Justin Bales March 18, 2014 BES 398 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

(KINGCO, 2013).

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).

Hypothesis

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.

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.

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.

Methods

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.

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.

Map  1  shows  the  previous  study  areas  of  an  invasion  study  of  I.  aquifolium  by  Stokes  et.  al  2014.  The  southern  red  portion  is  the  most  recent  2014  study  area,  and  the  northernmost  is  the  previous  study  area  (Map:  Mappery.com)  

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

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

locations.

References EarthCorps. (2014). Invasive Trees Found in the Puget Sound Region. <http://www.earthcorps.org/pdfs/resource/15/Invasive_and_Native_Trees.pdf> Evergreen. (2014). Invasive Plant Profile- Cherry-laurel, English-laurel, Prunis laurocerasus.

<http://info.evergreen.ca/docs/res/invasives/Invasive-Plant-Profile-Cherry-Laurel.pdf> 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. <http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-

identification/english-laurel.aspx> 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.