Transcript Slide 1

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, Orange City Iowa
Funded in part by an ISF research grant (#ISF-06-08).
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 Northwestern’s 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 Northwestern’s 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.
RESEARCH OBJECTIVES
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 Iowa’s native landscape seem appropriate
and warranted, particularly if much effort and expense is
being invested in removing and controlling the shrub.
1. 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.
2. 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.
3. To provide college students with valuable research
experience in conservation biology. Eleven students were
involved in the first year of research at Northwestern’s
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 Northwestern’s forest and at Oak
Grove County Park.
METHODS
Pairing treatment and control plots
Plots were surveyed in September 2005 at NWC’s 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 <13cm were
considered "saplings". Buckthorns between 0.5m and 1.5m
were also counted. We then paired treatment and control plots
(15 pairs at NWC’s 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.
RESULTS
Percent vegetative cover vs. # of Buckthorn
B
70
80
60
70
% vegetative cover
Northwestern’s 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).
Treatment vs. Control:
Percent vegetative cover
A
Mean % vegetative cover
DESCRIPTION OF STUDY AREAS
60
t=0.37
t=0.37
p=0.72
p=0.72
50
40
30
20
50
40
30
22
R =0.1
=0.10
R
p=0.26
p=0.26
20
10
10
0
0
Treatment
Control
0
50
100
Plot Type
150
200
250
# of Buckthorn
Figure 1. Results from NWC’s 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.
Treatment vs. control:
Species richness
A
Control plot species richness vs. # of buckthorn
B
10
9
9
8
t=1.01
t=1.01
p=0.33
p=0.33
7
6
5
Neither percent vegetative cover nor species richness
related to buckthorn density at Oak Grove (Fig. 3A&B).
7
6
R22=0.32
R
=0.32
p=0.03
p=0.03
5
4
3
3
Control
0
50
Plot Type
100
DISCUSSION
150
200
250
# of buckthorn in plot
Figure 2. Results from NWC’s 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.
Percent vegetative cover vs. # of buckthorn
A
7
6
R2=0.04
p=0.57
50
# of species in plot
Mean % vegetative cover
Species richness vs. # of buckthorn
B
60
40
30
20
5
4
2
1
0
0
50
100
150
200
250
# of buckthorn in plot
300
350
400
R2=0.22
p=0.17
3
10
0
Study area
8
4
Treatment
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 plasticbristled 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.
# of species in plot
# of groundcover species
per plot
10
0
No significant difference was found in vegetative
groundcover percentage between treatment (mean=
33.3%) and control plots (mean=31.1%; 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
(mean=6.6 species) and control plots (mean=7.0
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 (mean=89.3%) and control plots
(mean=89.9%; t=0.49, p=0.63), and no significant
relationship between canopy and buckthorn was found
(R2=0.17, p=0.13). Results of monthly pitfall trapping at
NWC’s forest are reported elsewhere.
50
100
150
200
250
300
350
400
# of buckthorn in plot
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.
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 NWC’s 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 Iowa’s 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 Iowa’s forests are imperiled by the invader.
REFERENCES
Boudreau, D and G. Wilson. 1992. Restoration & Management Notes 10: 94-95.
Fagan, M. & D. Peart. 2004. Forest Ecology and Management 194:95-107.
Heneghan, L., C. Clay, & C. Brundage. 2002. Ecological Restoration 20(2):108-111.
Schmidt, K. & C. Whelan. 1999. Conservation Biology 13:1502-1506.
USDA Agricultural Research Service. http://www.ars.usda.gov/Main/docs.htm?docid=9921
Yoo, H., R. O’Neil, D. Voegtlin, & W. Graves. 2005. Annals of Entomological Society of America 98:926-930.