DOES MAMMAL COMMUNITY COMPOSITION CONTROL RECRUITMENT IN NEOTROPICAL FORESTS?...
Lyman_Senior_Thesis
1. !1
SNOWSHOE HARE HERBIVORY AND OTHER FACTORS LIMITING SPRUCE
REGENERATION ON GREAT DUCK ISLAND, MAINE, U.S.A.
WADE LYMAN
email: wlyman@coa.edu
ABSTRACT:
Red and White spruce (Picea rubens and P. glauca) dominate the forests on Great Duck
Island (GDI), Maine. Tree core data suggest that spruce regeneration largely ceased in the 1960s,
at least two decades after the introduction of snowshoe hare. Instances of spruce forest decline
are widespread across northeastern North America, with causes ranging from introduced species
and invasives to acid rain. While the cause of the recruitment decline on GDI has not been
systematically studied, hypotheses among collaborators range from inbreeding depression to
edaphic factors, such as over-nitrification from seabirds. To test these predictions, a survey was
conducted on GDI in the summer of 2014 and in the spring of 2015. Data was collected on the
abundance of red spruce seedlings, vegetation cover and seed bank data in 152 plots across the
island. The spruce seed bank was quantified and compared to seedling abundance to determine
seedling survival rate at the various sites. LIDAR data were used to determine forest
characteristics across the island. Germination rate was then correlated with understory leaf area,
midday solar radiation, canopy density, canopy height, topographic convergence index and slope
aspect, as well as hare fecal pellet abundance. In addition, 32 exclosures were established in
separate vegetation plots at the end of the summer of 2014. These exclosures were re-surveyed
in the spring of 2015. None of the selected factors were strong predictors of pre-winter seedling
survival rate, but post winter surveying provided strong evidence for heavy influence of
herbivory. Exclosed sites experienced a 60 percent increase in seedling abundance overall, while
non-exclosed sites experienced 72 percent seedling disappearance. This study is the first of its
kind to yield direct evidence for the influence of snowshoe hare on spruce recruitment on GDI.
INTRODUCTION
Great Duck Island (GDI) is a 90 hectare
island 18 kilometers south of the mouth of
Frenchman Bay in the town of Frenchboro,
Maine (44°09'14.33"N, 68°14’53.46”W). The
island is co-managed by the Nature
Conservancy and the College of the Atlantic,
as the island is an important nesting habitat
for numerous bird species, including Leach’s
Storm Petrel (Oceanodroma leucorhoa),
Herring Gull (Larus smithsonianus), Great
Black-backed Gull (Larus marinus), and
Black Guillemot (Cepphus grylle), Savannah
Sparrow (Passerculus sandwichensis) and
Song Sparrow (Melospiza melodia). The
forest community, currently comprising 50
percent of the island area, consists of mostly
Red Spruce (Picea rubens) and White Spruce
(Picea glauca), with some Mountain Ash
(Sorbus americana) and Yellow Birch (Betula
alleghaniensis) in recently disturbed areas.
Understory species such as Bunchberry
(Chamaepericlymenum canadense) and
Canada Mayflower (Maianthemum
canadense) are also prominent in the forest,
while Wood Fern (Dryopteris spinulosa), Red
Raspberry (Rubus ideaus), and Wild
Sarsaparilla (Aralia nudicaulis) dominante
forest gaps.
In the early 1930s snowshoe hare
(Lepus americanus) were introduced to the
island as a food source for residents. Some
historical anecdotes suggest peak populations
of up to 500 individuals. However, a 2012
mark and recapture study suggested an end of
summer adult population of between 70 and
80 hare (Chen-Kraus 2012), which aligns
well with more recent subjective
observations.
Dendrochronology data show strong
evidence for a peak in spruce recruitment in
the late 1940s and early 50s followed by a
crash in the 60s (Phillips 2014). This is
2. !2
similar to a scenario described by Peterson et
al. (2005), where forest recruitment on Kent
Island, New Brunswick was effectively halted
by an introduced snowshoe hare population
shortly following introduction.
Lack of spruce recruitment and recent
forest mortality suggest that there is a high
probability of a severe reduction in forest
extent in the coming decades, which could
degrade the quality of the petrel nesting
habitat. Drury (1973) speculated that
browsing by sheep can alter the vegetation
and soil composition on islands in a way that
dramatically reduces petrel nesting viability.
Spruce forests are often associated with soil
and vegetation characteristics that align well
with petrel nesting conditions. The thick,
loose, humus layer and thin understory of
spruce forests may ensure easy burrowing for
the seabirds. Loss of the forest and associated
petrel habitat degradation due to browsing by
hare is therefore a critical conservation
concern for island managers. At the same
time, hare browsing is thought to have
temporarily resulted in a particularly thin
understory vegetation community - with
unknown and potentially beneficial short
term consequences on petrel nesting success.
Anderson (pers comm) suggested that a
thinner understory may allow for easier
nighttime navigation and burrowing for the
petrels, promoting exceptional petrel
population density on the island. However,
the effect of reduced understory vegetation
cover on petrel nesting is likely more
complicated. Corvid predation on petrels may
be higher on GDI due to reduced visual
protection from understory spruce foliage and
burrow erosion rates may be higher due
reduced soil stabilization from a lack of
superficial root systems.
Awareness of the spruce regeneration
issue in previous years culminated in the
establishment of 6 fenced exclosures (three 2
x 3 meter rectangular, and three 1.5 meter
diameter circular exclosures) for pairwise
comparison of browsed and unbrowsed
vegetation structure. Three exclosures were
placed in 1990 and an additional three
exclosures were established in 2006.
Preliminary observation of these exclosures
and their surrounding areas in the summer of
2014 suggested that excluding potential
herbivores (the hare being the only known
suspects) did in fact allow for spruce
regeneration. A comprehensive assessment,
however, was not completed due to missing
data about the exclosures establishment and
the need for a larger sample size.
The factors that influence seedling
recruitment on GDI, especially regarding
snowshoe hare, are not well understood.
Limiting influences such as edaphic factors
and herbivory on GDI have not been studied
until very recently. Negotia et al. (in review)
conducted a soil analysis of both GDI and its
sister island, Little Duck Island (LDI), which
has been free of herbivores since 1954, and
found no significant differences between
them. Plant community data suggested
herbivore driven differences in species
composition between the two islands - with
GDI showing a much higher abundance of
less palatable species. However, the islands
have highly disparate human, herbivore and
other ecological histories. GDI is known to
have been heavily grazed by domestic sheep
for longer than LDI and has a much longer
and more intensive known human use history
as well as seabird use history. Unfortunately,
it is not possible to differentiate between the
influences of human, sheep, seabirds and
snowshoe hare based on current plant
community composition alone. Therefore, a
more extensive look into the various top
down and bottom up factors that may
specifically influence spruce forest
regeneration is needed.
One factor that could be limiting
spruce seedling germination is light
availability. The relatively uniform, dense
canopy of the forest on GDI could be
significantly limiting solar irradiance and
thereby influencing spruce germination and
growth rates. While Baldwin (1934) stated
that direct light is not required for the
germination of red spruce, light can raise
temperatures and reduce soil moisture, thus
expediting or inhibiting germination
depending on intensity. In a field setting
Angell and Kielland (2009) showed that high
enough insolation was correlated with
temperature and soil characteristics that
adversely affect white spruce seedling growth
in the floodplains of interior Alaska.
Therefore, shade tolerant species such as Red
Spruce are expected to show a sharp decrease
3. !3
in viability in areas with high insolation on
GDI.
Since the Alice Eno Research Station
has been active, people have reported the
presence of spruce seedlings in the understory
at the end of every summer, but GDI forest
demography data show no trees younger than
40 years (Philips pers comm. see Figure 3).
During the summer of 2014 small populations
of seedlings - some over 10 years of age -
were confirmed in several isolated locations
above the browse-line (Figure 1). Assuming
that these areas do not differ significantly
from their immediate surroundings in edaphic
and top-down factors such as hare density,
and that they are in fact above the browse-
line, they suggests that hare have seriously
limited spruce recruitment.
In my study, I investigated the
relative effects of snowshoe hare exclosure,
canopy density, canopy height, light influx,
herbaceous understory growth and seed
abundance on spruce seedling survivorship,
as well as monitored the survival of specific
spruce seedlings before and after the winter
Figure 2. Great Duck Island satellite imagery
with the 152 randomly distributed study plots
with the spatial extent of each marked in red.
Figure 1a. The images above show an
abundance of spruce perched on a glacial
erratic, presumably above snowshoe hare
browse line. The photo on top shows a close-
up of erratic. This and other rare areas above
the browse line support the only known
spruce younger than 40 years and older than
4-5 years on the island.
Figure 1b. GDI Tree Demography.
Collected and complied by Christopher
Philips in the summer of 2014.
4. !4
of 2014-2015 with a large scale exclosure
experiment.
MATERIALS AND METHODS
Experimental Design
152 points were randomly generated on a
map of GDI’s vegetated area using ArcGIS
(see figure 2). A Trimble differentially
corrected GPS with sub-meter accuracy was
used to locate each of the random points in
the field. 1x1 meter plots were established at
each of these points over the course of 4
weeks in June of 2014. In each plot a 1
meter2 PVC frame was used to mark the
borders of the survey area. The edges were
aligned approximately to cardinal directions
and survey flags were placed permanently at
two opposite corners of each plot to insure
consistent future surveying. All seedlings
appeared to be in the very early stages of
development (1 to 3 years by my personal
estimate) and thus were counted within the
same age category. Plots were broken down
into smaller rectangular segments using
surveying flags to reduce probability of
double counting. High definition photographs
were taken of each plot during the second
week of July to ensure minimum
phenological influence on vegetation cover
data. Seedlings in all plots were counted
twice, once during the last week of June and
once during the last two weeks of July. Each
count series was taken between rainstorms to
avoid dramatic moisture related changes in
germination rate.
Hare fecal pellets were collected
during the seedling counts. All samples were
packaged in individual breathable paper bags,
dried in the sun and stored for future analysis.
Spruce cones were also counted but not
collected. Soil moisture and pH data were
taken as well, but were discarded, as the
meter did not register any differences among
plots despite obvious qualitative differences
in soil moisture content.
Thirty-two exclosures were placed
within individual vegetation plots. Each
exclosure consisted of 5 ft2 of aluminum
mesh fencing formed into a cube with one
open side facing down and the whole
structure fastened to the ground with iron
wire anchored in topsoil.
Soil samples were taken from a
subset of the plots containing seedlings. The
samples were taken using a 6cm diameter
bulb planter. The first 10 cm of the soil and
organic matter was taken - approximately 300
cm3 of material at each site. All samples were
packaged in individual breathable paper bags,
dried in the sun and stored for future analysis.
During the winter of 2014-2015 each sample
was sifted and spruce seeds were counted and
collected. A flotation method was attempted
for seed enumeration. Malone (1967)
describes using various salt solutions to
rapidly separate seeds from soil. This method
is very effective for separating soil from
organic matter, but was not well suited fro
spruce loam due to high undesirable organic
matter content. All seeds in varying stages of
decomposition were discarded. The
remaining selection of seeds were in ideal
physical condition, and presumably the most
viable.
The seedling counts were divided by
the sum of seedlings and seeds in each plot to
express seedling survival rate. This metric is
notably different than annual germination
rate, as annual accumulation of seedlings that
are older than 1 year are expected to
positively skew the ratio while an
accumulation seeds will tend to positively
skew it. To determine the seedling density per
meter, the seed samples were divided by
0.0056, the fraction of the survey plot that the
sum of soil samples covered.
In the spring of 2015, 18 of the 32
exclosures and their respective non-enclosed
plots were resurveyed for seedling
abundance. Not all plots were resurveyed due
to time constraints.
Light Surveying
Measurements were taken with a quantum
flux meter in 20 different locations with
varying seedling abundance across the island
to test this hypothesis. Light measurements
were taken between 1000 and 1400 hours.
Surveying days were selected based on cloud
cover. Surveying was conducted when there
was close to 0 visible cloud cover or when
direct sunlight values in the open read
between 1700 and 2000 µmol m−2 s−1 on the
flux meter. Each square plot was measured 10
times, with one measurement at each of its
four corners and one in its center, first 1
5. !5
meter above the ground to account for canopy
and mid-story influence and then at ground
level to account for understory influence.
Remotely Sensed Data
In winter of 2014-2015 canopy density,
canopy height, topographic convergence
index, and slope aspect rasters were
generated using ArcGIS. Averages for each of
the rasters were calculated for each of the
survey locations within a 5 meter buffer (See
Figure 2) these values were then used in
statistical analysis.
Canopy Density
Canopy density influences light penetration
and ground temperature. These in turn have
the potential to dramatically influence the soil
chemistry (Angell and Kielland 2009),
moisture and understory vegetation cover,
which could affect seedling germination.
LIDAR data from a 2010 survey was
downloaded from Earth Explorer and model
builder was used to organize a workflow.
ArcGIS Resources contains a comprehensive
explanation of canopy density calculation.
The methodology involves extracting
vegetation and ground point values
separately and converting them to rasters,
replacing all null values with zeros, and
dividing the vegetation raster by the sum of
the vegetation and ground rasters. Canopy
density rasters were tried at 1, 2, 3, 4, and 5
meter resolutions. A 1 meter raster was used
for analysis, while a 4 meter raster was found
to show density heterogeneity across the
island most clearly and was therefore chosen
for visual display. The final product was a
raster that described forest canopy density of
GDI in the summer of 2010 with 4 meter
resolution (See Figure 4).
Topographic Convergence Index
Topographic Convergence Index (TCI) uses
topographic data to determine where water
will tend to accumulate on a surface.
Sometimes referred to as Topographic
Wetness Index, TCI is defined as:
!
where a is the contributing upslope area in
square meters and ß is the surface slope
angle. This equation is applied to each cell of
a digital elevation model. The product is
considered a somewhat accurate proxy for
soil moisture (Western et al. 1999), where
higher convergence is associated with damper
soils. Germination limitations associated with
moisture should therefore be correlated with
topographic convergence. The major
limitation of the model is that it does not
incorporate soil characteristics that influence
hydrology, such as depth and porosity. The
tool can be found in the Topography Tools
pack from ESRI. The resulting data tends to
be highly positively skewed, so the raster was
log transformed for the figure (see Figure 5).
Aspect
Northern slopes may retain higher moisture
levels due to reduced insolation. Germination
Figure 3. Herbaceous vegetation percent cover. This
shows an interpolation of the degree of understory
vegetation cover in the 152 survey plots using a 10
meter interpolation sensitivity. Note that an extended
area of especially low values (seen in green) appears
to coincide with areas of highest germination rate (see
Figure 6).
6. !6
Figure 4. Canopy Density on Great Duck
Island June-July 2011.
Figure 5. Topographic Convergence Index on
Great Duck Island. Note areas of high
convergence in the island interior and the well
defined wetland areas as seen in figure 2.
Figure 6. Pre-Winter Survival Percentage. This
shows an interpolation of pre-winter seedling
survivorship in the 152 survey plots using a 10
meter interpolation sensitivity.
Figure 7. Hare Pellet Abundance. This shows
an interpolation of hare fecal pellet
abundance in the 152 survey plots using a 10
meter interpolation sensitivity.
7. !7
limitations associated with light and moisture
should therefore be correlated to aspect. The
Aspect tool in the ArcGIS Spatial Analyst
tool set was used to generate a simple raster
describing the bearing of each DEM cell for
GDI.
Vegetation Cover Analysis
The photographs of each vegetation plot were
classified using the Arc Supervised Image
Classification tool. Each image was given a
minimum of two classes; one consisting of
vegetation and one non-vegetation. Each
class was defined by a sample of 5000-20000
pixels. The Cell Statistics tool was used to
determine the sum of pixels in each class, and
a leaf area value was determined by dividing
the total vegetation pixels by the sum of
vegetation and all other classes. This method
very rapidly and accurately estimated
vegetation cover within each plot.
Statistical Analysis
Ranked correlations were performed using
McDonald’s (2012) online textbook and
spreadsheets. A Spearman’s ranked
correlation was used to account for outliers
and non-normality in seedling survivorship
ratios used in light intensity, canopy density,
topographic convergence, aspect and
vegetation cover correlations. In addition to a
Spearman’s ranked correlation, a linear
regression was used on the hare pellet data.
Percent seedling abundance change was
calculated for each enclosed and non-
enclosed plot. A Mann-Whitney U test was
performed on Systat 13 for pairwise analysis
of enclosed and non-enclosed area data.
Seedling number and seed number were log
transformed. Aspect data was converted
based on its deviation from north (i.e. 350°
and 10° are both equal to 10).
RESULTS
Combined Seed Germination and
Survivorship
The mean pre-winter seedling survivorship
on GDI was 4.8 percent with a standard
deviation of 4.7. The mean post-winter
survivorship outside of exclosures was 2.1
percent, with a standard deviation of 3.8.
Seed germination percentages ranged
between 1 and 5% on most of the island, with
characteristic seed densities of 200 to 4000
seeds per square meter.
Survivorship Percentage
The ranked correlation between seedling-seed
ratio and seed abundance was found to be
significant (n = 26, ρ = 0.85, p < 0.001).
There is a exponential relationship between
seed abundance and seedling abundance (n =
26, ρ = 0.71, p < 0001). See Figure 8 and
Figure 17.
Canopy Density and Canopy Height
A ranked correlation between canopy density
and seedling to seed ratio was not found to be
significant (n = 24, ρ = 0.05, p = 0.82). The
correlation between canopy height and
seedling to seed ratio was not found to be
significant (n = 24, ρ = 0.10, p = 0.63). See
Figure 12 and Figure 13.
Light Penetration
The correlation between light penetration and
seedling to seed ratio was not found to be
significant (n = 14, ρ = 0.14, p = 0.64). See
Figure 9.
Topographic Convergence Index and
Aspect
The correlation topographic convergence
index and seedling to seed ratio was not
found to be significant (n = 26, ρ = 0.23, p =
SeedligNumber
0
125
250
375
500
Seed Number
0 7.5 15 22.5 30
Figure 17. Seedling survival rate increases
with seed abundance, as seen in Figure 8.
Seed number is in seeds per 600 ml of soil.
Seedling numer is in seedlings per meter2.
8. !8
0
0.023
0.045
0.068
0.09
Fecal Pellets/
Meter
0 1.1 2.2
Figures 8-15. The y-axis is
seedling survivorship
(Seedlings/Seeds plus
Seedlings). See figure 17.
Figure 16. Percent Change in Seedling Abundance. All plots experienced significant
loss outside of exclosures. Plot 2 was the only plot to not see hare pellet deposition post
winter and was also the only plot to experience significant increase in enclosed
seedling abundance.
0
0.013
0.025
0.038
0.05
Solar Radiation
(µmol m−2 s−1)
0 200 400
0
0.023
0.045
0.068
0.09
Herbaceous Leaf
Area (Percent)
0 22.5 45 67.5 90
0
0.023
0.045
0.068
0.09
Topographic
Convergence
Index
0 130 260
0
0.023
0.045
0.068
0.09
Canopy Height
(Meters)
0 3.5 7 10.5 14
0
0.023
0.045
0.068
0.09
LOG Canopy
Density (returns/
m^2)
0 3.5 7 10.5 14
F.9
0
0.023
0.045
0.068
0.09
Seedling
Abundance
(seeds/meter)
0 12.5 25 37.5 50
F.13
F.8
F.12
0
0.023
0.045
0.068
0.09
Log Degree
Distance from
North (Aspect)
0 90 180
F.10
F.11
F.14 F.15
9. !9
0.26). The correlation between deviation from
north and seedling to seed ratio was not found
to be significant (n = 26, ρ = 0.03, p = 0.40).
See Figure 14.
Herbaceous Leaf Area Index
A ranked correlation between leaf area index
and seedling to seed ratio was not found to be
significant (n = 26, ρ = - 0.24, p = 0.24). See
Figure 11.
Hare Fecal Density
A ranked correlation between hare fecal pellet
number density and seedling to seed ratio was
not found to be significant (n = 24, ρ = -0.24,
p = 0.29), while a standard regression yields
significance ( r2 = 0.19, p = 0.04). See Figure
15.
Exclosures
Exclosure had a significant effect on seedling
survivorship. A Mann-Whittey U test showed
a significant difference between exclosed and
non-enclosed areas (n = 18, p < 0.001, u = 2).
See Figure 16.
DISCUSSION
Small island vegetation communities have
been shown to be drastically affected by
introduced herbivore populations (Van Vuren
& Coblentz 1987; Peterson et al. 2005). Yet,
an in depth 10 year study suggests that hare
herbivory has virtually no impact on
vegetation diversity and abundance, and that
nutrient availability is far more important
limiting factor in boreal vegetation
community composition (John & Turkington
1995; Turkington 2002). This study provides
evidence for the importance of herbivory
pressure on Great Duck Island, and
encourages a case-by-case approach to the
investigation of factors influencing vegetation
community development.
Percent Seedling Survival
The exponential relationship between seed
number and seedling number may be
indicative of hare browsing. Snowshoe hare
are typically solitary, and previous studies of
the hare on GDI show that individual hare
have very distinct home ranges with minimal
overlap (Chen-Kraus 2012). Therefore,
annual foraging may be affected by hare
density, allowing some seedlings to be
spared for some years in areas with
particularly high seedling abundance.
Although no precise seedling demography
data was collected during this study, my
qualitative assessment suggests that areas
with higher seedling abundance had a greater
age distribution - with some seedlings
showing 3-4 years of growth. Post-winter
surveying supports this idea, as areas with
higher seedling abundance experienced lower
percentage seedling disappearance. Over time
(perhaps over a period of 3-4 years) this
could result in the seed to seedling abundance
relationship seen in Figures 8 and 16.
Another explanation is that seedling
germination rate is higher in areas with more
seeds, perhaps because edaphic conditions
that favor germination improve spruce
fecundity.
In controlled conditions Baldwin
(1934) found red spruce germination rates
from 1 to 44 percent germination in spruce
loam and an average of 77 percent
germination in a sterile, humid environment.
Survivorship rates calculated from yearling
and seed abundance data on GDI were
expected to be lower than 44 percent due to
conditions and to a data skew from multi-year
seed accumulation.
Seed availability was expected to be
a major limiting factor for spruce
regeneration on GDI. Given that seedling
abundance was strongly correlated with seed
abundance, further investigation into
dispersal processes on GDI could help to
determine where regeneration rate will be
highest and thus where preservation efforts
should be focused.
Canopy Density and Light Intensity
The canopy density study was expected to
suggest two distinct effects on germination
rate. Direct solar radiation may be
detrimental to seedling germination,
especially in shade tolerant species such as
red spruce (Baldwin 1934; Gray & Spies
1997; Angell & Kielland 2009). This would
tend to drive a positive correlation between
germination rate and canopy density.
However, canopy density may pass above a
threshold that limits light availability.
Analysis suggests no clear relationship
10. !10
between seedling germination rate and
canopy density or light intensity on GDI.
Light penetration values were
expected to initially follow a positive
correlation with germination rate and to drop
off dramatically at high insolation intensities
- but no such trend was shown. Experimental
design could have been greatly improved for
this portion of the study. The insolation
readings probably underrepresented daily
radiation influx. Increased spatial sampling
resolution within each plot as well as a more
comprehensive temporal sampling regime -
throughout the day and throughout the year -
would have yielded more reliable results.
However, surface and soil temperature
measurements, taken throughout the day and
summer, may have been more useful than
direct photon influx for studying the
cumulative influence of sunlight and
substrate characteristics on germination.
Hare Pellet Abundance
Studies in western Wyoming show a strong
positive correlation between snowshoe hare
fecal pellet density and hare population
density in uncleared survey plots (Krebs
2001; Murray 2002; Berg & Gese 2010).
Prugh and Krebs (2004) showed that up to 70
percent of hare fecal pellets remain intact in
spruce forests after 2 years of weathering and
decomposition. Hare pellet density may
therefore act as a proxy for recent hare
presence on GDI. Assuming that hare
foraging area and hare excretion area are
spatially similar, one might expect to find
unusually low seedling survivorship ratios in
areas of high hare pellet abundance. Seedling
survivorship and hare pellet abundance on
GDI were found to be inversely correlated,
with slight statistical significance. In
addition, comparison of Figures 6 and 7
shows that areas with highest germination
rate and areas of highest hare pellet
abundance do not coincide with one another.
Some of the hare pellet distribution
data may have been confounded by surveying
bias. Hare pellets are more difficult to locate
in areas with high understory vegetation
cover, and high vegetation may constrain hare
movement to narrow trails, leading to a
distribution in hare pellets that is prone to
under sampling.
Remote Sensing
Raster sampling methodology was not well
calculated. Instead, a 5 meter circular buffer
was arbitrarily chosen for sampling.
Experimenting with numerous buffer sizes,
starting with single cell size and expanding
outward, would reveal which size captures a
stable, yet distinguishing average and range
within each sampling site.
Topographic Convergence and Aspect
Soil moisture gradients are expected to be
influenced mainly by surface and subsurface
drainage, surface evaporation, and
transpiration. Topographic convergence is
expected to be a major determinant of soil
drainage (Western et al. 1999), while slope
aspect presumably modulates solar radiation
driven evaporation, and has been positively
correlated with evaporation rate (Ng and
Miller 1980). Germination rate was therefore
expected to be positively correlated with high
topographic convergence and conformity to
northern aspect. No strong trend was found,
but the random plots I distributed did not pick
up on some ecological features of the
landscape that may have suggested otherwise.
Outside of survey sites, seedling abundance
was found to be exceptionally high on the
northern slope midway up the island.
Bryophytes were also prevalent in these
areas, with very high coincidence of
seedlings and Sphagnum sp. Northern aspects
may create microclimates that are conducive
to bryophytes, which in turn may aid in
spruce development (Dibble and Gese 1999).
Herbaceous Leaf Area Index
Herbaceous understory vegetation can
compete with coniferous seedlings for light
and moisture in their critical, early
development stages immediately following
spring and summer germination (Western et
al. 1999). Herbaceous leaf area index may
therefore have numerous conflicting
implications for substrate moisture content
and light availability. High ground-level leaf
area may reduce solar influx enough to
reduce daytime ground temperature to levels
that inhibit spruce germination. At the same
time, higher leaf area index may indicate
higher potential for nighttime hydraulic
uplift, and improved soil moisture and
temperature conditions during the day
11. !11
(Caldwell et al. 1998). This may be critically
important to seedlings in canopy gaps, where
temperature and evaporation rate are likely
elevated (Angell & Kielland 2009). Specific
herbaceous understory species may influence
seedling germination as well. Dryopteris spp.,
which were abundant within several of the
survey plots, are also known to leach
allelochemicals that inhibit Red Spruce
germination (Klein et al. 1991). Leaf area
was therefore not expected to yield a clear
correlation.
Exclosures
In addition to showing significant
redeposition of hare pellets, resurveying all
exclosure plots in the spring of 2015 showed
consistent reduction in non-enclosed seedling
abundance, and consistent increase in
enclosed seedling abundance. Exclosure data
suggest that snowshoe hare are responsible
for removing a range of 30 percent to 95
percent of P. rubens and P. glauca seedlings
and 77 percent seedling disappearance
overall.
The exclosures were intended to only
prevent the influence of snowshoe hare
herbivory on seedling survivorship. However,
the exclosures may have influenced
microclimate characteristics, as the physical
properties of the metal fencing may promote
condensation, thus modifying temperature
and moisture evaporation and deposition
regimes. This was not controlled for, but is
not expected to have had a significant impact
on germination, as dew deposition does not
contribute to a significant portion of soil
moisture in temperate climates (Kidron
1998).
Future Directions
Future investigations on GDI on this topic
could focus on what may enable the highest
seedling survivorship rates, particularly those
clustered in the sphagnum-rich substrate.
Temperature and moisture are critical factors
for germination and may be linked to the
seedling rich, north-facing slope just south of
the slough.
Controlling the influence of
snowshoe hare on spruce regeneration at a
large scale, whether by eradication or
exclosure, while studying the change of hare
populations and vegetation change may yield
valuable insights into the effects of herbivory
on forest regeneration on GDI. Given that
humans are continually fragmenting forest
habitats (Haddad et al. 2015), studies on
landscape scale influences of herbivory in
island - and otherwise isolated - systems may
have important implications for land use,
preservation and conservation in the future.
CONCLUSION
The snowshoe hare on GDI are limiting
recruitment of the spruce forest. Additional
factors this study investigated, though not
statistically significant, may still significantly
influence spruce regeneration rate, but
probably do not limit regeneration overall.
The relationship between seed and
seedling abundance may suggest that hare
territories are spatially constrained, leading to
incomplete annual browsing in areas with
exceptionally high seedling abundance.
Because the exclosures proved effective at
preventing hare herbivory of seedlings, a
large scale exclosure strategy could be used
for future forest management efforts on GDI.
Based on the current study, future forest
management efforts might be affected
through enclosing areas with larger seedling
age ranges on the margins of existing
historical hare territory (as determined by
hare fecal presence). While the hare on GDI
are significantly impacting the forest
community, and potentially undermining
integrity of the petrel habitat, there may be
ways that forest habitats on the island can be
regenerated and sustained without full hare
extermination efforts.
ACKNOWLEDGEMENTS
I am deeply grateful for the support of Dr.
John Anderson in the design and execution of
this project and for the research experiences
at Alice Eno Field Research Station leading
up to it. I thank the Maine Space Grant
Consortium, and the National Aeronautics
and Space Administration for their generous
support of College of the Atlantic’s student
research projects, and the COA faculty, Dave
Feldman, Sarah Hall, and Steve Ressel who
approved my proposal. I thank the GDI
research team of 2014 - Heather Brown,
12. !12
Emily Engelking-Rappeport, Rachel Karesh,
Porcia Manandhar, Christopher Phillips and
Liam Torrey - for supporting a successful
research season in 2014; Christopher Phillips,
Liam Torrey, Porcia Manandhar and Meaghan
Lyon for help in the field; Chris Petersen for
help with statistical analysis; and Luka
Negotia, Katherine Shlepr and Chloe Chen-
Kraus for general support and comments on
the manuscript. Permission to work on the
bulk of Great Duck Island was provided
under a cooperative agreement between the
College of the Atlantic, the Nature
Conservancy, and the State of Maine Dept. of
Inland Fisheries and Wildlife. Additional
funding for my food, housing and some
equipment was provided by the College of
the Atlantic W.H. Drury Jr. Research Fund.
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