Evaluation of invasion strategies of Prunus laurocerasus (Cherry laurel), and Ilex aquifolium (English Holly)
1. Justin Bales
March 18, 2014
Evaluation of invasion strategies of Prunus laurocerasus (Cherry laurel), and
Ilex aquifolium (English Holly)
Description of Target Species
Prunus laurocerasus [Cherry laurel] is an invasive evergreen tree species native to
southeastern Europe and Asia Minor. It was introduced to North America as an ornamental
hedge. P. laurocerasus is typically found in sun or partial shade, and prefers moist, unsaturated,
acidic, and well-drained soil (Evergreen, 2014; KINGCO, 2013). It can grow to 10 meters in
height, and forms a dense evergreen canopy (KINGCO, 2013). It is competitive with native
species in forest environments, and is often dispersed by birds (EarthCorps, 2014). It is the
second most common invasive tree species in northwest forests, next to Ilex aquifolium
Ilex aquifolium [English Holly] is an invasive evergreen tree species native to Eurasia
and northern Africa that can grow to 23 meters in height (Stokes et. al 2014). It is commonly
dispersed by birds, and also forms a dense evergreen canopy (Jones and Reichard, 2009; Zika,
2010). Holly is generally very tolerant of differently soil types, but is typically considered
intolerant of wet soils (Peterken and Lloyd, 1967).
There appears to be a correlation between the distributions of Prunus laurocerasus, and
Ilex aquifolium in Puget Sound area urban forests such as St. Edwards Park in Kenmore,
Washington. The goal of this study is to determine if this association is simply spontaneous, or if
these similar distributions are caused by one or more specific factors.
2. One of the most commonly cited dispersal methods for both of these species appears to
be common, berry-consuming birds. While there has been minimal research into the exact
invasion characteristics of P. laurocerasus, it has been been cited as being at least partially
dispersed by birds in Pacific Northwest forests (Evergreen, 2014; KINGCO, 2013). I. aquifolium
has been studied more extensively. In Zika 2010, a study observing the consumption of seeds on
various naturalized and cultivated Ilex plants in the Seattle area, there were an observed 4,975
events of berry consumption by birds. Of these events, the American Robin consumed 95.12% of
berries and the European starling ate 3.56% (Zika, 2010). Holly has also been cited as bird
dispersed by other organizations, including King county and EarthCorps. Due to the possible
similarity of dispersion between the plants, and the likelihood that that they are spread by birds,
it appears as though bird dispersion could play a critical role in the dispersion of both P.
laurocerasus and I. aquifolium within St. Edwards Park.
In addition to looking at bird dispersion, this study will also investigate several other
dispersion factors, and spacial concepts. The first is to see if there actually is a correlation
between the locations of plants. While collecting geospatial information for I. aquifolium plants
in St. Edwards Park, we speculated that we were finding P. laurocerasus plants in many of the
same spots. We need to collect GPS coordinates, size, height, spread, sex type, and age of P.
laurocerasus and I. aquifolium within St. Edwards Park. Then we need to compare their spreads
with GIS analysis to see if these two species of plants actually co-occur as frequently as we
seemed to observe.
The second goal is to see if there actually appears to be any form of bird dispersion
method within St. Edwards Park. We will need to observe birds such as the American robin, to
see how they predate seeds of both I. aquifolium, and P. laurocerasus within St. Edwards Park.
3. Determining if birds are actively distributing seeds in the park can help in locating areas with a
high potential for invasion.
Lastly, the study will try to identify any environmental factors that could be limiting the
growth of both I. aquifolium, and P. laurocerasus. Personal communication with David Stokes
has identified four factors that could be potentially advantageous to I. aquifolium [Proximity to
edge of forest and/or edge of trails, presence of coniferous forest, and avoidance of water
corridors]. While these four factors certainly don’t automatically apply to P. laurocerasus, they
do offer a good starting point into possible explanations of dispersion characteristics of P.
laurocerasus based on all of the morphological similarities of these two species. It appears that
bird dispersion, in harmony with these environmental factors, sets precedence for when and
where I. aquifolium and P. laurocerasus will likely be successful within the park.
The first step of this process is to collect data on P. laurocerasus in previous study areas
of I. aquifolium at St. Edwards Park. By collecting data in a previously used area, we can
minimize cost and time consumption of trying to plot an entirely new study area while trying to
map not one, but two invasive tree species. I would recommend targeting either the southern area
covered in Winter of 2014, or several of the other study areas covered in previous years, as
shown in Map 1. Data collection methods for this portion of the study will be very similar to the
methods of Stokes et. al 2014. This is important for several reasons. Most significantly, this will
allow for consistency between both studies. Changing data collection methods with a study on P.
laurocerasus would essentially nullify any attempt to effectively use the existent I. aquifolium
data. Additionally, the system set up by Stokes et. al 2014 has proven to be very effective, and
well utilized over the course of their studies.
4. Data will be collected
by taking GPS coordinates of
each P. laurocerasus plant
within the study area. If there
is more than one plant within
25 meters, only one tree will
have GPS coordinates taken,
and then a compass bearing and
distance reading will be taken
for each other plant. Stokes et
al. 2014 has found this to be more accurate than GPS mapping every plant. In addition to this,
the height, canopy coverage, and canopy density will be covered. Measurements will also be
taken to find the cross sectional size at breast height, base height, and 20cm from the base on
each plant. Finally, cuttings will be taken to find the age of the plants.
Collection of this broad range of data is important because by finding the GPS
coordinates of the P. laurocerasus, you can establish if the plant is following similar dispersion
trends as I. aquifolium, using basic spatial analysis techniques in ArcGIS. There is also a
potential for the tree age information to be very useful, because similarly located trees with
comparable ages could have been introduced by the same bird, or influenced by the same
ecological factors. By gathering this data, we can establish if there is actually an association
between the environmental preferences and distributions of both I. aquifolium, and P.
laurocerasus in this portion of forests in St. Edwards Park.
5. While looking at the GIS data of the P. laurocerasus and I. aquifolium it is also very
important to test the environmental factors that seem to be leading to the dispersion of these two
species. As stated earlier, Professor Stokes has identified four potential factors that could be
potentially limiting to I. aquifolium, which include its proximity to edge of forest or the edge of
trails, the presence of coniferous forest, and its avoidance of water corridors. I would like to
apply these traits to the GIS data found for P. laurocerasus and see if there are any similarities
between the two species. Each of these four criteria would be selected in ArcGIS, and then buffer
zones could be used to determine if any of these traits share similarities between the two species.
This is important because if we can identify any similar trends between environmental factors of
the two plants, we can identify common dispersion methods, and common habitats that are likely
to be utilized by both of these invasive trees.
It seems possible that bird dispersion rates across the park could be relatively constant
throughout the preferred habitat areas of the American robin, however the environmental factors
within this area could be leading to increases and decreases of seed germination rates within the
park. In other words, birds could be applying a canvas of seeds throughout the park, but the seeds
are more likely to germinate in preferred ecological niches, which are potentially similar in both
I. aquifolium, and P. laurocerasus.
It is very important to observe seed predation habits in the park. The most effective way
to do this would be to establish a study area outside of the sampling area, with both I. aquifolium
and P. laurocerasus. The next step would be to find seeding trees of both species within that
area, and wait for birds to show up and forage on the trees. Zika 2010 reported that the most
effective method for noting seed consumption was to find a study area in November to February,
because this is prior to the breeding season of birds, where birds switch to mostly invertebrate
6. prey instead of berries (Zika 2010). Collecting consumption data would involve the capture of
GPS coordinates of a specimen tree, and then the observation of any birds that ate a seed. Each
seed would be tallied, and it would be noted if birds showed up individually or in a group. I
would also note where the birds flew after the consumption of the seeds, because this could
potentially lead to finding areas where defecation and seed release are most common.
Collecting consumption data serves two main purposes. First and foremost, we could see
if birds are consuming the seeds in St. Edwards Park. If there appears to be no seed foraging
behaviors within the park, then it is likely that some other factor is leading to the dispersion of
seeds throughout the park. If seed forage by birds is apparent and abundant, then that potentially
means that bird dispersion within the park is probably a leading cause of the spread of both I.
aquifolium and P. laurocerasus.
This study is important for analyzing the spread of invasive I. aquifolium, and P.
laurocerasus. It attempts to combine previous predictions of the spread patterns of both species,
while acknowledging the overall environmental characteristics that seem to be driving
dispersion. By researching the ways both I. aquifolium and P. laurocerasus spread, we are
presented with an opportunity to understand where these plants will likely invade, their impacts
on Pacific Northwest forest ecosystems, and how they will most likely disperse to these
EarthCorps. (2014). Invasive Trees Found in the Puget Sound Region.
Evergreen. (2014). Invasive Plant Profile- Cherry-laurel, English-laurel, Prunis laurocerasus.
Jones, C., Reichard, S. (2009). Current and Potential Distributions of Three Non-Native Invasive
Plants in the Contiguous USA. Natural Areas Journal.Vol 29, No. 4.
[KINGCO] King County. (2013). English Holly Ilex aquifolium. King County Noxious Weeds.
Mappery. St. Ewards State Park. Map.
Peterken, G., Lloyd, P. (1967). Biological Flora of the British Isles- Ilex aquifolium. Journal of
Ecology, Vol 55, No. 3.
Stokes, D., Church, E., Cronkright, D., Lopez, S. (2014). Pictures of an invasion: English Holly
(Ilex aquifolium) in a Semi-natural Pacific Northwest Forest.
Stokes, D. (2014). Professor. Personal Communication. University of Wahington, Bothell.
Zika, P. (2010). Invasive Hollies (Ilex, Aquifoliaceae) and Their Dispersers in the Pacific
Northwest. Madroño, Vol 57 No. 1.