Monitoring the Impact of European Buckthorn in Two Forest Communities in Northwestern Iowa T. Tracy, D. Dockter, E. Vander Broek, J. Boersma, R. Weeks, L. Furlong, A. Kolb, G. Dubois, J. Olson, and L. Rensink Biology Department, Northwestern College,

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Monitoring the Impact of European Buckthorn in
Two Forest Communities in Northwestern Iowa T.
Tracy, D. Dockter, E. Vander Broek, J. Boersma,
R. Weeks, L. Furlong, A. Kolb, G. Dubois, J.
Olson, and L. RensinkBiology Department,
Northwestern College, Orange City Iowa
Funded in part by an ISF research grant
(ISF-06-08).
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ABSTRACT The key objective of this study is
to elucidate the short- and long-term impacts of
the invasive European buckthorn (Rhamnus
cathartica) and its removal on the flora and
fauna of two forests in Sioux County, Iowa. In
2005, we established 30 research plots in
Northwesterns five-acre floodplain forest near
Alton, Iowa, and in 2006, we established 10 plots
within a 75-acre upland forest near Hawarden,
Iowa. At both sites we initially surveyed each
plot for buckthorn density, overstory
composition, and vegetative cover and
composition, and we collected soil samples from
each plot for analysis of nitrogen content. We
then paired plots according to similarity of
buckthorn density and overstory composition. We
removed all buckthorn from treatment plots and
treated stumps with herbicide. In 2006, we
re-surveyed all plots in Northwesterns forest
and also surveyed invertebrates every 5-6 weeks
from April through October using pitfall traps.
We intend to survey all plots in both forests
annually for the foreseeable future to observe a)
whether differences exist/emerge between the
control and treatment plots, and b) whether
buckthorn density relates to soil properties and
floral and faunal diversity within the control
plots.
METHODS Pairing treatment and control
plots Plots were surveyed in September 2005 at
NWCs forest and September 2006 at Oak Grove by
identifying and counting every tree and shrub at
least 1.5m tall within each 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. We then paired treatment and control
plots (15 pairs at NWCs forest and 5 pairs at
Oak Grove) according to proximity and
similarities in buckthorn and tree/sapling
composition, using Peterson's Homogeneity index.
Surveying groundcover Composition of the
herbaceous understory and the percent groundcover
in each of the plots were determined in late
summer by photographing a 30cm x 50cm quadrat
frame placed in three randomly chosen locations
within each plot. Digital color photos of all
three quadrats per plot were printed out, and
each quadrat was independently scored by four
researchers for percent groundcover, species
abundance, and species richness. All three
quadrats were also assessed simultaneously for
total species richness per plot. Regression
analyses were performed to examine the
relationship between buckthorn density in control
plots and a) ground cover within plots, b)
groundcover species richness within plots, and c)
species abundance within plots.

• RESEARCH OBJECTIVES
• To elucidate the short- and long-term effects of
  the invasive European buckthorn on forests in
  Iowa. Monitoring will include overstory
  vegetation, herbaceous understory species, soil
  chemistry, and forest floor macroinvertebrates.
• To determine whether expensive and time-consuming
  buckthorn eradication efforts are worthwhile. If
  long-term negative ecological effects of
  buckthorn are minor, then these efforts would be
  largely unnecessary. In addition, we need
  information regarding potential negative impacts
  of buckthorn eradication measures on native
  species.
• To provide college students with valuable
  research experience in conservation biology.
  Eleven students were involved in the first year
  of research at Northwesterns forest, and 50
  students were involved in this project during the
  summer and fall of 2006.
• 4. To educate the general public about the
  European buckthorn and invasive species in
  general. Clear differences between treatment and
  control plots can already be seen in
  Northwesterns forest and at Oak Grove County
  Park.

INTRODUCTION 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
host for crown rust, USDA host for overwintering
soybean aphids, Yoo et al. 2005). 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.

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DESCRIPTION OF STUDY AREAS   Northwesterns
forest is a 5-acre secondary successional
floodplain forest along the Floyd River, near
Alton, Iowa, on a land that produced corn as
recently as 1968.  Almost 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), and white mulberry (Morus
alba), with European buckthorn 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), Virginia creeper (Parthenocissus
quinquefolia) creeping Charlie (Glechoma
bederacea), and black snakeroot (Cimicifuga
racemosa).  Oak Grove County Park is a
101-acre multipurpose recreation area located six
miles north of Hawarden, Iowa, along the Big
Sioux River. The park contains a 75-acre upland
forest, within which we set up 10 permanent
non-contiguous 20m2 plots using plastic-bristled
marking flags staked into the ground to delineate
the corners of each plot. The forest is
dominated by bur oak (Quercus macrocarpa),
hackberry (Celtis occidentalis), white ash
(Fraxinus americana), and slippery elm (Ulmus
rubra), with European buckthorn, black snakeroot,
Virginia creeper, Virginia waterleaf, and wood
nettle dominating the understory and groundcover.
RESULTS No significant difference was found in
vegetative groundcover percentage between
treatment (mean 33.3) and control plots
(mean31.1 Fig. 1A). Likewise, we found no
significant relationship between percent
vegetative cover and the number of buckthorn in
control plots (Fig. 1B). Although species
richness did not significantly differ between
treatment plots (mean6.6 species) and control
plots (mean7.0 species Fig. 2A), a significant
direct relationship between species richness and
number of buckthorn was found among the control
plots (Fig. 2B). Percent canopy cover
measurements did not differ between treatment
(mean89.3) and control plots (mean89.9
t0.49, p0.63), and no significant relationship
between canopy and buckthorn was found (R20.17,
p0.13). Results of monthly pitfall trapping at
NWCs forest are reported elsewhere. Neither
percent vegetative cover nor species richness
related to buckthorn density at Oak Grove (Fig.
3AB).
B
A
t0.37 p0.72
R20.10 p0.26
Figure 1. Results from NWCs forest. A) Plot
means for percent vegetative groundcover are
shown. Lines connect paired treatment and
control plots. No significant difference was
found. B) Comparison of percent vegetative cover
and number of buckthorn in each control plot. No
significant relationship was found.
A
B
t1.01 p0.33
R20.32 p0.03
DISCUSSION While buckthorn has elsewhere been
reported to decrease herbaceous groundcover
(e.g., Boudreau Wilson 1992), our findings
suggest a significant direct relationship between
buckthorn density and groundcover diversity in
NWCs forest. A survey of much of our forest
performed 11 years ago showed no buckthorn
present, so it is possible that the differences
between ours and other studies reflect that most
of the buckthorn in our forest are not as mature
as the buckthorn in the other studies.
Preliminary data from Oak Grove suggest no
significant relationship between buckthorn
density and groundcover, but a more thorough
vegetative sampling is warranted. The
extent of the potential long-term impact of
European buckthorn on Iowas flora and fauna (and
even agriculture) has not been well studied, and
the results of short- and long-term monitoring at
our two sites will provide a better understanding
of the extent to which Iowas forests are
imperiled by the invader.
Figure 2. Results from NWCs forest. A)
Groundcover species richness values for each plot
are shown. Lines connect paired treatment and
control plots. No significant difference in
species richness was found. B) Comparison of the
number of groundcover species and the number of
buckthorn in each control plot. A significant
direct relationship was found.
B
A
R20.04 p0.57
R20.22 p0.17
Figure 3. Results from Oak Grove. A) Comparison
of percent vegetative cover and the number of
buckthorn in each plot. No significant
relationship was found. B) Comparison of the
number of groundcover species and the number of
buckthorn in each control plot. No significant
relationship was found.
REFERENCES Boudreau, D and G. Wilson. 1992.
Restoration Management Notes 10 94-95. Fagan,
M. D. Peart. 2004. Forest Ecology and
Management 19495-107. Heneghan, L., C. Clay,
C. Brundage. 2002. Ecological Restoration
20(2)108-111. Schmidt, K. C. Whelan. 1999.
Conservation Biology 131502-1506. USDA
Agricultural Research Service.
http//www.ars.usda.gov/Main/docs.htm?docid9921 Y
oo, H., R. ONeil, D. Voegtlin, W. Graves.
2005. Annals of Entomological Society of America
98926-930.
Study area