An Investigation of the Structure in Two Forested
Communities and Their Boundaries Along a Topographic
Gradient in Grand Valley State University Ravines
Kelsey Byxbe, Brittany Karavas, Miranda Kasprowicz, and Lauren Mroz
GVSU, SCI 336 Extended Investigation
Introduction
The Grand Valley State University Campus is situated within a natural woodland ravine environment
alongside the Grand River. A variety of habitats and communities exist within the Grand River ravines
including streams, shrublands, mature and young forest communities. Along the eastern side of the
ravines next to the Grand River, elevational changes exist within a variety of topographic gradients in
the transition downslope from upland to bottomland communities. The physical structures associated
with upland and bottomland communities differ in tree species composition, abundance, and vertical
stratification layers. Varying tree species throughout the topographic gradient demonstrate differences
in biomass, height and location within stratification layers affecting the amount of light penetrating the
forest floor.
Structural differences in communities and community boundaries can be explained through the views
of botanists, Frederick Clements and H.A. Gleason. Clements developed an organismic concept of
communities. He proposed that communities and boundaries are distinct and well-defined with narrow
transitions (Matthews, 1996). Clements proposed that individual species interact and behave as an
organized whole (Matthews, 1996). Gleason, however, discussed the individualistic nature of species
distribution in his individualistic or continuum concept. He focused on the idea of gradual changes
along environmental gradients with any distinct boundaries caused by abrupt environmental changes.
Gleason emphasized that species acting individualistically produce communities with transitions
difficult to identify (Matthews, 1996).
As stated by Barnes and Wagner, Jr. (1981), typical tree species present in bottomland communities
include maple (Acer sp.), ash (Fraxinus sp.), elm (Ulmus sp.), poplars (Populus sp.), pawpaw (Asimina
triloba), and certain species of oak (Quercus sp.). In upland communities, several typical tree species
include maple (Acer sp.), beech (Fagus grandifolia), hemlock (tsuga), oak (Quercus sp.), poplars
(Populus sp.) and dogwood (Cornus sp.) (Barnes and Wagner, Jr., 1981).
Byxbe, Karavas, Kasprowicz, Mroz 2012
According to Kricher and Morrison (1988), Eastern North America contains a variety of tree and shrub
species in woodland environments, where oak, maple, and beech may dominate. Climate variances in
woodland environments throughout Eastern North America influence tree species composition and
abundance (Kricher and Morrison, 1988). Climate is a determining factor for distribution of plant
species. Amount of precipitation changes the acidity of the soil influencing soil moisture and pH
measurements. The amount of sunlight exposure influences tree species that are able to survive and
reproduce within forested communities. Sugar maple and beech trees have high shade tolerances and
typically dominate locations associated with soil containing moderate and well-balanced amounts of
moisture (Shanks, 1953). Oaks prefer drier soil conditions, while white ash and elm grow best when soil
is more moist (Shanks, 1953).
According to the Iowa Department of Natural Resources, growing conditions such as soil moisture, soil
pH, and light intensity vary depending on topographic position (upland, bottomland, slope, flat).
Varying conditions within sites primarily determine whether tree species will grow and reproduce.
While variances in site characteristics influence tree species composition and abundance, amplitude
(the range of environments that species are able to adapt to and survive) are typically more narrow of
bottomland communities (Bell, 1974). Species located in upland communities have broader amplitudes
(Bell, 1974).
Depending on the ratio of photosynthetic tissue to supportive tissue characteristics of tree species,
stratification of canopy layers will be evident in forested communities. The tallest trees comprise the
upper canopy, while the shorter, younger trees comprise the lower canopy and understory. Once the
canopy is fully developed, it is possible only 1% of sunlight makes its way to tree and shrub species
growing on the forest floor (Kricher and Morrison, 1988). Due to the range of light exposure
throughout a forested community’s canopy layers, species are present due to their light tolerance level.
Ward and Worthley (2004) explain that among different tree species, the amount of sunlight necessary
for survival varies. The minimum amount of light a species requires is referred to as shade- tolerance, a
highly limiting factor in tree species development that varies from shade-intolerant to shade-tolerant
with moderately tolerant capabilities in-between (Ward & Worthley 2004). Plant species adapted to
high-light environments (require quite significant amounts of light to survive and reproduce) are
identified as shade-intolerant species. Shade-tolerant species are adapted to low amounts of light
exposure and are able to survive and reproduce in these environments. According to Tubbs and
Houston, beech and sugar maple tree species are highly shade tolerant and are often the climax species
in northern hardwood forests, such as the Great Lakes States. On the other hand, shade-intolerant
species present in the Great Lakes States typically dominating the upper canopy layer in forested
locations, include species of oak and aspen (Barnes and Wagner, Jr., 1981).
Byxbe, Karavas, Kasprowicz, Mroz 2012
The Grand River ravines are composed of a beech-maple forest ecosystem, containing a number of
associated tree species. We were interested in investigating changes that occur in species composition
and abundance as we progress downslope along a topographic gradient from an upland to bottomland
site. Because basal area is a representation of biomass, we were interested in calculating basal area of
the upland and bottomland communities. We were also interested in changes we might find in soil
moisture, soil pH, and light intensity while we progress downslope. Our specific questions are presented
below.


What is the species composition and abundance of the upland and bottomland sites?
Are there two different, separate communities? And if so, are the boundaries distinct and the
transitions narrow?

Is there a difference in the mean soil moisture, soil pH, and light intensity between the upland
and bottomland sites and along the topographic gradient downslope?

Is there a difference in the mean basal area of the tree community in the upland and
bottomland sites?
We predicted different tree species will compose the upland and bottomland sites signifying two distinct
communities. We predicted a gradual transition would be evident in the composition and abundance of
species as we progress downslope along the topographic gradient from upland to bottomland sites.
Specifically, species from the upland site would decrease in abundance and new species would increase
in abundance as we progress down the slope towards the Grand River. We also predicted there would
be differences in soil moisture, soil pH, and light intensity throughout the topographic gradient. In
reference to community structure of the upland and bottomland sites, we predicted a difference in
community structure related to basal area and biomass including number of tree species present within
the stratified layers of upper canopy, lower canopy, and understory.
METHODS
We selected a site along a topographic gradient with seemingly distinct upland and bottomland sites at
the top and bottom of the slope. We sampled the topographic gradient using 5x5m plots running along
three contiguous transects that went from the top to the bottom of the slope (Figure 1). The first and last
three 5x5m plots compose the upland and bottomland sites, respectively. The number of plots in each
transect varied due to the irregularities of the landscape features. We used Google Earth to determine
that the highest elevation of our site was 205m and the lowest elevation was 176m.
Byxbe, Karavas, Kasprowicz, Mroz 2012
Figure 1. Location and elevational changes of sampled site.
Grand River
We identified tree species using Barnes and Wagner (1981) and Flynn and Holder (2001), and recorded
species composition and abundance in each plot. We divided the height of the tallest trees at each site
into thirds to represent an upper canopy, lower canopy, and understory. We used a clinometer to
measure the height of one tree in each site that was representative up of upper canopy. We recorded the
stratification layer of each tree in every plot and measured the circumference of each tree 1.4 meters
from the ground in order to determine the tree’s basal area.
By roughly dividing each sample plot in half, we recorded one measurement of light intensity, soil
moisture, and soil pH in each half of all sample plots. We used a Kelway Soil Tester to measure soil
moisture and soil pH, and a Plant Light Intensity Meter to measure light intensity.
We used t-Tests to determine if there were significant differences in mean light intensity, soil moisture,
soil pH, and basal area between the upland and bottomland sites.
RESULTS
We found a change in the species composition and abundance progressing down the topographic
gradient (Table 1). The upland site is plots 1-3 with each plot representing a new 5x5m plot
progressing downslope towards the bottomland site, plots 24-26.
Byxbe, Karavas, Kasprowicz, Mroz 2012
Table1. Tree species composition and abundance along the topographic gradient from the upland site to
the bottomland site.
Plot:
1
2
3
4
sugar maple
10
4
3
2
Northern red
2
0
0
1
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
2
2
2
2
1
1
1
2
2
8
2
1
1
4
2
1
1
1
American elm
18
11
1
red maple
5
3
8
cottonwood
3
13
pawpaw
15
18
swamp white
1
6
3
4
Species:
oak
white ash
1
balsam poplar
1
American beech
2
1
2
1
1
0
1
1
1
4
4
1
9
2
1
2
quaking aspen
1
American linden
2
(basswood)
2
oak
American
3
mountain ash
2
1
2
Sugar maple and American beech are present throughout the gradient but not in the bottomland site.
Tree species common near the upland become less abundant along the gradient, and some bottomland
species become more abundant closer to the Grand River.
We found that there are differences in the species composition and abundance within the stratification
layers between the upland and bottomland sites (Tables 2 and 3).
Table 2. Upland tree species composition, abundance, and stratification layers.
Species
sugar maple
Upper Canopy
Lower Canopy
6
Understory
6
Total
5
17
Byxbe, Karavas, Kasprowicz, Mroz 2012
white ash
1
0
0
1
Northern red oak
2
0
0
2
balsam poplar
1
0
0
1
Mean
2.5
1.5
1.25
5.25
Range
5
6
5
16
2.38
3
2.5
7.85
Standard Deviation
Table 3. Bottomland tree species composition, abundance, and stratification layers.
Species
Upper Canopy
Lower Canopy
Understory
Total
red maple
6
4
6
16
American mountain ash
6
3
0
9
swamp white oak
7
0
0
7
American elm
5
18
7
30
cottonwood
0
8
8
16
12
14
7
33
Mean
6
7.83
4.67
18.50
Range
12
18
8
17
3.85
6.94
3.67
10.7
pawpaw
Standard Deviation
Upland sites are composed of four species compared to six species in the bottomland. The tree
abundance in the bottomland is 81 and in the upland is 21. The most abundant species in the upland is
sugar maple and in the bottomland are cottonwood and American elm. The maximum height of the
upper canopy in the upland is 22 meters and in the bottomland is 16 meters.
According to the statistical analysis, there were differences in the mean soil pH (p < 0.001), soil
moisture (p < 0.001), light intensity (p < 0.001), and basal area (p = 0.019) between the upland and
Byxbe, Karavas, Kasprowicz, Mroz 2012
lowland sites. Upland and bottomland sites differences in soil moisture were 56% and 80% respectively.
The upland had a mean pH of 6.5 and the bottomland 6.2. In addition, the light intensity of the upland
was lower than the bottomland at 4.2 foot-candles compared to 6.7 foot-candles.
Conclusion
Results of our investigation suggest that there are two relatively distinct communities with a changing
pattern of species composition and abundance along the topographic gradient transect. According to
Nesom (2005), Northern red oak is a moderately shade- tolerant species, and mature white ash is highly
shade- intolerant, influencing the physical structure of the site. In the upland site, Northern red oak and
white ash compose the majority of the upper canopy layer but are not producing any young. We expect
that in the future, as these trees die, they will be replaced by species, such as sugar maple, that are more
shade tolerant and are currently reproducing. (Nesom, 2005). In this site the number of trees that
composed the upper canopy was large relative to the total number of trees in the site (10 of 21). There
was also a higher basal area which suggests that the trees are much larger and more mature and thus
contain a larger biomass. This contributes to a thick canopy layer which decreases the amount of light
that reaches the forest floor, resulting in a lower mean light intensity (4.2 foot-candles).
The dominant tree species in the bottomland are eastern cottonwood and American elm. Both species
grow best in conditions with moist soil and high light intensity levels (Nesom, 2005), characteristic of
our bottomland site conditions; these include mean soil pH (6.2), mean soil moisture (80%), and mean
light intensity (6.7 foot-candles). The Ecological Research Center of Shelby Farms Park (2008) found
that bottomland forest species diversity is high and spread throughout the stratification layers. We
found a similar result in that there is a more even distribution of trees throughout the three
stratification layers in our bottomland site relative to the upland (Tables 2 and 3). The mean basal area
for the bottomland site is also significantly lower than the upland, suggesting that the trees are smaller
and less mature, and thus contain a smaller biomass.
We have concluded that there are two relatively distinct communities because of substantial differences
in community structure and projected trends in reproduction (Figure 2). The upland community
consists of the flat terrain at highest elevation of our site and progresses approximately three-quarters
of the way down the slope. The bottomland community includes approximately one-quarter of the
slope and continues along the flat terrain at the lowest elevation of our site.
Byxbe, Karavas, Kasprowicz, Mroz 2012
Figure 2. Changes in tree species composition along a topographic gradient.
The species composition of the upland community is small at six species (sugar maple, Northern red
oak, white ash, balsam poplar, American beech, and American mountain ash) compared to eleven
species (sugar maple, balsam poplar, American beech, quaking aspen, American linden, American elm,
red maple, cottonwood, pawpaw, swamp white oak, and American mountain ash) in the bottomland
community. Species abundances are also larger in the bottomland community (154 total trees to 96
total trees). There are differences in maximum canopy height between the upland and bottomland
communities, 22 meters and 16 meters, respectively. The upland community has a thicker canopy layer
which is supported by the light intensity level (mean light intensity of 6.7 foot-candles). Trees are more
evenly distributed within the stratification layers in the bottomland, whereas in the upland, more trees
comprise the upper canopy layer (Tables 2 and 3). Basal height differences indicate that there is a larger
biomass in the upland community compared to bottomland community (p < 0.001). A smaller biomass
in the bottomland community suggests it is at an earlier stage of succession than the upland community.
The dominant tree species in the upland community are American beech, Northern red oak, and sugar
maple. In the bottomland community the dominant species are American elm, American mountain ash,
cottonwood, and pawpaw. Dominant upland species found reproducing are American beech and sugar
maple due to high tolerances for low light intensity levels. However, in the bottomland the dominant
species are not the only ones reproducing. Several other species are also reproducing. These trends in
reproduction indicate that the bottomland community species composition and abundances will change
in the future while the upland community remains relatively the same.
Byxbe, Karavas, Kasprowicz, Mroz 2012
We have two relatively distinct communities but the transition from one community to the next is not
clearly defined. Based on the physical and environmental conditions, we can make an educated
assumption about the boundary between these two communities. However, our sample site is too small
(15m x 15m) to make a definite conclusion supported with results. We assume that a relatively distinct
and relatively narrow boundary occurs approximately three-quarters of the way down the slope and is
no larger than three plots (15m). This relatively distinct boundary is due to an abrupt change in the
physical environment, supported by Gleason’s individualistic view of communities. Physical
environment changes are near the bottom of the slope where the site changes from a gradient to flat
terrain. This area also included a small pond that changed the environment from terrain to aquatic
before the lowest elevation. These abrupt changes in the environment influenced the abundance and
composition of trees between the two communities. Since a relatively narrow transition occurs, the
upland and bottomland communities have several species in common (American beech, American
mountain ash, balsam poplar, and sugar maple). These species have a large amplitude and higher
tolerances for the environmental conditions of both communities. In order to make any more
conclusions about the transition and boundaries between these two relatively distinct communities, a
larger sample site would be needed.
We were able to answer all of our questions in regards to differences in soil moisture, soil pH, and light
intensity between the upland and bottomland sites. Analysis of our results lead to new questions we feel
could be further investigated. Our sample size was relatively small in comparison to the ravines; a
larger sample size might provide new evidence that could further support or disprove our hypothesis
and provide more evidence for the transition between the two communities. An investigation of the
trend in tree species that are reproducing throughout the topographic gradient could help us analyze
what the future composition of the forest might look like. Research and attempts at collecting evidence
of soil composition indicate that species composition and abundances change based on the type of soil.
An analysis of the difference in soil composition between the upland and bottomland communities
could provide more evidence for these relatively distinct communities.
Looking back, we wish we would have done a few things differently. Thorough research of tree species
prior to our investigation would have been beneficial in identifying various tree species based on bark
and leaf structure and characteristics. Binoculars would have been helpful in seeing the leaves in the
upper canopy to aid in classification. Also, we would have liked to spend more time in the upland and
bottomland communities to study a larger area (to increase accuracy) and collect more samples of soil
Byxbe, Karavas, Kasprowicz, Mroz 2012
pH, soil moisture, light intensity, basal area, and canopy height.
Byxbe, Karavas, Kasprowicz, Mroz 2012
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