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The Effects of European Buckthorn (Rhamnus cathartica) on a
Floodplain Forest in Sioux County, Iowa
TODD T. TRACY, NIC BOERSMA, LISA WALTERS, GENA DUBOIS, MARK BRADLEY, ASHLIE ARTHUR, MICHELLE GARRELS,
JONATHAN JANSEN, CHRISTINA MICK, TIMOTHY SWART, ALLYSON TENOLD
Department of Biology, Northwestern College, Orange City, Iowa 51041
Groundcover survey
Methods: The composition of the herbaceous understory and the percent
groundcover in each of the plots were determined by using a modified lineintercept technique for vegetative sampling. We ran a tape measure from
corner to corner diagonally across each plot and estimated the extent of
coverage for each herbaceous species falling within 5cm of the tape.
Estimated coverage values were made in 1-m increments, such that ~28
increments were examined per diagonal transect. Two transects were
surveyed in 8 plots and only 1 transect was surveyed in 17 plots because of
time constraints. Ground-cover surveys were performed in late September
and early October 2005, before the first killing frost. Percent groundcover and
species richness were determined for each of the plots, with richness scores
averaged for plots with two transects surveyed. Regression analyses were
performed examining the relationship between species richness and
buckthorn density, and between % ground cover and buckthorn density, to
determine whether buckthorn appears to be limiting the growth and diversity
of the herbaceous groundcover.
Tree and buckthorn survey
Methods: Trees and buckthorn shrubs were surveyed
by identifying and counting every individual at least
1.5m tall (~shoulder height) within each of the 30 study
plot. Those 13cm in circumference at 1.5m height
were considered "mature", and those at least 1.5 m tall
but <13cm were considered "saplings". Buckthorns
between 0.5m and 1.5m were also counted. Surveys
were performed from early to mid-September 2005.
Regression analyses were performed comparing the
number of buckthorn to the number of mature silver
maple and boxelder trees in each plot.
Results: Groundcover percentages ranged from 11.9% to 88.6% and species
richness ranged from 6 to 14 species. No significant relationships were found
between either variable and buckthorn density, suggesting that buckthorn is
not inhibiting the growth of the herbaceous under story. However, our
analysis does not examine the relationship between buckthorn and individual
groundcover species, so buckthorn may indeed be affecting the species
composition of the under story.
# of saplings
5
762
European buckthorn
Silver maple
245
2
Boxelder
461
81
White mulberry
77
353
Green ash
10
10
Plains cottonwood
1
0
100
3
Slippery elm
4
0
Hackberry
1
5
807
1213
Totals
# of buckthorns vs. # of
mature silver maples
250
200
150
100
# of buckthorns vs. # of
mature boxelders
R2=0.536, df=29, p=4.22x10-6
250
f buckthorns
300
0
200
150
100
R2=0.346, df=29, p=0.00062
R2=0.065, df=24, p=0.220
15
80
R2=0.041, df=24, p=0.329
60
40
20
0
13
11
9
7
5
0
American elm
Ground-cover species
richness vs. # of buckthorns
Ground cover vs. # of
buckthorns
Species richness
# of
mature trees
Ground cover (%)
Species
f buckthorns
ABSTRACT
We performed a preliminary study and commenced a
long-term study to examine the impact of European
buckthorn (Rhamnus cathartica) on Northwestern
College’s field station, a 5-acre floodplain forest near
Alton, Iowa. We set up a grid of thirty 20-m2 plots
within the forest and surveyed the trees, saplings, and
herbaceous growth in each plot, and we estimated the
percent ground and canopy cover within each plot. We
also trapped small mammals and tested soil nitrogen
content in each plot. We found a significant direct
relationship between buckthorn and silver maple (Acer
saccharinum) density and a significant inverse
relationship between buckthorn and box elder (A.
negundo) density. We also found a significant direct
relationship between buckthorn density and canopy
cover. We found no significant relationships between
buckthorn density and ground cover, species richness,
or species diversity. After all surveys were performed,
we paired treatment and control plots (15 total pairs of
plots) and removed all buckthorn from the treatment
plots. These plots will be resurveyed annually to
observe whether differences emerge between the
control and treatment plots, further elucidating the
impact of buckthorn on our forest ecosystem.
Results: Over 60% of the saplings in the study area
were buckthorns, while very few "mature" buckthorns
were found. A total of 1983 buckthorns were found,
1216 of which were between 0.5m and 1.5m tall. A
significant direct relationship was found between the
number of buckthorns and silver maples in each plot,
and a significant inverse relationship was found
between the number of buckthorns and boxelders in
each plot.
50
100
150
# of buckthorns
200
250
0
50
100
150
200
250
# of buckthorns
Soil-nitrogen testing
Methods: Two soil samples were collected from locations ~1m apart near the center of each plot on
November 15, 2005. Organic litter was removed and a trowel was used to dig up ~150g of the top layer of soil
for each sample. Samples were deposited into airtight plastic containers and frozen until analyses could be
performed. All samples were collected prior to the cutting of buckthorn and treatment of stumps with herbicide.
Samples were thawed and dehydrated overnight at ~50°C before assays were performed. Hach NitraVer 5
reagent and a spectrophotometer were used to determine the level of nitrogen in each sample. Regression
analysis was then used to determine the relationship between buckthorn density and soil nitrogen levels in our
study plots.
# of buckt
Soil nitrogen level vs. # of
buckthorns
100
50
0
RESEARCH HYPOTHESES
We hypothesized that European buckthorn (Rhamnus
cathartica) would both directly (via direct competition)
and indirectly (via allelopathy and chemical changes in
the soil) affect the composition of the forest understory,
and we predicted decreased ground cover and altered
soil nitrogen content in areas of our forest with higher
buckthorn density. We also predicted that
groundcover density and diversity, soil nitrogen levels,
and habitat use by deer mice would relate to buckthorn
density. We also hypothesized that European
buckthorn would exhibit certain habitat "preferences"
and predicted that buckthorn density would relate to
the openness of the canopy and the species
composition of the mature trees in our study plots.
0
10
20
30
40
9
Soil nitrogen (ppm)
# of mature silver maples
Canopy cover
Methods: Mean percent canopy cover was determined for
each plot by using a spherical canopy densiometer to take
measurements from the center of each plot while facing all
4 compass directions. Canopy measurements were made
on October 4, 2005. Regression analysis was performed to
examine the relationship between numbers of young
buckthorn (0.5m  height < 1.5m) and % canopy cover to
determine whether canopy cover might affect recruitment of
buckthorn in the plots.
8
R2=0.163, df=17, p=0.097
7
6
5
4
3
0
50
100
150
200
Canopy cover vs. number of
mature boxelders
DISCUSSION--European buckthorn and glossy buckthorn
(R. frangula) are invasive plant species initially imported to
the U.S. in 1849 as ornamental shrubs. Researchers
throughout the Eastern U.S. have found various negative
ecological effects of buckthorn (e.g., increased nest
predation in songbirds, Schmidt & Whelan 1999; increased
soil nitrogen content, Heneghan et al. 2004; decreased
herbaceous groundcover, Boudreau & Wilson 1992;
decreased juvenile recruitment by canopy trees, Fagan &
Peart 2004). Since the effects found in one area of the U.S.
or in one habitat may not be the same as those found in
others, studies that specifically examine the local habitat
preferences of European buckthorn and the impact of
buckthorn on Iowa’s native landscape seem appropriate and
warranted, particularly if much effort and expense is being
invested in removing and controlling the shrub.
# of young buckthorns (height
< 1.5m) vs. % canopy cover
Canopy cover vs. # of mature
silver maples
R2=0.275, df=29, p=0.0029
100
200
90
80
# of young
buckthorns
Canopy cover (%)
95
90
85
80
75
70
65
60
55
50
R2=0.27, df=29, p=0.0032
70
60
50
10
20
30
# of mature boxelders
40
R2=0.241, df=29, p=0.0058
150
100
50
0
0
10
20
30
40
50
60
# of mature silver maples
70
80
90
Canopy cover (%)
Mammal trapping
Methods: A Sherman trap baited with oatmeal and a
carrot chunk was placed at the center of each of the 30
plots. Traps were checked daily between 8 and 10 a.m.
for the duration of the study. To facilitate individual
identification upon recapture, captured deer mice were
uniquely marked inside their earflaps and beneath their
armpits with a combination of red, black, and blue Sharpie
markers and were released at the point of capture. All
captures and recaptures were recorded. Mammal
trapping commenced on October 25, 2005, and extended
over the next 4 nights. Regression analysis was
performed comparing the number of deer mice captured
in each plot (excluding recaptures within the same plot) to
the total number of buckthorn in each plot to determine
whether buckthorn density relates to habitat use by deer
mice.
# of mice captured within plot
vs. # of buckthorns
4
R2=0.0028, df=29, p=0.782
3
Results: Deer mice were the only mammals captured,
although a few traps showed signs of squirrel activity.
Eleven different mice were trapped in a total of 15 plots
over the 4-day period. No significant relationship
between deer mouse activity and buckthorn density was
found.
# of mice
0
250
Total # of buckthorns
Results: A significant direct relationship was found
between numbers of young buckthorn and % canopy cover.
It appears that buckthorn are more readily recruited into
shadier areas of the forest. However, canopy cover was
also found to be significantly directly related to silver maple
density and significantly inversely related to boxelder
density, so the purported relationship between buckthorn
recruitment and shading may simply reflect an effect of the
species composition of the forest overstory. Furthermore,
there were signs that the boxelder trees had begun losing
some leaves by the time we took canopy measurements,
so we plan to take canopy measurements earlier in the
season in 2006 in order to verify the relationships detected
in our 2005 study.
Canopy cover (%)
DESCRIPTION OF STUDY AREA
Northwestern College's forest is a 5-acre floodplain
forest along the Floyd River, near Alton, Iowa. This is a
secondary successional forest on a land that produced
corn as recently as 1968. The land was disked and
seeded with prairie seeds in 1969, and small trees
were hand-planted in much of the area in the early
1970's. None of the early plantings survived, and
virtually all of the present-day trees in the forest
originated as seeds blown in from nearby areas. The
forest canopy is dominated by silver maple (Acer
saccharinum), boxelder (A. negundo), white mulberry
(Morus alba), and green ash (Fraxinus pennsylvanica),
with European buckthorn (Rhamnus cathartica) and
small black currant (Ribes nigrum) shrubs dominating a
patchy woody understory. The heterogeneous
groundcover is dominated by wood nettle (Laportea
canadensis), Virginia waterleaf (Hydrophyllum
virginianum), creeping Charlie (Glechoma bederacea),
reed canarygrass (Phalaris arundinacea), and black
snakeroot (Cimicifuga racemosa). Within the forest,
the corners of thirty 20-m2 plots have been
demarcated with numbered iron fence posts
permanently pounded into the ground.
150
2
1
0
0
50
100
150
# of buckthorns
200
250
100
In our study, we found that buckthorn density was directly
related to maple density and inversely related to boxelder
density. These relationships may reflect a more hospitable
environment for buckthorn recruitment beneath maples than
boxelders, or they may simply reflect a spurious relationship
reflecting similar locations of seed sources for maple and
buckthorn. There is no evidence that buckthorn is negatively
impacting the herbaceous groundcover of our forest,
although a more in-depth analysis of groundcover
composition is warranted. A virtually significant inverse
relationship was found between soil nitrogen levels and
buckthorn density, although it is unclear whether this
relationship reflects a change effected by buckthorn or
simply that buckthorn recruitment is higher in areas with
higher soil-nitrogen levels. Finally, we found no evidence
that buckthorn is influencing habitat use by deer mice. Since
the majority of buckthorns in our study are quite young, it is
possible that differences between high- and low- density
areas have not yet emerged. To determine the longer-term
effects of buckthorn, we have commenced a long-term study
within our forest by pairing treatment and control plots with
similar buckthorn and tree composition and removing all
buckthorn from treatment plots, with the goal of resurveying
the plots annually. We are currently surveying the
invertebrates collected in soil samples from our plots, and
we are studying the predation rates of artificial nests set up
in buckthorns vs. tree saplings. Results of these studies will
help us more fully understand the potential ecological impact
of European buckthorn and help Iowans understand the
extent to which the invasive shrub imperils our native
forests.
REFERENCES
Boudreau, D and G. Wilson. 1992. Buckthorn research and control at
Pipestone National Monument. Restoration &
Management Notes 10: 94-95.
Fagan, M. & D. Peart. 2004. Impact of the invasive shrub glossy buckthorn
(Rhamnus frangula) on juvenile recruitment
by canopy trees.
Forest Ecology and Management
194:95-107.
Heneghan, L., Clay, C., & Brundage, C. 2002. Rapid decomposition
of buckthorn litter may change soil nutrient levels.
Ecological
Restoration 20(2):108-111.
Schmidt, K. & C. Whelan. 1999. Effects of exotic Lonicera and Rhamnus
on songbird nest predation. Conservation
Biology 13:1502-1506.