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STATISTICAL ANALYSIS
OF UNDERSTORY VEGETATION DATA
FROM VALLEY FORGE NATIONAL HISTORICAL PARK
1993–2003
Technical Report NPS/NER/NRTR—2008/118
Duane R. Diefenbach1, Wendy C. Vreeland2,
and Kristina M. Heister3
1U.S. Geological
Survey
Pennsylvania Cooperative Fish and Wildlife Research Unit
Pennsylvania State University
University Park, PA 16802
2 Pennsylvania Cooperative Fish and Wildlife Research Unit
Pennsylvania State University
University Park, PA 16802
3 National Park Service
Valley Forge National Historical Park
1400 North Outer Line Drive
King of Prussia, PA 19406-1009
U.S. Department of the Interior
National Park Service
Northeast Region
Philadelphia, Pennsylvania
______________________________
Executive Summary
A fixed-plot
monitoring system was implemented in 1992 to evaluate vegetative communities
in two large wooded areas at Valley Forge National Historical Park.
The objectives of this monitoring system are to: 1) describe the existing
understory plant community on Mount Misery and Mount Joy in terms of
species richness and abundance; and 2) determine changes in abundance
and species composition of understory plant communities in fenced and
unfenced plots over time. This report summarizes the data collected
in these plots in 1993, 1995–1996, 1998, and 2003, and presents
the results of statistical analyses of the data to determine if specific
vegetative changes have occurred over time.
Thirty vegetation sample sites were randomly located on Mount Misery
and Mount Joy (15 in each area). At each sample site, paired plots were
established where one plot was fenced to exclude deer but no other herbivores.
The unfenced control plots were located 36.5 m (119.75 ft) from the
center of the fenced plots in a random direction (except three plots
were located 20– 27.4 m [65.6–90 ft] away). Each plot was
2×2 m (6.5×6.5 ft) in size. Most tree, shrub, vine, and
herbaceous vegetation was identified to species, although some vegetation
was identified only to genera. The number of tree seedlings was enumerated
in all plots.
Species richness was greater on Mount Joy than on Mount Misery for each
of the years, and in both areas generally increased over time in fenced
plots and exhibited a slight decline over time in unfenced plots. Between
1993 and 2003 on Mount Misery, the total number of species increased
from 31 to 41 species in fenced plots, and decreased from 27 to 23 species
in unfenced plots. On Mount Joy, the total number of species increased
from 56 to 71 species in fenced plots and declined from 51 to 48 species
in unfenced plots. On average, only 28% of species that were present
in the fenced plots (mean no. species = 10.4) were present in the unfenced
plots.
Over all years, the number of fenced and unfenced plots containing exotic
species exhibited similar changes observed for overall species richness.
In general, exotic species were present in more fenced plots than in
unfenced plots and individual fenced plots contained more exotic species
than did individual unfenced plots. Twenty-two of the 24 exotic species
recorded from plots have been present in at least one fenced plot during
one or more of the four sampling periods. Since 1995, two species were
first observed in fenced plots in 1998 and two species were first observed
in fenced plots in 2003. The overall increase in the number of exotic
species in plots between 1993 and 2003 is due solely to an increase
observed in fenced plots on Mount Joy (from 78 to 95 [number of species
occurrences per plot summed over all the plots]).
Mean tree seedling counts in 2003 were greater in fenced plots, but
the large variability in seedling counts among plots precluded any statistically
significant evidence of a change over time or differences between Mount
Joy and Mount Misery. By 2003, there were, on average, 7.1 (SE=1.56)
and 11.3 (SE=1.54) more species of tree seedlings in fenced plots than
in unfenced plots on Mount Misery and Mount Joy, respectively.
Guidelines for acceptable abundance of tree seedlings for forest regeneration
are 25 seedlings per 12.57 m2 (135 ft2 [McWilliams
et al. 2002]). The percentage of plots with adequate seedling abundance
is referred to as a stocking rate. We calculated stocking rates using
all tree species combined, only native tree species, and only deer-preferred
tree species. When either exotic tree species or species that deer do
not prefer are excluded from the calculations there was minimal effect
on stocking rates. In 1993, three percent of both unfenced and fenced
plots had adequate seedling abundance. However, by 2003 the stocking
rate in fenced and unfenced plots was 27% and 0%, respectively.
In 2003, four (Jack-in-the-pulpit [Arisaema triphyllum], wild
sarsaparilla [Aralia nudicaulis], sweet cicely [Osmorhiza
claytoni], and Indian cucumber root [Medeola virginiana])
of six herbaceous species known to occur in the park that have been
proposed as potential indicator species of the effects of deer browsing
(Latham et al. 2005) occurred in nine of 30 fenced plots (1–3 species
present per plot), and one species (Jack-in-the-pulpit) was present
in six of 30 unfenced plots. Whenever Jack-in-the-pulpit was present
in an unfenced plot, it also occurred in the paired fenced plot.
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