ABSTRACT

1
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 line-intercept 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. 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.
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 lt13cm 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 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.
ABSTRACT We performed a preliminary study and
commenced a long-term study to examine the impact
of European buckthorn (Rhamnus cathartica) on
Northwestern Colleges 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.
Species of mature trees of saplings
European buckthorn 5 762
Silver maple 245 2
Boxelder 461 81
White mulberry 77 353
Green ash 10 10
Plains cottonwood 1 0
American elm 3 0
Slippery elm 4 0
Hackberry 1 5
Totals 807 1213
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 50C 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.
2
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.
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 lt 1.5m)
and canopy cover to determine whether canopy
cover might affect recruitment of buckthorn in
the plots. 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.
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 Iowas
native landscape seem appropriate and warranted,
particularly if much effort and expense is being
invested in removing and controlling the shrub.
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.
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.
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. 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.
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
19495-107. 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 131502-1506.