2011 NORTHEASTERN NATURALIST 18(4):509–520
The Effects Of Deer Exclosures On Voles and Shrews In
Two Forest Habitats
David Byman*
Abstract - White-tailed Deer (Odocoileus virginianus) have overbrowsed much of the
hemlock-mixed northern hardwood forest in northeastern Pennsylvania. I investigated
the possible deleterious effect of this overbrowsing on 4 ground-cover-dependent smallmammal
species through the use of deer exclosures. From May through September
1996–2005, 4 x 10 Sherman live-trap grids were placed in two 0.65-ha exclosures and their
adjacent control sites in forest heavily browsed by deer. One exclosure was located beneath
a primarily oak-maple canopy and the other exclosure under Tsuga canadensis (Eastern
Hemlock). More Myodes gapperi (Southern Red-backed Vole) were captured in the oakmaple
grids than in the hemlock grids and more M. gapperi were taken in the oak-maple
exclosure than in the neighboring control. Microtus pinetorum (Woodland Vole) first appeared
at the grids six years after the exclosure construction, and a large majority were taken
in the oak-maple exclosure. Of Blarina brevicauda (Northern Short-tailed Shrew) captured
at the grids, 80% were taken in the oak-maple grids and 58% in the oak-maple exclosure.
Beginning in 2000, most of Sorex cinereus (Masked Shrew) captured were taken in the oakmaple
habitat. Most of those trapped under the oak-maple canopy were taken in the control.
These observations suggest that heavy deer browsing may depress populations of M. gapperi,
M. pinetorum, and B. brevicauda, but not Sorex cinereus.
Introduction
It is well documented that Odocoileus virginianus Zimmermann (Whitetailed
Deer) populations in forested regions of the northeastern United States
can adversely affect plant regeneration and diversity (Alverson and Waller 1997,
Behrend et al. 1970, Brenner 2006, Marquis and Grisez 1978, Russell et al. 2001,
Tilghman 1989, Townsend et al. 2002). Studies have revealed the negative effects
of deer browsing on young trees (Anderson and Katz 1993, Buckley et al. 1998,
Healy 1997), shrubs (Levri et al. 2009, Townsend and Meyer 2002), and herbaceous
plants under the forest canopy (Anderson 1994, Augustine and Frelich 1998,
Shelton and Inouye 1995). Such heavy browsing by White-tailed Deer has had a
particularly deleterious effect on plant regeneration in an uplands hemlock-northern
hardwoods forest of northeastern Pennsylvania (Townsend et al. 2002).
With the exception of bird populations (McShea and Rappole 2000), the indirect
negative effects on wildlife from reduced understory diversity and cover
density due to heavy deer browsing are not well studied. Browsing by deer could
adversely affect small-mammal populations through the depletion of protective
ground cover, modification of ground microclimate conditions, and competition
for preferred foods. Presumably, semi-fossorial voles and shrews should be
among the small mammals most sensitive to the loss of ground cover.
*Penn State Worthington Scranton, 120 Ridge View Drive, Dunmore, PA 18512; dxb14@
psu.edu.
510 Northeastern Naturalist Vol. 18, No. 4
In the second-growth mixed conifer-hardwood upland forests of northeastern
Pennsylvania, the most prevalent voles and shrews are Myodes gapperi Vigors
(Southern Red-backed Vole), Microtus pinetorum LeConte (Woodland Vole),
Blarina brevicauda Say (Northern Short-tailed Shrew), and Sorex cinereus
Kerr (Masked Shrew) (Merritt 1987). The Southern Red-backed Vole has been
described as a sometimes cyclic, opportunistic feeder whose distribution in the
forests of the northeastern United States is determined by moisture availability
as well as by litter abundance (Kirkland and Griffin 1974, Merritt 1981, Miller
and Getz 1977). The semi-fossorial Woodland Vole normally requires a thick
ground cover over well-drained soil (Connor 1953, Hamilton 1938, Miller and
Getz 1969, Smolen 1981). The Northern Short-tailed Shrew has broad habitat
requirements, but seems to prefer moist, deep litter in hardwood forest (George
et al. 1986; Getz 1961; Miller and Getz 1977; Pruitt 1953, 1959; Yahner 1982,
1983). The Masked Shrew is associated with soil moisture maintained by soil organic
matter and abundant understory vegetation (Getz 1961; Pruitt 1953, 1959;
Whitaker 2004). Platt and Blakely (1973) suggested that Masked Shrew population
levels are inversely correlated with those of the larger Southern Short-tailed
Shrew, which does prey on Sorex spp. as well as its primarily invertebrate food
species (Eadie 1949, Hamilton 1940).
As the Woodland Vole is highly dependent on leaf litter and herbaceous cover
for security, the removal of much of this cover by deer should adversely affect this
rodent. As the abundance of the Red-backed Vole is dependent on moisture levels
as well as cover and as this vole is also highly cyclic in numbers, high deer densities
may not obviously affect this species to the degree that would be true for the Woodland
Vole. As the body of the Northern Short-tailed Shrew is roughly twice as long
and its weight three times as heavy as that of the Masked Shrew in Pennsylvania
(Merritt 1987), the bigger animal is likely to require the denser cover for protection
against predators. Since White-tailed Deer have depleted the forest ground cover in
the Eastern Hemlock-White Pine-hardwoods forest of northeastern Pennsylvania,
I predicted that improved habitat within deer exclosures would lead to increased
numbers of the cover-dependent voles and shrews. I hypothesized that the populations
of voles and shrews in the exclosures would increase through increased
reproduction, survivorship, and/or immigration. To test this prediction, I compared
vole and shrew populations inside and outside deer exclosures.
Prior to the post-winter appearance of the leaves of deciduous trees, plants
growing underneath deciduous canopies receive more light in the early growing
season than do species underneath evergreen canopies. If as a result, deer exclosures
under deciduous and evergreen canopies differ in ground cover density and
diversity, vole and shrew densities may then differ between these protected sites
as well. I predicted that cover-dependent voles and shrews would achieve higher
populations in a deer exclosure placed in deciduous forest than in an exclosure
under an evergreen overstory.
Field-Site Description
Lacawac Sanctuary (41°23'N, 75°17"W) is a 202-ha nature preserve located
on the Pocono Plateau in northeastern Pennsylvania (Townsend et al. 2002).
2011 D. Byman 511
Anecdotal observations of high deer abundance at the sanctuary date from the early
1970s (Townsend et al. 2002). In 2000, deer density was estimated at 19 to 29 individuals
km-2 (Townsend et al. 2002). At Lacawac, there was an obvious browse line
at 1.5 to 1.6 m, with a scarcity of understory plants throughout the sanctuary.
Within the sanctuary, Tsuga canadensis (L.) Carrière (Eastern Hemlock) was
the dominant canopy tree, with Acer rubrum L. (Red Maple) and Quercus rubra L.
(Red Oak) as subdominants. Quercus prinus L. (Chestnut Oak), Fagus grandifolia
Ehrhart (American Beech), and Acer saccharum Marshall (Sugar Maple) were
also important canopy trees. The understory at Lacawac was almost completely
devoid of sizeable seedlings or sprouts or small saplings of these overstory trees
(Townsend et al. 2002). Seedlings and sprouts of mature understory trees and the
tall shrubs Hamamelis virginiana L. (Witch Hazel), Ilex verticillata (L.) A. Gray
(Winterberry), Rhododendron maximum L. (White Laurel), Vitis labrusca L.
(Fox Grape), Kalmia latifolia L. (Mountain Laurel), and Vaccinium corymbosum
L. (Highbush Blueberry) were nearly absent throughout the sanctuary and were
always small (≤20 cm) when present. Flowering herbaceous vegetation was also
severely depleted in species composition and abundance.
Methods
Small mammal trapping
I followed guidelines of the American Society of Mammalogists for the capture
and handling of mammals (Animal Care and Use Committee 1998). This
study was approved by the Penn State University Institutional Animal Care and
Use Committee. Two 0.65-ha deer exclosures had been constructed in 1994–1995
at Lacawac Sanctuary. The oak-maple exclosure was in a site dominated by
Red Oak (36% of stand density) and Red Maple (30% of stand density), with
Chestnut Oak (10%), Eastern Hemlock (8%), Pinus strobus L. (White Pine)
(6%), American Beech (4%), and Betula lenta L. (Sweet Birch) (3%) as associates
(Townsend and Meyer 2002). The Eastern Hemlock exclosure was on a
site with Eastern Hemlock as the primary overstory species (>70%). The two
exclosures were 306 m apart, separated by 120 m of hemlock-dominated forest
and 186 m of primarily oak-maple forest. The exclosure fences were constructed
from livestock fencing (lower 1.2 m), and chicken wire (upper 1.2 m) attached
to steel poles at 5-m intervals. Most of the bottom apertures within the livestock
fence were 15.5 x 20 cm. However, some apertures on each side of each fence
were larger, ranging up to 25 cm in width and 20 cm in height. The bottom wire
of the fence generally lay on the ground, although the largest gap between fence
and ground was 25 cm for the hemlock fence and 21 cm for the oak-maple fence.
This fence design was adopted because it was easy to repair and would be a minor
impediment to cross-fence travel by fox-sized and smaller animals.
In contrast to the forest outside the exclosures, plant succession proceeded
within the exclosures following the erection of the fences (Townsend et al. 2002).
New growth of a flowering herbaceous understory and tree seedlings occurred
throughout the oak-maple exclosure. In the hemlock exclosure, development of
a new understory and ground cover was less extensive, only occurring below the
512 Northeastern Naturalist Vol. 18, No. 4
few breaks in the tree canopy and under scattered deciduous trees. Throughout
the 10-year study, exclosure fences were checked for damage every week, and the
exclosures were monitored for visible deer, deer pellets, and deer tracks in order
to determine whether deer had entered the exclosures.
Live-trap grids were placed within the oak-maple and Eastern Hemlock exclosures
(OMX and HEX, respectively) in 1996. An additional two live-trap grids,
the oak-maple (OMC) and Eastern Hemlock (HEC) controls were placed immediately
outside each exclosure fence. The two control grids were within 20 m of
the exclosure fence and were constructed so that they would parallel the grids
within the exclosure fences.
All four trap grids contained 40 large (23 x 9 x 7.5 cm) Sherman live traps
(H.B. Sherman Traps models LFA, LFATDG, LFG, and LNG) in a 10 x 4 pattern
with 10 m between traps. Traps were set during 3 consecutive evenings each
2-week period from May through September in each year. In 1996–1999, traps
were baited with peanut butter and rolled oats. In 2000–2005, traps were baited
with a mixture of sunflower seeds, oats, and raisins. When first captured, a mouse
was marked by toe clipping from 1996 through 2000 and by ear tags (Scott Roestenberg
- Western Tag) from 2001 to 2005. Shrews were marked by toe-clipping
throughout the study. After each capture and before release, voles and shrews
were weighed with a spring scale (Pesola) and voles were sexed.
Data analyses
In order to examine the effect of deer exclusion on the number of Southern
Red-backed Voles and Woodland Voles captured over the 10-year duration of the
study, a series of 2 (Grid) by 10 (Year) repeated-measures analyses of variance
(ANOVA, SPSS 13.0 for Windows), with Grid as the between-subjects factor
and Year as the within-subjects factor. An alpha level of 0.05 was used to test
significance of indicated differences in the repeated-measured ANOVAs when
examining grid site effects. The effect of deer exclusion on the number of shrews
captured over the 10-year duration of the study was examined with the Wilcoxon
signed-rank test (Ott 1988).
Results
In addition to the Southern Red-backed Vole, Woodland Vole, Northern
Short-tailed Shrew, and Masked Shrew, mammals captured in the OMX, OMC,
HEX, and HEC grids between 1996 and 2005 were Tamias striatus (L.) (Eastern
Chipmunk), Sciurus carolinensis Gmelin (Eastern Gray Squirrel), Tamiasciurus
hudsonicus Erxleben (Red Squirrel), Glaucomys volans (L.), Southern Flying
Squirrel, Peromyscus leucopus Rafinesque (White-footed Mouse), Napaeozapus
insignis Miller (Woodland Jumping Mouse), and 1 immature Didelphis virginiana
Kerr (Virginia Opossum). The White-footed Mouse made up more than 90%
of the captures on the four live-trap grids.
Voles
Southern Red-backed Voles were captured in both OMX and HEX during 1996,
the first year of the study, and then in OMX and OMC in 1997 (Fig. 1). This species
2011 D. Byman 513
was not captured again until 2001, with 1 individual in OMC, and 2002, with 8 individuals
in the OMX and 4 in OMC. No Southern Red-backed Voles were taken in
2003, 2004, or 2005. Other than the 3 Southern Red-backed Voles in 1996 that were
taken in HEC, all the remaining animals were taken in the oak-maple grids, 12 in
OMX and 7 in OMC. As 19 of the 22 Southern Red-backed Voles captured in the four
grids were taken in the oak-maple traps, the repeated measures ANOVA was only
calculated for the oak-maple exclosure and control grids.
Within-subjects analysis. Examination of the fluctuations of the Southern
Red-backed Vole populations over the ten-year study period revealed a uniformly
significant year effect in all comparisons for both oak-maple grids,
which was unsurprising given the large yearly fluctuations in numbers of animals
captured (Fig. 1).
Between-subjects analysis. Examination of the effect of different trapping
grids on numbers of individual Southern Red-backed Voles captured over the
10-year research program failed to reveal a significant year x grid interaction for
OMX vs. the OMC, Wilkes lambda = 0.910, F(1,9) = 1.454, P = 0.228. This result
was despite the 2-to-1 disparity in the number of Southern Red-backed Voles
captured in OMX vs. OMC in 2002.
The first Woodland Vole captured at any of the grids was taken in OMC in
2001, 7 years after the construction of the exclosure fences and 6 years after the
onset of this study (Fig. 1). In 2002, 1 Woodland Vole was taken in OMX and 1
in HEX. Beginning in 2003, 9 years after the construction of the exclosure fences,
20 individuals were taken in the OMX, 1 in OMC, and none in either hemlock
grid. As 23 of the 24 Woodland Voles captured in the 4 grids were taken in the
oak-maple traps, the repeated measures ANOVA was only calculated for the oakmaple
exclosure and control grids.
Within-subjects analysis. Examination of the fluctuations of Woodland Vole
populations over the 10-year study period revealed a uniformly significant year
effect in all comparisons for both oak-maple grids, the result of large yearly fluctuations
in numbers of animals captured (Fig. 1).
Between-subjects analysis. Examination of the effect of the exclosure fence
on numbers of individual Woodland Voles captured over the 10-year research
program on the oak-maple trapping grids found a significant year x grid interaction
for OMX vs. OMC, Wilkes lambda = 0.812, F(1,9) = 7.124, P < 001.
The 10-year trends in the numbers of the 2 vole species taken in the oak-maple
grids were different (Fig. 1). Red-backed Voles were taken in the first 2 years of
the study and then only seen again in 2001 and 2002. In contrast, the Woodland
Vole first appeared in 2001 and sustained a trappable population for the last 5
years of the study.
Shrews
Northern Short-tailed Shrews were first captured in the 4 grids in 1999
(Fig. 2). In every year from 1999 to 2005, more Northern Short-tailed Shrews
were taken in the two oak-maple grids than in the 2 hemlock grids, with a consistently
increasing disparity in the last 4 years of the study. With the exception of
2001, more Northern Short-tailed Shrews were captured in the OMX than in the
514 Northeastern Naturalist Vol. 18, No. 4
OMC. The difference in number of captures in the two oak-maple grids in favor
of OMX also steadily increased from 2002 to 2005, with a marked disparity of
19 animals in 2005: 26 in OMX vs. 7 in OMC.
Figure 1. Yearly totals of individual Southern Red-backed Voles and Woodland Voles
captured in each of four forested live-trap grids from 1996 through 2005. The four grids
were placed in oak-maple and hemlock deer exclosures and nearby control sites.
2011 D. Byman 515
The one-sided Wilcoxon signed-rank test indicated that over the 10-year duration
of this study significantly more Northern Short-tailed Shrews were captured
in OMX than in OMC (n = 7, T = 1, P < 0.025). It was not possible to use this
Figure 2. Yearly totals of individual Northern Short-tailed Shrews and Masked Shrews
captured in each of four forested live-trap grids from 1996 through 2005. The four grids
were placed in oak-maple and hemlock deer exclosures and nearby control sites.
516 Northeastern Naturalist Vol. 18, No. 4
test to evaluate the effect of the exclosure on Northern Short-tailed Shrews in the
hemlock habitat. There were only 4 yearly observations (capture totals) with a
non-zero difference, and 5 are required by the Wilcoxon signed-rank test.
Masked Shrews were first captured in the 4 grids in 2000 (Fig. 2). No more than
2 Masked Shrews were taken each year from 2000 to 2003. As with the Northern
Short-tailed Shrew, the number of Masked Shrews captured greatly increased in
the last 2 years of the study, with more animals taken in the oak-maple sites than
in the hemlock sites. However, within the oak-maple sites, the relative numbers of
Masked Shrews taken in the OMX vs. OMC differed markedly from that observed
with the Northern Short-tailed Shrew, with more Masked Shrews trapped in the
control than in the exclosure (Fig. 2). In 2004, 7 Masked Shrews were captured in
OMC and 4 were taken in the neighboring OMX. In 2005, 14 Masked Shrews were
trapped in OMC and only 3 captured in OMX.
It was not possible to use the Wilcoxon signed-rank test to compare the numbers
of Masked Shrews captured over the 10 years of this study between either
exclosure trap grid with its corresponding control trap grid. In both the oak-maple
and the Eastern Hemlock habitats, there were fewer than 5 pairs of observations
(capture totals) with a non-zero difference.
The observed increase in the number of both shrew species captured in the
last 2 years of the study, 2004 and 2005, occurred as 3 lepidopteran species,
Lymantria dispar (L.) (Gypsy Moth), Malacosoma disstria Hubner (Forest Tent
Caterpillar), and Malacosoma americanum Fabricius (Eastern Tent Caterpillar)
entered into an outbreak phase (T. Marasco, Division of Forest Pest Management,
Pennsylvania Bureau of Forestry, Harrisburg, PA, pers. comm.). The “ground
crawling” mid-to-late larval stages of the Gypsy Moth were particularly abundant
in 2005. None of these 3 Lepidopteran species had been in outbreak phase
in Wayne County, PA and in adjacent Pike County since the early 1990s.
Discussion
Although sample sizes and the lack of site replication limit the conclusions
that can be drawn concerning the effects of habitat type and deer exclusion on
small-mammal abundance, this study supported the hypothesis that high deer
density negatively affects the population density of the Southern Red-backed
and Woodland Voles inhabiting a hardwoods forest habitat. So few individuals
of either species were captured in the Eastern Hemlock habitat that it was impossible
to determine anything about exclosure effects under that canopy (Fig. 1).
That the repeated measures analysis of variance did not find a significant difference
in the numbers of Southern Red-backed Vole captured in the oak-maple trap
grids may be due to the prevalence of years with no captures at all at either site
or the small number of individuals captured. This species is reported to exhibit
cyclic fluctuations in numbers, with peaks ranging from 6 to 10 years (Grant
1976, Patric 1962). Two population peaks may have been observed during this
study, in 1997 and 2002, and a few Red-backed Voles visited the oak-maple grids
during those years. The numbers of animals captured were low, but after 7 years
of deer-free plant succession, the number of Northern Red-backed Voles taken in
2011 D. Byman 517
OMX more than doubled. The 2-to-1 ratio of Northern Red-backed Voles in the
oak-maple exclosure vs. the adjacent control in 2002 suggests a preference for
habitat conditions associated with a lack of deer.
The repeated measures analysis of variance did find a significant difference in
the numbers of Woodland Voles in the oak-maple grids in favor of OMX. This result
would be expected from the 20 to 1 disparity in Woodland Voles taken inside
and outside the exclosure fence during the 2003–2005 trapping seasons (Fig. 1).
The capture data suggest that under the conditions at Lacawac Sanctuary, a minimum
number of 6 years of plant succession was required within the exclosure
before the habitat was suitable for this vole species. It should be noted, however,
that it is possible that a) it simply took 6 years for Woodland Voles emigrating
from a population refuge to “find” the exclosure, or b) this pattern of captures
is the result of cycles in abundance of Woodland Voles. As this semi-fossorial
species requires thick ground cover of either litter or vegetation (Connor 1953,
Miller and Getz 1969, Paul 1970) while eating forbs, grasses, roots, and seeds
in forested habitats (Benton 1955, Cengel et al. 1978, Hamilton 1938), it is to be
expected that this species would prefer OMX over its companion control grid after
a sufficient number of years of succession provided adequate cover and food.
It seems likely that a high deer density in the Pennsylvania’s mixed hardwood
forest negatively affects Woodland Vole populations.
This study suggests that the eastern hardwood forest is superior to the Eastern
Hemlock forest as habitat for the Northern Short-tailed Shrew (Fig. 2). However,
as Northern Short-tailed Shrews were captured in the Eastern Hemlock as
well as the oak-maple grids from 1999 on and as in both habitats more Northern
Short-tailed Shrews were generally caught in the exclosures than in the controls,
it can be assumed that by 1999, all 4 grids had at least minimally adequate habitat
for this shrew. In addition, both exclosures may have had better habitat for
the Northern Short-tailed Shrew than their corresponding controls. The results
of the Wilcoxon signed-rank test for the oak-maple grids suggest that the superiority
of the OMX over its control as Northern Short-tailed Shrew habitat after
2002 was more pronounced than was the case of the hemlock exclosure when
compared with its corresponding control.
It is impossible to distinguish which of 2 possible effects of high deer density
negatively affected the Short-tailed Shrew populations in the two controls. The
deer removed protective cover, but also may have adversely affected the shrew’s
invertebrate prey habitat. The outbreak of the Gypsy Moth in 2005 may give
some clarity. If it is assumed that the density of the larvae in the soil was roughly
the same on both sides of the exclosure, then the considerably higher number
of Northern Short-tailed Shrews inside the fence was made possible by the superior
protective ground cover inside the exclosure. The apparent superiority of
the ground cover in the deer-free habitat was evident since 2002, but became
dramatic when it could protect an unusually high Northern Short-tailed Shrew
population, made possible in 2005 by an improved food supply.
Although Sherman traps probably are not the most effectual means of
assessing shrew abundance, comparison of captures of Masked and Northern
Short-tailed Shrews revealed markedly different effects of high deer populations
518 Northeastern Naturalist Vol. 18, No. 4
and the resulting ground-cover depletion. Although the Masked Shrew first appeared
in 2000, a year after the first captures of the Northern Short-tailed Shrew,
few Masked Shrews were caught until 2004 (Fig. 2). In 2004 and 2005, the years
of greater insect food abundance outside as well as inside the exclosure fence, the
oak-maple exclosure fence appeared to have a dramatic but unexpected effect on
the Masked Shrew population, as the Masked Shrew catch was markedly higher
on the control grid. Even though a few more Masked Shrews were captured inside
the exclosure in 2004 and 2005 than had been the case in previous years, the
Masked Shrew did not appear to greatly benefit from the improved ground cover
inside the oak-maple exclosure.
One explanation for the greater abundance of the Masked Shrew outside the
exclosure fence in the high insect food supply years of 2004 and 2005 would be
the combination of the Masked Shrew’s small size and the large numbers of the
Northern Short-tailed Shrew inside the fence. Since the Masked Shrew is roughly
one quarter the size of the Southern Short-tailed Shrew, it seems possible that the
smaller shrew was less likely than the bigger shrew to be spotted by predators in
the limited cover available outside the fence. Since Northern Short-tailed Shrews
prey on Sorex spp. (Eadie 1949, Hamilton 1940), the Masked Shrews may have
tended to simply avoid contact with the large numbers of Northern Short-tailed
Shrews inside the exclosure, “preferring” the predation risk created by the reduced
cover outside the exclosure to that created by the unusually high numbers
of Northern Short-tailed Shrews within the fence. However, it is also possible
that the Masked Shrew preferred the deer-affected vegetation structure and/or the
microclimate outside the exclosure.
Now that 16 years have passed since the construction of the exclosure fences,
it would be valuable to examine the relative abundances and diversity of coverdependent
voles and shrews, ground-cover species, and soil invertebrates on the
oak-maple grids to improve our understanding of the effects of high deer density
on small mammals. It can then be determined whether the trends in vole and
shrew abundance on the different sides of the deer exclosure fences described
in this study have been sustained. If the trends continue, it will be interesting to
examine whether the exclosure-based disparity in voles and shrew numbers is
coincident with an increased disparity in vegetative ground cover and soil invertebrates
on either side of the exclosure fences.
Acknowledgments
I wish to thank Dr. Daniel Townsend of the University of Scranton for his leadership
in the construction of two deer exclosures in Lacawac Sanctuary. Without his invaluable
efforts, this research would have been impossible. I would also like to thank Janice Poppich,
the Executive Director of Lacawac Sanctuary and its Board of Directors for their
support and interest in my research. Finally, I would like to acknowledge the invaluable
financial support from Penn State Worthington Scranton, in both Research Development
and Richard Matthews grants.
2011 D. Byman 519
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