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The role of understory plant community in seedling establishment of three invasive evergreen species

Modeling the spread of invasive species is essential in understanding the impact non-natives could have on native ecosystems. In this study, we aim to understand the spread of English holly (Ilex aquifolium), a common invader in Pacific Northwest ecosystems, at the level of seedling establishment.  Specifically, we investigated whether the native understory plant community has an effect on seedling establishment of this invader. In addition, we searched for the presence of cherry laurel and Portuguese laurel, two other invasive evergreens, whose presence in Pacific Northwest ecosystems is becoming rapidly more noticeable. We surveyed 10 plots in St. Edward State Park in Kenmore, Washington over a 4-week period in February 2016. Five of the plots had an understory dominated by swordfern (Polystichum munitum) and Five of the plots had an understory dominated by Salal (Gaultheria shallon). In each of these plots we measured the presence of these invasive evergreen seedlings. Our data suggests that the understory plant community does have an effect on establishment. Fewer seedlings were found in the salal dominated plots than in the swordfern dominated plots, despite being in close proximity to mature fruiting holly. Because of the time limitations of our study, we cannot conclusively say that our results are significant. This study acts as a pilot study to seek out further questions that should be answered. Our results suggest that salal dominated communities should be investigated to determine what features they possess that hinder holly establishment.
The invasion of non-native species has had many impacts on ecological and human systems worldwide (Pysek & Richardson 2010). Included in these impacts are the loss of ecosystem biodiversity, disruption of ecosystem processes and structure, and the weakening of ecosystem services utilized by humans (Mack et al. 2000; Stokes & Lopez 2015). Invasive species are often difficult or impossible to remove, and have become a problem for land managers and those hoping to preserve native ecosystems (Stokes et al 2014). In the Pacific Northwest (PNW), while many invasive species have been recognized as a problem by ecologists and land managers alike, little research has been done on the exact impacts these species may have on native ecosystems (Stokes et al 2014). In order to predict these impacts and create effective management protocols, we must first understand the nature of non-native spread and establishment (Lockwood et al 2007).
English holly (Ilex aquifolium) has been recognized as a quickly proliferating invasive species in the Pacific Northwest (Stokes et al 2014). It is an evergreen shrub native to Europe and Northern Africa that was introduced to the Pacific Northwest by humans as an ornamental plant in 1869 (Stokes et al 2014).  I. aquifiolium was first recorded to be present in PNW natural environments in 1953 (Zika 2010), and since that time, research on its spread has shown that this species has the potential to change forest structure on a massive scale (Stokes et al 2014). Some counties in the area have recognized the species as a threat based on its unique hardiness in closed canopy forests, its successful reproduction through seed and vegetative spread, and its difficulty to successfully remove (Portland 2013; King County 2006; NWCB 2010). St. Edward State Park in King County, Washington, where English holly has been deemed a “Weed of Concern” (King County 2006), is a location of ongoing research on this non-native.
Research in the park has shown that the English holly population has been growing at an exponential rate in the last 50 years (Stokes et al 2014). There are features about Pacific Northwest forests that allow English holly to thrive here (King County 2006), but these features have not been precisely determined. What we do know is that this unique species has no analogue in PNW ecosystems (Stokes et al 2014). There is no other native species that grows the way it does, making it difficult to determine the impacts this invader may have. Without control efforts, invasive species like holly have the potential to spread within the park and threaten native plant communities (Smith 2006). That’s why it is crucial to understand the nature of holly spread and establishment. Potential factors that determine holly establishment success in the park have been discovered, including local environmental conditions and types of species surrounding a plant, along with dispersal mechanisms (Stokes & Lopez 2015). This research tells us that while holly populations in the park are growing, we have begun to determine areas where it doesn’t grow and reasons for this.  Determining the features of these areas that prevent holly establishment could lead to a greater understanding of its spread within the park.
To continue this research, this study hopes to address what effect different native understory plant communities have on English holly seedling establishment.  Previous research done in the park shows a patchy pattern of establishment of holly (Stokes et al. 2014). What we want to know is if the native understory community plays some role in determining where holly establishes and where it doesn’t. Seedling establishment is the phase of spread with the highest mortality of any point in most woody plants life cycles (Arrieta & Suarez 2006). Research in Denmark on the niche filled by holly suggests that interactions with surrounding species at critical phases such as establishment should be analyzed in order to understand the role it plays in a community (Banuelos et al. 2003). Determining this could allow us to predict what features make an area vulnerable to invasion and lead to better holly management protocols. In addition to surveying for English holly, we have also surveyed for seedlings of Portuguese laurel (Prunus lusitanica) and cherry laurel (Prunus laurocerasus), two non-native evergreens whose presence in the park is becoming rapidly more noticeable.
To answer whether or not understory species play a part in seedling establishment of these non-natives, we have measured the presence of seedlings in two different community types in St. Edward State Park. The areas we chose to survey were those dominated by (1) Swordfern (Polystichum munitum) and (2) Salal (Gaultheria shallon). We hypothesized that areas dominated by salal would have fewer holly seedlings than those dominated by swordfern. Salal tends to form dense thickets and has extensive underground rhizome structures that have been known to limit the growth of conifer seedlings due to nutrient competition (Messier & Kimmins 1990; Fraser et al. 1995). While holly has been known to establish in a wide variety of environments (Peterken & Lloyd 1967), we believed that the limited nutrients and heavy shading in salal dominated communities would limit establishment. According to Peterken & Lloyd (1967) holly seedlings are limited by heavy shading.  In areas dominated by swordfern, heavy shading would not be an issue, so we hypothesized further that these areas would have a higher number of holly seedlings than salal dominated communities.  In addition, we hypothesized that areas with higher species richness would have fewer holly seedlings because species richness tends to correlate with lower invasibility (Lockwood et al. 2007).
To test these hypotheses, we performed a comprehensive search for seedlings in 5 paired areas of each community type in St. Edward State park. At each plot we characterized the plant community, canopy coverage, presence of seedlings, ground cover and distance to the nearest large invasive. When a seedling was discovered, the height, basal diameter, local species present, and local species density and richness was recorded. We analyzed the collected data for correlations between the presence of seedlings and differing species community, density, and richness.
Data Generation
Study area and determination of plots
Data was collected in 5 paired plots in St. Edward State Park in Kenmore, Washington, over a 4-week period in January and February of 2016. We selected the plots by walking the trails throughout the park, searching off-trail for plots of swordfern and salal to pair that were (1) close in proximity (within 10 m) to one another to decrease variation in soil composition (2) at least 3m away from trails (3) similar in canopy coverage and canopy species (4) at least 8×8  in size (5) visually appearing to be at least 35% covered in salal (GASH) or swordfern (POMU), with no other groundcover species being equally dominant, and (6) free of large, easily visible invasives. We evaluated the plots solely on these six criteria to prevent selection bias and to increase the value of our paired comparison.
In each plot we found, we first determined the boundaries of the plot by marking off four corners with flagging tape that had been labelled for the site (ex: POMU 1, GASH 1). These boundaries were determined by (1) limits of POMU/GASH dominance in the understory (2) physical boundaries such as trails or large clusters of evergreens (3) changes in topography.
Determination of seedling presence and measurement of seedlings
After setting the boundaries, we performed a comprehensive search of the plot. Two researchers walked 1-2m apart (depending on vegetation) in transects across the plot, moving slowly and checking underneath understory plants for invasive seedlings.
When a seedling was discovered a series of measurements were taken. We characterized a “seedling” by any holly found that was under 1m in height. Holly clusters that were determined to have originated from one plant through vegetative spread were counted as 1 seedling. To determine if they were from seed or vegetative spread, the root structure was analyzed. We measured each seedling for (1) Height or average height in the case of holly clusters using a meterstick (2) basal diameter using calipers (3) percent ground cover within 1  by species using a 1  quadrat (4) percent bare ground within 1  (5) species richness within 40 .

Characterization of plots
After surveying each plot for seedlings, we performed a series of evaluations to characterize the site. We measured each site for (1) percent canopy cover by visual analysis (2) canopy species composition (3) percent groundcover, including the percentage all identifiable groundcover species and percentage of bare ground (4) plot dimensions measured with a metertape (5) distance from plot to nearest large invasive (6) presence of berries on nearest large invasive.
Data Analysis
We have not yet done our statistical analysis because we don’t know how! We are currently working on this but it wasn’t done in time to put it into this paper. I did the best I could with the data we collected.
Seedling presence in two different community types
We analyzed the presence of seedlings in the two different community types by running a paired t-test to determine if the results we acquired were significant. The results were graphed with bars to indicate the error.
Species richness and presence of seedlings
We averaged the species richness in the 5 POMU sites and the 5 GASH sites to determine whether species richness correlated with invasability.
Large holly trees and proximity to plots
We ran a paired t-test to determine if plot proximity to large invasives was statistically significant, and whether the proximity to large invasives was a determinant of the presence of holly seedlings.
Location of determined plots
Five paired plots of each community type were located and their estimated locations were marked on a map (Fig. 1).
Fig 1. Trail map of St. Edward State Park in Kenmore, WA. Trails 2, 4, 6, 7, 8, 9, 13,  and 14 were searched along to find our plots, as well as uncharted trails in the southeastern areas of the park. Our selected salal and swordfern paired sites are indicated in red.
Seedling presence in two community types
Out of 500 square meters (estimate, exact numbers to be calculated) of the swordfern dominated community that we surveyed, we found 3 seedlings (Table 1). Two seedlings were in POMU 1 and were both English holly. One of the seedlings stood alone and appeared to have originated from seed and one of the seedlings was actually a cluster of small holly plants that appeared to be reproducing vegetatively. The other seedling was a cherry laurel seedling, found in POMU 2.
Out of 750 square meters (estimate, exact numbers to be calculated) of the salal dominated community that we surveyed, we found zero seedlings (Table 1).
Seedling presence in differing species richness
            Average understory species richness in the swordfern dominated community was 6.6 species per plot (this will be per sq meter when we have calculated it) (Table 1). In the salal dominated community the average understory species richness was 4.8 species per plot. It appears that, although there were more species in the swordfern dominated plots, they had higher numbers of invasive seedlings.
Holly seedling presence in proximity to mature fruiting holly
It appears there is no correlation between presence of holly seedlings and proximity to mature fruiting holly. All five of the salal dominated plots were within 22.5m of mature fruiting holly and none of these plots had seedlings in them. We found a berry on the ground in one salal plot, suggesting that, while there is potential for holly seedlings to establish in these plots, something is keeping them from establishing. Neither of the swordfern plots that had seedlings in them were in close proximity to mature fruiting holly.
Bare ground in the two different community types
The plots dominated by swordfern had an average of 25% bare ground. This is ground not covered by fallen logs or inhabited by other species. The plots dominated by salal had an average of only 10.6% bare ground (table 1).

Table 1. Site characterization in 5 different plots of two understory community types
While research has been performed on the rate of English holly spread at St. Edward State Park, little is known about the nature of its spread within the park (Stokes 2014). Invasive species spread is complicated to understand because it requires knowledge of so many different aspects. In addition to understanding the physiological features and life history of a species in its own range, we must understand how it will interact within the native ecosystem (Lockwood 2007). A broad study was performed in 2015 on the nature of holly spread in the park (Stokes & Lopez 2015). This study determined that holly establishment is positively affected by the presence of coniferous evergreens and proximity to forest edges, and negatively affected by proximity to water sources (Stokes & Lopez 2015). While this confirmed that species characteristics and local environmental conditions greatly affect seedling establishment, still more must be determined in order to predict the impact this species will have in the park and to create management protocols. Understanding if there is a correlation between holly seedling establishment and understory plant community is one piece of the puzzle.
Holly seedling establishment in two different understory communities
Based on our results, the understory community plays a role in the establishment of seedlings. Of the five plots we surveyed where salal (Gaultheria shallon) was the prominent understory species, we found zero seedlings. We hypothesized that holly would not establish in areas where salal was the prominent understory species because holly seedling mortality often has to do with seedling shading (Peterken & Lloyd 1967) and salal often forms dense thickets. While we cannot say for certain that seedling shading is responsible for the lack of holly in these plots, the data supports our hypothesis. Further research should be done on holly establishment in salal-dominated understories to determine exactly what keeps the holly out of these areas. It’s possible that salal hinders holly establishment in a way that could be useful in managing this invader.
Holly seedlings seem to have greater success establishing in areas where swordfern (Polystichum munitum) is the prominent understory species. According to our results holly density in swordfern dominated understories is xx seedlings per square meter. This is xx times greater than in salal dominated understories. We hypothesized that areas with species that did not form dense ground cover would have more holly seedlings. In the swordfern dominated communities, there was an estimated 14.4% more bare ground than in the salal communities (table 1). Having areas of bare ground could make a community more invasable to English holly because the seedlings would not have to compete for resources such as light in those areas.
Species richness
We found that species richness had little effect on the seedling establishment of English Holly. In the swordfern dominated plots, communities with high species richness had no fewer holly seedlings on average than those with lower species richness. We had hypothesized that areas with high species richness would be less invasable by English holly based on the niche model (Lockwood 2007). This does not appear to be the case. It does seem that percent groundcover, however, does play in to the occurrence of holly seedlings. Areas with less than 15% bare ground had no holly seedlings, regardless of community type. This indicates that holly may be unable to establish in forests where percent groundcover is high.
Limitations & further research
The biggest limitation in our study was time. We were only allotted 4 weeks for data collection, which did not allow us to survey the number and variety of understory plant communities we would have liked to. In 2006 Hans Smith created a summary of vegetation within St. Edward State Park. Within the park he designated six different plant communities based on their prominent species and mapped each community. Further research should be done on the establishment of holly each of these community types, as well as in communities of differing understory plant species.
Another limitation was that we weren’t able to determine what features of salal communities hindered holly establishment. This study created a baseline for characterizing salal communities in terms of percent ground cover and species richness, but sheds little light onto the reasons why holly doesn’t establish in these communities. More research should be done on the properties of salal dominated communities to determine what limits holly establishment.
This study aimed to determine whether native understory communities play a role in dictating where English holly seedlings establish in St. Edward State Park in Kenmore, WA. While we were only able to survey two community types throughout the park, it appears that the understory community may play a role in the invisibility of an area by English holly. In addition, it is significant that a higher percentage of groundcover correlates with fewer holly seedlings. Both of these findings can act as stepping stones to discovering what features make an area invasable by holly, but more research must be done on the features of these holly-free zones. Creating a profile for holly and how it spreads within the park will be useful not only to create successful management protocols and determine impacts of this invader, but also to create a broader understanding of invasive species in Pacific Northwest forests.
We would like to thank Dr. David Stokes for making this research possible, and the students in his investigative biology class for participating in discussions about our topic. Thanks also goes out to Dr. Heather Galindo, whose input in the creation of this paper has been invaluable. Our tools were provided by the University of Washington Bothell and by Dr. David Stokes himself.
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Banuelos MJ, Kollmann J, Hartvig P, Quevedo M. 2003. Modeling the distribution of Ilex aquifolium at the north-eastern edge of its geographical range. Nordic Journal of Botany 23(1) pp. 129-142.
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Messier C, Kimmins JP. 1990. Above and below-ground vegetation recovery in recently clearcut and burned sites dominated by Gaultheria shallon in coastal British Columbia. Forest Ecol and Mgmt 46 (1991) 275-294.
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Pysek, P,  Richardson D. 2010. Invasive species, environmental change and management, and health. Ann Rev of Environ and Resources 35 (1): 25–55.
Portland. 2013. [Internet] Alien Plant Invader: English holly. [cited 26 Feb 2016] Available from: https://www.portlandoregon.gov/bes/article/474845.
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