INVASIVE ENGLISH HOLLY AND EFFECTS ON SOIL
Jacqueline Watts, Darlene Zabowski
School of Environmental and Forest Sciences
INTRODUCTION
English holly (Ilex aquifolium) is commonly found in disturbed and undisturbed forests. Its adaptability and
shade tolerance accounts for fast growth in a wide range of environmental conditions and allows for
germination beneath dense forest canopies. My research project determined if the alkaline leaf litter from
holly alters soil pH, soil carbon and nitrogen, and C/N ratio in O and A horizons.
MAP 1
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Research Area Map Showing Soil Types in a 1 1/4 mile stretch of Grays Harbor County
23 - Carstairs
2
142 - Tebo
METHODS
Unused Soil Types
The research area was located on coniferous forest located outside the city limits of McCleary in Grays Harbor
County (Map 1). Three soil types were selected for plot placement; Carstairs, Lyre Variant and Tebo, to
investigate if soil type could influence pH (see Map 1).
Experimental (Holly
1 )Plots
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Water
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Control Plots
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Establishment of Experimental and Control Plots
Ten sets of experimental and control plots were located within each soil type. Selection of experimental plots
was dependent upon age of the holly plants. Holly plants ten years or older were selected from research plots
previously established by a graduate student. For experimental plots two soil samples were collected a distance
of 2 feet from the holly plant. One sample consisted of O horizon while the second sample was A horizon
retrieved 5 cm below the O horizon. Control plots were placed on a transect 30 feet away at a random compass
from the experimental plot. Soil samples were collected from the O and A horizon 2 feet away from the nearest
tree at the end of the 30 foot transect. The distance between control and experimental plots decreased likelihood
of English holly roots growing within control plot and influencing soil pH.
pH & C/N Testing
All collected soil samples underwent pH testing using a standard pH meter. A total of 12 soil samples from
experimental plots with the largest DBH (diameter at breast height) holly plants and corresponding control
plots were processed in a CHN analyzer to determine percentage of carbon and nitrogen.
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72 - Lyre Varient
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MASON COUNTY
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RESULTS AND DISCUSSION
• Results showed a general trend of alkaline pH in experimental plots with holly, however, there was
no signifiant difference of the mean between experimental and control plots.
• Observed higher percentage of % nitrogen and (C/N ratio) in experimental vs control plots.
• While my project did not decisevely prove English holly affected soil pH, continuing research
demonstrates English holly is invasive and affects native vegetation though suppression and
competition.
Control O-Horizon
Holly Mineral
Control Mineral
Overall average pH of experimenal
O horizon was 5.18 vs control 5.03.
Mineral soil pH in experimental
plots averaged 4.81 with control
pH 4.83. T test indicated no
statical correlation, (p> 0.10).
6
3
Tebo
Lyre Variant Carstairs Total Average
Soil Type
Average pH of all soil types for
experimental O horizon measured
5.23 vs control 5.07. A horizon pH
of experimental plots average 4.72
vs control pH 4.59. A paired t-test
indicated no signifiance, (p> 0.10).
2.5
FIGURE 4 : C/N Ratio
2.0
Average carbon per parts of nitrogen
of each soil type. Experimental plots
averaged 24.66 parts carbon per 1
part nitrogen vs 22.07 C/N in control
plots. Testing of A horizon revealed
average 21.07 parts carbon to
nitrogen in experimental plots while
control plots averaged 22.07 parts
carbon / nitrogen. Again a paired
t-test (p> 0.10) indicated no
statistical signifiance.
1.5
1.0
0.5
0.0
LITERATURE CITED
Peterken, G., Lloyd, P. 1967. Ilex Aquifolium L. Journal of Ecology. 55 (3): 841-858
5
4
FIGURE 3 : Average % Nitrogen
Holly Age> 10 years
Nitrogen content of O horizon in
experimental plots averaged to
1.86 % while control averaged
1.75 %. A horizon nitrogen content
averaged 0.73 % vs control 0.71 %.
Paired t-test indicated no
signifiance, (p> 0.10).
FIGURE 2 : Average pH of
Holly > 10 Years
Tebo
Lyre Variant Carstairs Total Average
Soil Type
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6
pH
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pH
FIGURE 1 : Average Soil pH
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4
3
Tebo
Lyre
Carstairs Total Average
Soil Type
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30
Percentage
CONCLUSIONS
Holly O-Horizon
Percentage
Although there was a trend of higher pH under holly plots, there was no statistical difference in pH of O or A
horizons using a paired T-test (p>0.10)(Figure 1). There was also no difference in the effects of holly on soil pH
among the 3 soil types tested. The trend of increased pH was more evident when soil samples from holly plants
older than ten years were tested (Figure 2). Likewise, there was no significant difference in wither total soil
nitrogen or C/N ratio (Figure 3 and 4). Thus holly may not significantly impact soil properties unless the plant has
been present on a site for 25 years or more. Changes of pH and C/N ratio could impact health and the competitive
ability of understory vegetation and trees adapted to acidic forest soil conditions, however, it is not evident
in my study.
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25
20
15
Tebo
Lyre Variant Carstairs Total Average
Soil Type
USDA Soil Survey of Grays Harbor County Area, Pacific County and Wahkiakum County, Washington (1979)
ACKNOWLEDGEMENTS
Watts, A. (2013) An Examination of Stand Attributes and the Presence of English holly in a Pacific Northwest Forest, Grays Harbor County,
Washington Proquest UMI, Dissertations Publishing University of Washington
Andy & Joyce Watts, Joyce & Dave Flores , Charlie & Kathy Watts, Port Blakely Tree Farm and to my sisters, Andrea and Jenny, for their help with my project.
A big thank you to SEFS department for awarding the quarterly capstone fund award.
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