Effects of English Holly on Native Plant Species in St ... · Effects of English Holly on Native...
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Effects of English Holly on Native Plant Species in
St. Edward State ParkElliott Church
& Dr. David Stokes
Meeting The Challenge: Preventing, Detecting, and Controlling Invasive Plants
UW Botanical Gardens | September 17th 2014
A Nightmare Before Christmas (Tim Burton)
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Outline
• Research need
• Methods
• Results
• Some discussion
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Research need
• English holly
(Ilex aquifolium)
• Evergreen shrub/tree
• Shade tolerant
• Native in British Isles
through southern EuropeKing County 2014
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Research need
• Holly is spreading in western Washington
forests
• Reports of negative impacts
• Has potential to radically impact forests
But…
• Little information available regarding its
effects on natives
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Photo: David Stokes
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Hypotheses
1. Native vegetation abundance and diversity
are reduced under holly
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Hypotheses
1. Native vegetation abundance and diversity
are reduced under holly
2. Holly shade and litter fall are mechanisms
contributing to suppression
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Methods
1m2 quadrat
(control)
• Sampled native
Woody and evergreen
Vegetation
• Sampling method designed to
– Compare outside vs. under holly canopy
– Differentiate between shade and litter effects
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Methods
1m2 quadrat
(control)
• Sampled native
Woody and evergreen
Vegetation
• Measured percent
cover for each
species in quadrat
• Summed to get total
cover for quadrat
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Results
• Species
–19 native woody and evergreen species
–Three most common (# plots present):
• Gaultheria shallon (salal)
• Rubus ursinus (trailing blackberry)
• Polystichum munitum (sword fern)
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Results
Looking at under vs. outside holly canopies
Average
of
two
quadrats
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Results
Total cover - Under vs. outside holly canopies
31.63 ± 17.55
91.75 ± 35.82Mean difference:
60.125
P < .000
(paired t-test)
under outside
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Results
Total cover - Under vs. outside holly canopies
31.63 ± 17.55
91.75 ± 35.8295% confidence interval
for difference:
44.72 - 75.53
outsideunder
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Results
Total cover - Under vs. outside holly canopies
outsideunder
36.60 ± 19.77 %
Native cover underneath holly was36.60 % of native cover outside of holly canopy
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ResultsDiversity - Under vs. outside holly
Mean difference:
.33
P < .000
(paired t-test)
0.39 ± 0.23
0.72 ± 0.31
↓ 46%
outside under
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ResultsCover of top 3 species under vs. outside holly
Salal (Gaultheria shallon)
© Starflower Foundation
http://www.wnps.org/landscaping/herbarium/p
ages/gaultheria-shallon.html
© Heidi Bohan
http://www.wnps.org/landscaping/herbarium/pag
es/polystichum-munitum.html
Trailing blackberry
(Rubus ursinus)
© Heidi Bohan
http://www.wnps.org/landscaping/herbarium/
pages/rubus-ursinus.html
Sword fern
(Polystichum munitum)
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ResultsCover of top 3 species under vs. outside holly
SIGNIFICANT DIFFERENCE
Average difference:
23.5 points
P=0.00098, Wilcoxon signed-rank
test
n=17
33.68 ± 18.01
11.62 ± 12.62
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ResultsCover of top 3 species under vs. outside holly
n=17
11.77 ± 9.95
4.12 ± 3.18
SIGNIFICANT DIFFERENCE
Average difference:
8.24 points
P=0.001641, Wilcoxon signed-rank
test
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ResultsCover of top 3 species under vs. outside holly
DIFFERENCE NOT
SIGNIFICANT
P=0.1152, Wilcoxon
signed-rank test
n=14
14.11 ± 14.37
6.25 ± 7.52
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Some discussion
• Clear reduction in woody and evergreen
native cover under holly
• Suggests holly invasion has capacity to
alter
–biodiversity
– succession
– structure
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Some discussion
• Difference between response of top 3
species suggests that one possible impact of
holly is to alter native vegetation community
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Contact
Elliott Church
[email protected] Student
School of Environmental and Forest Sciences
University of Washington
Thanks to: Carolyn Brennan at the UW Bothell Office of Researchthe UWB Undergraduate Research Scholarship
UWB Founders fellowship award for undergraduate research