Examining the mechanistic pathways of interactions between shrubs and annual plants in semi-arid and arid ecosystems
1. EXAMINING THE MECHANISTIC PATHWAYS OF
INTERACTIONS BETWEEN SHRUBS AND ANNUAL
PLANTS IN SEMI-ARID AND ARID ECOSYSTEMS.
Alessandro Filazzola
PhD Candidate
Progress Report – March 6th
6. Dominant-mediated distribution
Survival of experimentally planted seedlings of Taraxacum
officinale within and outside Azorella monantha cushions at
3200 m a.s.l.
Cavieres et al. 2007
7. Purpose
To examine or decouple the different
mechanistic pathways and drivers of
plant-plant interactions in arid systems
8. Chpt Title Progress
1 A systematic review and conceptual framework for
the mechanistic pathways of nurse plants
Published in GEB, Summer 2014
2 Contrasting effects of drought-stress and consumer
pressure on plant-plant interactions in an arid
shrub-understory system
Submit to Ecology
3 The effect of a dominant shrub on annuals plants in
a desert system changes along a gradient of
continentality.
Active – repeat census in March 2015
4 Shrubs directly and indirectly increase seed
densities under canopy in arid environments
Data collection complete
5 Shrubs expand the species distributions of annual
plants in arid environments
Begin survey – April 2015
9. Chapter 3
Purpose: To determine how desert shrubs influence
annual plant productivity along a regional gradient of
climate.
H: The magnitude and frequency of positive
interactions between shrubs and annuals will be
greatest at intermediate levels of stress along a
regional gradient of continentality.
C3
16. Chapter 5
Purpose: To examine the role of shrubs in expanding
the range of annual plants.
H: Shrubs may be able to expand the realized niche
of an annual species beyond their distribution by
reducing the abiotic stresses that would otherwise limit
their establishment.
C5
24. Future work
Chpt Title Goals
3 The effect of a dominant shrub on annuals
plants in a desert system changes along a
gradient of continentality.
Conclude census in March-April 2015
Data inputted June 2015
4 Shrubs directly and indirectly increase seed
densities under canopy in arid environments
Conduct statistical analysis & write-up
Fall 2015
5 Shrubs expand the species distributions of
annual plants in arid environments
Begin survey – April 2015
Field manipulations – Jan 2016
By now we know positive plant-interactions are the crux of my thesis. There are multiple pathways where a dominant plant can affect those around it. This includes the popular stress amelioration pathways, such as increasing soil moisture, reducing heat stress and modifying soil nutrients. But it can also include lesser examined pathways such as seed trapping or increased pollinator visitation. My research looks at tackle multiple different pathways and seeing the effect on community composition.
Part of my thesis, is to try and decouple these different types of mechanisms. In looking at papers that study plant-plant interactions, typically most studies only examine one pathway. There are accumulating evidence of other papers examining two or three, but often this is just measuring a similar factor. For example, soil moisture and temperature would be considered two mechanisms.
This is particularly important because inclusion of certain mechanisms, such as consume pressure can dramatically alter the outcome of plant-plant interactions. Here is a paper by Smit et al. detailing the interaction between hebivores and nutrient limitation. RNE represents the effect on a plant by its neighbours. While increasing salinity from low to high, increases the positive neighbour effect, in the presence of herbivores, the is a consistently positive neighbour effect. This could be because neighbouring plants act to deter the herbivore.
Another under-examined factor in plant-plant interactions is the effect of regional drivers such as climate. The location of two interaction plants will strongly determine the interaction that occurs between them. There has also been much debate over trying to generalize the occurrence of positive interactions. Recent evidence suggests that when the climate gets too severe, positive interactions collapse and competition becomes the relevant driver again. Yet conversely, there are instances of positive interactions even in the most stressful environments including a island off Antarctica. Trying to better understand what predicts these interactions is thus important given a changing climate.
A related mechanism that I described, is that of dominant or shrub-mediated distribution. Certain plant species specifically associate with these dominant plants, especially on the peripheries of the species distribution. This is because the microclimate the shrub creates could be the difference between a plant surviving or not when compared to an open area. In the high Andes, Cavieres grew non-native dandelion which normally could not survive alpine environments in cushion plants and found that there was a 50% survival rate. This may suggest that dominant plants can perhaps mediate range shifts of plant species by reducing inter-annual variability.
For my dissertation I am focusing around five chapters, with one being a review and the other four being field experiments. My first chapter was accepted last year in Global Ecology and Biogeograhy and outlines the different mechanistic pathways that I am testing within my thesis. My second chapter aimed to decouple abiotic and biotic stress as drivers of shrub-annual interactions. I have finished my analysis and write up. My next step is to revise some comments and submit to Ecology. My fourth chapter examines seed-trapping which is very poor studied and is compete but still needs to analyzed and written. This leaves my chapters 3 and 5 which are currently active and that I will be talking about today.
My chapter three intends to determine how shrubs influence annual plant productivity along a regional gradient of climate. I expect that positive interactions will be most frequent and strong at intermediate levels of stress, but decline at extremes such as too much precipitation or too little.
To do this I needed to find a gradient of precipitation. Working with a collaborator in France we came up with three possible options using Worldclim data for cities in southern California and a PCA. The red arrow represents the largest progression in vegetation composition through four different deserts, however it did not relate to overall climate. The green arrow represented the bimodal rainfall occurrences of the Sonoran Desert. Albeit a significant trend, the variability between years was too high. We therefore settled on the blue arrow which presents a gradient of continentality and decreasing precipitation.
My next step was to find a dominant shrub and frequently observed annual for a phytometer. I had already done some work with Ephedra and therefore used CalFlora to find instances of it along the gradient in Southern California. I chose Amsinckia as the phytometer because it is a widely distributed plant that I have observed frequently and is particularly thorny which I hoped would deter herbivory, reducing the impact of consumer pressure.
I chose five sites based on a series of criteria. These being that it had ephedra in somewhat frequent abundance, it was along the regional gradient, the elevation was within a 500 meter range of each other and permissions to conduct research. At each of the 5 sites I put 0.5 grams of seed in two plots under shrubs and in open. At one of the plots, I left existing annuals intact and in others I removed. I subsequently watered and set up an irrigation unit to keep the soil moist.
Here are my five sites along the gradient. Temperature does increase to as much as 10 degrees between the first and last site. Precipitation does decrease with the exception of site 3, however site 3 actually had a significant amount of rainfall this year. This is because although the surrounding area in the 30 arc-second pixel is very dry, the site I chose ephedra had a higher amount of precipitation. This shows that while the world clim data is an excellent starting point, its resolution is a draw back. Particularly in Mojave where I’ve work in sunlight while thunderstorms have raged only a few kilometeres away.
My chapter 5 is a bit of a work in progress. I would like to show how shrub association can increase along a gradient. I do not want this to be another stress gradient hypothesis paper, but instead one that implies that shrubs can extend the distribution of an annual species by functioning like habitat islands. I think that shrubs may be able to expand the fundamental niche of an annual species, similar to that of the Cavieres paper.
The gradient I have tentatively chosen for this experiment is a road within the Mojave that climbs from 300 m asl to 1200 asl. With this increase in elevation temperature decreases and humidity increases. Analogously, the variation within readings also increases dramatically. These climatic differences may be threshold points for certain species that cannot grow higher than a certain temperature.
The gradient I have tentatively chosen for this experiment is a road within the Mojave that climbs from 300 m asl to 1200 asl. With this increase in elevation temperature decreases and humidity increases. Analogously, the variation within readings also increases dramatically. These climatic differences may be threshold points for certain species that cannot grow higher than a certain temperature.
I will also collect seed of particular species that shows increasing shrub association and replace it at all the sites along the road. If the plants grown under shrub are most successful, than the shrub is successfully extending its distribution.
I will also collect seed of particular species that shows increasing shrub association and replace it at all the sites along the road. If the plants grown under shrub are most successful, than the shrub is successfully extending its distribution.
I will also collect seed of particular species that shows increasing shrub association and replace it at all the sites along the road. If the plants grown under shrub are most successful, than the shrub is successfully extending its distribution.
Currently I have 5 sites along this road that I have previously collected data on. Collectively they create this sigmodal curve. I would like to increase the number of sites, perhaps to 7 to more accurately depict this curve, but also because interannual variability. While the threshold of a plant one year may be 25 degrees, it can change if the rainfall is poorer or some other confounding effect. I hope by adding more sites will strengthen the confidence in our response.
This is all to be linked to the bigger climate picture. California has an extremely variable climate and despite receiving heavy rains this year is still experiencing a terrible drought. It is therefore important to understand how plants interactions can limit the loss of biodiversity by maintaining species within an area otherwise unsuitable to them.
My next plans for work is to conclude my census for chapter three and input the data. Chapter 4 needs to be analzyed and written which I intend to do this fall. Lastly, Chapter 5 will begin with a survey this year and field manipulations next year.
