthetic process (Monson 2000).
The maximization of photosynthetic potential is accounted for by
physiological differences in plants, differences such as density of
chlorophyll pigments, leaf thickness, LAI, and density of stomata per
38

52. leaf area (Monson 2000). Increased density of chlorophyll pigments,
roughly translatable into the ―greenness‖ of the leaf, allows the plant to
absorb energy faster than lighter-colored leaves that have a lesser
amount of chlorophyll per leaf area. Thicker leaves allow the plant to
capture more Q. These details strongly influence the plants‘ ability to
make maximum use of the photon energy. Furthermore, the overall
budget of potential CO2 assimilation of a plant depends on its LAI.
Additionally, distribution of stomata takes different densities according
to the urgency to minimize water loss. For example, tropical leaves
compared to xerophytic leaves have dense versus sparse
concentrations of stomata, respectively. Because leaf surfaces are the
interfaces of plant correspondence and mass and energy exchanges
with the overlying boundary layer, investigating all leaf processes is
important.
For a plant, the visible wavelengths are not the only solar energy
spectrum of interest. All wavelengths outside the visible range are
important to the plant, because they culminate in the total amount of
energy available at the surface of the plant‘s habitat. Thermal energy,
which partially translates into air temperature, is another factor that
determines the rate of photosynthesis. Optimal leaf temperatures [TL]
39

53. for C3 plants usually range between 30 and 40 C, but plants can also
alter their optimum to match their typical environment (Nobel 1999).
The overall intensity of solar radiation that reaches the plant
depends on the solar angle, which is a function of the time of day and
year, latitudinal position, and leaf orientation. Additionally, depth and
density of the atmosphere above the plant determine the amount of
energy (and actual CO2 concentration, which depends on atmospheric
pressure, and may therefore be considered lower at High Creek Fen
than at sea level) that arrives at the surface of the earth. Intuitively, the
sun‘s intensity will lessen with cloud cover. A thin atmosphere, present
over high elevation sites, allows for less absorption of solar radiation
during its way through the atmosphere, and thus has a more intense
impact on the surface compared to thicker cloud cover, or an
environment at sea level.
The net radiation (Rn) consists of the incident short-wave
radiation that strikes an area (K) minus the amount that is reflected off
that surface (K), plus the incoming long-wave radiation (L) minus the
amount that is radiated from that same area (L), the latter is a function
of the surface temperature and emissivity at a particular location.
Hence, we have the equation
Rn = (K- K) + (L - L) (1).
40

54. Energy at the surface can be expressed in Watts per square meter
(W m-2), or in micromol per square meter per second (mol m-2 s–1).
The energy available for absorption (transmittance, and reflectance) by
the leaf is a strong determining factor in the photosynthetic process and
the energy balance over an area.
Micrometeorologists like to follow the fate of the net radiation in
its distribution at the impacted surface, because it is a distinct way of
looking at the environmental dynamics of an area. The net radiation is
partitioned into three main terms, i.e. the energy is distributed into the
heating of air (H), the transformation from water into water vapor,
(evaporation or E), and into the heating of the ground (soil heat flux
[G]). It follows that
Rn = H + E + G (2).
Usually, due to the dense ground cover at High Creek Fen, the
lesser part of the net radiation goes into the heating of the ground.
(Over areas with bare soil, however, the partitioning changes.) The
distribution of Rn between H and E is often expressed as the Bowen
ratio (), where  = H/ E. Generally, the Bowen ratio takes on
numbers between 0 and 5, where the latter would typify an extremely
xeric, and the former an intensely humid environment. Another effect of
Rn at the surface is upon Tair and the temperature dependent
41

55. atmospheric water vapor deficit [D]. D exerts another strong control
over plant transpiration. As stated above, water vapor diffuses from
intercellular air spaces and the stomata into the atmosphere. The flux
rate is subject to the differences in water vapor concentration between
the inside of the leaf (assumed to be 100 %) and the surrounding air;
the steepness of the gradient determines the flow rate. Diffusion of
water vapor from the plant into the atmosphere, based on the second
law of thermodynamics, or the law of entropy, can therefore
mathematically be expressed as follows:
E = -K cH2O / z, (3)
where K is the molecular diffusion coefficient for water vapor (from
higher to lower concentration), and cH2O / z is the difference in water
vapor concentration over the height of the leaf boundary layer, which
again is a function of wind speed. Strong winds will thin the boundary
layer over the leaf, increasing the gradient. A low relative humidity,
usually present at the daily peak of Q, forces water out of the plant
faster than a high relative humidity, which is generally common for the
morning hours. Hypothetically, the relatively constant wind at High
Creek Fen delivered warm, dry air from the arid Mosquito Range and
Park area in the west, and therefore increased the evaporative demand
42

56. at the surface. Hence, the large E above the fen is combined with dry
air (D max = 5 kPa).
Due to physiological constraints, a strong demand for water
vapor out of the leaf will likely lead to stomatal depression or full
stomatal closure. This adaptation allows a plant to control the amount
of water vapor leaving its stomata, since too great of a demand for
water vapor out of the leaf would result in cautation of water inside the
xylem and death of the plant. Soil moisture [ ] at the fen was plentiful
during the whole growing season, assuring the plants in their respective
locations a generally lesser stressed summer than may be expected
from plants located in semi-arid environments.
The daily pattern of  varied considerably between sites; soil
moisture recharge occurred either through atmospheric deposition, e.g.,
rain or dewfall (surface recharge) or through groundwater movement
(subsurface recharge). Intuitively, soil moisture can be expected to
gradually decrease during a day where photosynthesis occurs, reaching
a minimum at the photosynthetic peak, both due to root water extraction
and evaporation from the bare soil surface. At the densely vegetated
fen, however,  stayed high throughout the day, and was only slightly
influenced to a downward direction throughout a period of little rain at
the end of July 2001, when  measured at the tower showed a
43

57. minimum  of 93 %, which is to be considered saturated soil. In
contrast, investigating soil moisture control in non-saturated locations
allowed for testing of differences in intra-specific stomatal responses to
living in drier versus wetter areas of the fen. Summer 2001‘s studies on
B. glandulosa and S. candida both showed soil moisture control on g
and E. Attention to such physical and physiological factors as detailed
above is paramount in assessing the processes that govern plant
processes. These observations will now be communicated in light of
the above.
Photograph 3.1. Cumulus cloud (Cu) over High Creek Fen (view to
NE) in Summer 2001. Although never again in this exact shape, Cu
commonly form in areas adjacent to the fen during the summer season
in early or late afternoon.
44

58. 3.3. STUDY SITE DESCRIPTION
In the following paragraphs, the research site is described from
personal observation and as communicated through the literature.
First, a general description of the site‘s topography, hydrogeology, and
history, and last a focus on the environmental factors given by its
geographical location and local dynamics, including the energy balance,
microclimate, and soil moisture will be given.
High Creek Fen (Photograph 3.1.) is the largest remaining
natural fen in the South Park region of Colorado (Brand and Carpenter
1999). It is currently a nature preserve that has been managed by TNC
since 1990. The 750- acre wetland is located at 3906‘00‖N,
10557‘30‖W at an elevation of 2850 m, between the towns of Fairplay
and Buena Vista Figure 3.1.).
3.3.1. TOPOGRAPHY, HYDROGEOLOGY, AND HISTORY
Topographically, South Park lies in a flat valley surrounded by
the Mosquito Range to the west, the Kenosha and Taryall Ranges to
the north, and the Rampart Range to the east. The wetland, located
just east of Black Mountain (igneous remnant), shows a gentle change
in elevation from its highest (2850 m a.s.l.) northwest corner to its
lowest (2810 m a.s.l.) southeast corner.
45

59. Geologically, (visible from a geologic map of the area) High
Creek Fen is underlain by easterly dipping Cambrian through
Pennsylvanian sedimentary rocks (quartzite, shale, and dolomite)
deposited on a Precambrian basement complex of gneiss and schist
(the Idaho Springs Formation). These easterly dipping sedimentary
rocks represent the eastern limb of the Sawatch Anticline to the west.
The bedrock geology is obscured at High Creek Fen by surficial
deposits of Quarternary gravels and alluvium, and the underlying
geology has been inferred by projecting the geology of the adjacent
Mosquito Range to the east (Misantoni 2002).
Hydrogeologically, the fen is subject to complex variables. The
ground water pattern is influenced by both the Creek as well as the
make up of the material described above. Following the gentle slope,
High Creek supplies the fen grounds with fresh (and relatively warm)
spring water from the northwest, and leaves the area to the southeast.
Additionally, the underlying formations contain several aquifers, e.g.,
the Leadville and Quarternary aquifers. Several scenarios concerning
the delivery of ground water into the alluvial substrate and fen soil are
viable: (1) ground water is recharged from aquifers through several
Paleozoic strata by ways of faults and fractures (Shawe 1995, Appel
1995) that reach into the alluvium through its semi-permeable bottom
46

60. layer, or (2) ground water is recharged from one formation only, (e.g., a
layer of shale forms an aquifer) topped again by a semi-permeable
layer reaching into the alluvium, or (3) the alluvium is itself an aquifer
with an impermeable bottom layer, and recharge is either not yet
necessary (last glacial period only ended 10,000 years ago), or is
partially achieved from surface water. While the shallow ground water
level at High Creek Fen may be due to any, all of, or additions to the
above scenarios, the ground water level was relatively constant
throughout the years 1995 – 1998 (Johnson 1998) and 2000/ 2001
(tower data). The water supply to the fen, however, may be threatened
by water-use projects such as the ―South Park Conjunctive Use Project‖
(now fallen through), in which the city of Arvada would have been
supplied with water from this region. While it is unknown whether a
drop in the water table at the fen would likely occur after one or 100
years, such projects present a definite threat to sufficient supply of  for
the already dry environments surrounding the fen, including several
ranches, i.e. livelihoods of the locals.
The high E during the summer months as well as relatively
constant  even after atmospherically dry days both mandate a
perpetually active groundwater recharge. A transect of  taken
diagonally across the fen with a water content reflectometer revealed
47

61. values between 8% outside the fen and 60% within the fen with soil
texture ranging from clay to silt with varying organic matter contents.
This transect of  taken throughout the fen in summer 2001 (Figure
3.1.) and an accompanying photograph to gain perspective on the
transect (Photograph 3.2.) can be viewed below.
Photograph 3.2. View across the fen from NW (transect survey pole)
to SE shows approximate transect location; the location of the
meteorological tower is included on transect. Note: this picture was
taken in Winter 2001/ 2002, while the transect data graphed below
(Figure 3.1.) was collected July 1st 2001.
48

62. 60
Tow er
50
Volumetric Soil Moisture [%]
40
30
20
10
0
0 200 400 600 800 1000 1200
Dist ance [m]
Figure 3.1. Soil moisture transect from southeast (0) to northwest
(1000 m) taken across the fen on July 1st, 2001. With distance
increments of 33 m, 31 data points were recorded. Low  values
represent areas outside the fen.
49

63. Photograph 3.2. View across the fen from NW (transect survey pole) to
SE shows approximate transect location; the location of the meteorological
tower is included on transect. Note: this picture was taken in Winter 2001/
2002, while the transect data graphed above (Figure 3.1.) was collected
July 1st 2001.
Historically, small portions of High Creek Fen were disturbed
during a short period of peat mining from the 1970s until the mid- 1980s
(Schulz 1998), when 22 of the 750 acres were mined. Since 1992,
attempts have been made to restore plant communities (Sanderson,
pers.comm. 2001). Disturbance also occurred while High Creek Fen
was open to grazing by cattle and sheep since 1860 and prior to that by
50

64. bison, elk and antelope (Brand and Carpenter 1999). Apart from the
above, High Creek Fen has remained undeveloped and largely
undisturbed.
3.3.2. CLIMATE AND ENERGY BALANCE AT HIGH CREEK FEN
The harsh climate of High Creek Fen is characterized by intense
solar radiation, strong winds, and little precipitation. Due to its high
elevation, on cloudless days, High Creek Fen is exposed to a solar
peak of 2500 mol m-2 s -1 during 10:00 and 15:00 hours mountain
daylight time (MDT) throughout the height of the growing season; this
amount is 1.25 times higher than the average sea-level peak of 2000
mol m-2 s –1. Winds typically originate from the northwest; peak
observations of up to 150 km per hour have been made on the ridges to
the N and W, e.g., Boreas Pass and Windy Ridge (Cusack, personal
communication 2001). While the Mosquito Range to the west of the fen
functions as a rain shadow most of the time, convective clouds
(Photograph 3.1.) are common in the summer time; they supply most of
the precipitation recorded throughout the year. As stated above,  is
generally recharged by the ground water of High Creek Fen and barely
influenced by local precipitation. The mean total annual precipitation
between 1961 and 1997 at the nearby weather stations Antero
51

65. Reservoir and Fairplay was measured to be 234 mm and 352 mm
respectively (Brand and Carpenter 1999). Those long-term recordings
also show that 40% of this precipitation falls in July and August. On-
site measurements, while on a different scale, indicate that 121 mm
precipitated onto the fen in the summer of 2001. Thus, High Creek
Fen‘s location exhibits extreme conditions of little precipitation and high
solar radiation; high soil moisture (Figure 3.1.) and special soil
chemistry and nutrients are conditional for the relatively dense and lush
vegetation present throughout the site (Blanken, pers. comm. 2001).
While High Creek Fen is exposed to the above-mentioned
regional meteorology, its microclimate differs from those of the
surrounding areas. During the photosynthetically active hours of the
days of this study, TS ranges were small, e.g., 2.5 or 3.5 C; such small
difference between minimum and maximum TS during daylight hours is
mainly due to the high volumetric moisture content of the soil,
perpetuated by an insulating, dense ground cover. Further, the diurnal
trend of D over the fen has a distinct shape and large amplitude. In the
morning, D has been measured as low as 0.2 kPa (in this case, 80 %
relative humidity). At the warmest part of the day, D can be as high 2.3
kPa (in this case, 25% relative humidity), both due to the solar heating
of the air, and the increasing, dry winds typically from the northwest.
52

66. Maximum D was measured by the porometer over S. monticola at 5
kPa with a TL = 36 C and Q = 1800 mol m-2 s-1 and  = 40 %.
Due to its high elevation, the vegetation of High Creek Fen is
comparable to that of high-latitude wetlands of the boreal and tundra
regions (with exception of the perma-frost layer), where, as mentioned
above, E can comprise close to 80% of the net radiation. At High
Creek Fen, preliminary measurements of E using the Bowen Ratio
suggest that E is an important component of the wetland‘s water cycle,
and also, that the source of the water that is available for plant
transpiration cannot solely be local precipitation, but must primarily be
supplied by deeper rock units, or adjacent uplands.
3.3.3. VEGETATION AT HIGH CREEK FEN
The growing season lasts from early June until mid- September;
the ground is thawed from May throughout October. The vegetation
pattern can broadly be divided into upland and wetland types (Brand
and Carpenter 1999). The vegetation of the wetland exhibits great
variety in comparison with the adjacent upland areas (Cooper 1996,
Sanderson and March 1996). A description of both upland and wetland
species can be found in Cooper (1996) and Brand and Carpenter
(1999).
53

67. Wetland habitats include hummock communities, meadow
communities, spring fen communities, and a sodic flat community
(Cooper 1996). Dominant shrubs of the wetland are several willow
species, including silver willow (Salix candida), myrtleleaf willow (Salix
myrtillifolia), planeleaf willow (Salix planifolia), mountain willow (Salix
monticola) and barren-ground willow (Salix brachycarpa). Also
abundant are dwarf birch (Betula glandulosa), which inhabit mostly the
hummock and meadow communities, but also border the drier sodic flat
communities, as well as the moist spring fen areas. While kobresia is
the dominant grass throughout the fen, abundant especially at the
wetland‘s platform are sedges, mainly water sedge (Carex aquatilis)
(Photograph 3.3.). Furthermore, the existence of several state-rare and
globally-rare plants at High Creek Fen, including porter feathergrass
(Ptilagrostis porterii) and pale blue-eyed grass (Sisyrinchium pallidum)
supports TNC‘s recent suggestion that the fen is a globally significant
site. The species diversity at High Creek Fen is exceptional, deserves
scientific attention, and may be dependent upon protection from
anthropogenic disturbance such as a lowering of the water table.
54

68. Photograph 3.3. Dense ground-cover of willow, birch, and sedge at
High Creek Fen, Summer 2001. Blue spruce in the background greatly
influence turbulence at the site.
3.4. THE FOUR SITES AND THEIR INHABITANTS
All sites served as environments to investigate the importance of
soil moisture, water vapor deficit of the atmosphere, leaf temperature,
and solar radiation on stomatal conductance and plant transpiration.
Spatially,  is highly variable, and while some plants, e.g., B.
glandulosa seem to be tolerant of a wide spectrum, others, such as S.
candida are restricted to a narrower range.
55

69. The research sites were chosen to control for , plant composition
and accessibility. Measurements of leaf conductance, transpiration,
vapor pressure deficit, leaf temperature, and solar radiation were taken
on several randomly chosen days dispersed throughout the growing
season from early June until late August 2001. Additionally, soil
moisture measurements were taken at each plant. Data were collected
from sunrise until sunset, weather permitting. This study focused on six
plant species abundant in the fen: Betula glandulosa, Salix candida,
Carex aquatilis, Salix monticola, Salix brachycarpa, and Salix planifolia.
B. glandulosa (Photograph 3.4.) grows on sites varying in  from
15% to 60%, constituting a good indicator for potential soil moisture
control on its stomatal conductance and transpiration.
56

70. Photograph 3.4. Betula glandulosa (Swamp Birch) in a drier location
at High Creek Fen, Summer 2001. This species occurs in a range of
locations where 15 % <  < 60 %.
In contrast, S. candida (Photograph 3.5.) was not found in areas
with less than 35% average volumetric soil moisture. However, it was
chosen as a study organism since these plants are state-rare glacial
relicts, which are not found anywhere else in the Southern Rocky
Mountain region but at the South Park fens. Assessing their
environmental constraints is of great interest to the botanical
community, and existing work on this plant species in Manitoba,
Canada (Blanken and Rouse 1996) allowed a general comparison
between the plant‘s behavior on a latitudinal gradient.
57

71. Photograph 3.5. Close view of the thick, dark-green leaves of Salix
candida (silver willow). Although not measured, leaf appearance
suggests a multi-storied photosynthetic apparatus and dense
chlorophyll pigmentation.
C. aquatilis is the most abundant sedge in portions of High Creek
Fen, offering necessary data for future mapping of transpiration
throughout the fen. A sample of one specimen can be seen in
Appendix A. S. monticola (Photograph 3.6.) is the most abundant
58

72. willow of the South Park region (Sanderson, pers. comm. 2001), and
comparing its environmental constraints with those of the rare S.
candida was an integral part of this project, as this allowed a look for
potential constraints to S. candida’s occurrence in these latitudes. S.
brachycarpa (Photograph 3.7.) and S. planifolia were chosen to further
the investigation of on-site willows for comparison of stomatal response
of different willow species to varying environmental factors.
Photograph 3.6. S. monticola Photograph 3.7. S. brachycarpa
59

73. 3.5. STUDY HYPOTHESES
The research presented here investigates interactions of the
environmental factors explained above. It explains the nature of the
correlations between stomatal conductance [g] and transpiration [E]
from the leaf with the meteorological and soil moisture conditions that
exert limitations and affect the magnitude of transpiration. This
research is expected to explain several processes and therefore to
enhance the understanding of the interrelationships between
meteorological and plant physiological processes. In particular, it
shows a spatial variability of E corresponding to the heterogeneity of
the vegetative surfaces. It strives to explain the nature of the
correlation of g and E from the leaf with the meteorological conditions
that exert limitations on the plant physiological processes. This
research expands former analyses to include the effects of  on the
magnitude of E;  is expected to be also highly variable throughout the
fen. This research focused on testing three specific hypotheses, which
are outlined below.
60

74. 3.5.1. PROBLEM STATEMENT 1: DOES HEIGHT ABOVE GROUND
INFLUENCE PHYSIOLOGICAL RESPONSES WITHIN AN
INDIVIDUAL SPECIES?
Stomatal conductance and E from distinct heights in an
individual plant above ground may vary because light absorption in the
leaf depends on the magnitude and partition between direct and diffuse
radiation that reaches to the vertical leaf layers of a plant, and because
the plant itself creates its own microclimate that may, for example, alter
the vapor pressure deficit of the air surrounding the leaf [D] so that a
leaf at the top of the plant may experience a higher D than a leaf in the
middle of the plant. Such differences would lead to diverging values of
g and E from different heights above ground, and if sufficiently large,
would have to be considered when extrapolating from the leaf to the
canopy level. Hence, the magnitudes of g and E from three leaves of
the same plant (S. monticola) at heights of z = 40, 70, and 100 cm
above ground were compared.
It was hypothesized that no significant differences in both g and
E from the three leaf levels of the same plant would be found.
61

75. 3.5.2 PROBLEM STATEMENT 2: DOES SOIL MOISTURE
CONTROL RATES OF STOMATAL CONDUCTANCE AND
TRANSPIRATION FROM THE SAME SPECIES IN DIFFERING
LOCATIONS?
Soil moisture in High Creek Fen is incomparably higher than that
of its immediate surroundings, i.e. most of the Southern Rocky
Mountains. One goal of this study was to assess a species‘ sensitivity
to water stress, and to suggest scenarios that may occur with an abrupt
lowering of the water table due to increasing anthropogenic water
demand. Hence, the control of  on the magnitudes of g and E was
quantified for both B. glandulosa and S. candida. B. glandulosa was
chosen because of its occurrence in locations with a wide range of  as
well as its abundance within the Southern Rocky Mountain region, and
S. candida was chosen both because of its narrow range of  and its
extraordinary occurrence in the latitudes where this fen is located.
To investigate S. candida‘s response to  (Problem Statement
2.a), g and E from two individuals were compared. Their respective
mean equaled ~45 % at the drier, and 50 % at the wetter site. The
plants were 20 m apart, were approximately the same height, and
appeared to be of similar age. A significant difference in the magnitude
62

76. of the average g and E from the plants in the different soil moisture
categories was hypothesized.
To test discrepancies in g and E from B. glandulosa (Problem
Statement 2.b), nine plants located in differing soil moisture conditions,
three with mean= 18 %, three with mean= 35 %, and three within fully
saturated soil (mean= 60 %) were compared. The plants were within a
radius of 50 m of each other.
3.5.3. PROBLEM STATEMENT 3: WHEN EXPOSED TO THE SAME
MICROCLIMATE, DO DIFFERENT SPECIES VARY IN STOMATAL
CONDUCTANCE AND TRANSPIRATION?
Variability in g and E from different species must be understood
when quantifying or modeling E above a site like High Creek Fen,
where a great variety of species is represented. A comparative
investigation was designed to assess the physiological differences
between species, to determine plant sensitivity to water stress, and to
identify certain plants as early-warning indicators to changes in the
amount of plant available soil moisture at the fen. A site representative
of the fen was chosen to record g and E from different species, i.e. B.
glandulosa, S. candida, C. aquatilis, S. monticola, S. brachycarpa, and
S. planifolia that were within a radius of five meters of each other in
63

77. order to minimize microclimatic and site differences. Especially, the
effects of Q, TL, D, and  on the magnitudes of the g and E from the six
plants were investigated. A significant difference between the six rates
of g at any point in the day was hypothesized, and E was expected to
differ among species.
The testing of all three hypotheses was to enhance the
understanding of arctic and high elevation wetland species, allow for a
comparison of physiological distinctions between common and rare
plants of the area, and help assess the sensitivity of high elevation
plants to microclimatic variability, soil moisture availability, and
disturbance.
64