Abstract
Environmental gradients affect where plant communities are able to either thrive
or lead a marginal existence. Physical factors such as light and water availability,
average temperature, and topographic position determine to a large extent the ecological
niche of plant species. In recording tree species and basal area data along a slope
gradient in upstate NY, the impact of slope position on species composition became
evident. Different forest types were encountered along the transect line. It became
obvious to even the most novice observer that slope position directly determined what
tree species were found at the 0.01ac sample points.
Introduction
The natural world is a diverse array of countless different kinds of plants, animals,
and land. Uniformity in setting is not the norm and the reason that such biodiversity
exists is because of the changes in water availability, hours of daylight, elevation,
drainage, and soil texture. These are but a sampling of the ecological factors responsible
for the enormous scale by which life exists.
A good example of this is the impact of latitude on plant species. Black cherry
(Prunus serotina) is common in the area surrounding Syracuse, NY. However, one
would be hard pressed to locate such a specimen in Florida. The range of the tree is only
as far south as the Appalachian Mountains of West Virginia. In this way, both
temperature and elevation play a role in determining the realized niche of the black
cherry.
In a study done in Louisiana and Florida, herbaceous/herb data collection in
quadrats located along an elevational gradient found three separate plant communities
which could be delineated (Drewa et al, 2002). These separate communities were along a
gradient of only several meters. These plant communities were closely linked to
available soil moisture (Drewa et al, 2002). In addition, in a study which examined the
physical factors of forest edges versus interiors, the authors found that maximum air
temperatures and relative humidity changed significantly (Cadenasso et al, 1997). These
changes were observed as one moved from the forest edge into the interior of the forested
stand. These results were observed at both a northwest-facing slope and a northeastfacing slope. In this case, the fragmentation of a forest led to a change in the
environmental factors (i.e. maximum air temperature and relative humidity) as one
moved along a gradient from a forest edge into a forest.
Hypothesis and Objectives
In order to test the theory of change along an environmental gradient, a study was
performed in Heiberg Forest located in Tully, NY. By recording tree species and size
data along a gradient, it was hoped that conclusive evidence would be found to disprove
the null hypothesis. HO being that a slope gradient does not impact tree species or
determine tree populations. The alternative hypothesis (HA) being that a slope gradient
does impact tree species and determines, to some extent, tree populations.
The objective of this study was to discover, in analyzing the field data, whether
conclusive evidence could be found to refute the null hypothesis. Slope position
determines microsite conditions such as available water, depth to the water table,
available light, and soil nutrient availability. The dynamic nature of these attributes
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across a gradient is drastic enough to warrant a change in successful forest types. This is
what the study was determined to find.
Results
After completion of the fieldwork, the hard data was entered into a spreadsheet
and the basal area was calculated for each DBH (diameter at breast height) recorded. The
formula for this calculation is .005454(D)^2 where D = diameter. Using a bar graph,
these basal areas were visually displayed. Two bar graphs were made; one using basal
area as the measurement (Chart A) and the other simply using a tree count per species at
each of the five sample points (Chart B). In both cases, there are evident differences in
tree species distribution as one moves from plot 1 towards plot 9. Hard maple (Acer
sacharum) and black cherry (Prunus serotina) were found in greater numbers in plot 1
and plot 3 then in plots 7 and 9. Also, hemlock (Tsugas canadensis) was found in a
greater proportion in plots 7 and 9 then in plots 1, 3, or 5 where is was almost
nonexistent. Although, it should be noted that there was a lot of advanced hemlock
regeneration on plot 5. These are two of the more identifiable trends along the transect
although there are some other less obvious ones as well. American beech (Fagus
grandifolia) was observed in appreciable numbers in plots 5 and 7 but disappeared by plot
9. One yellow birch (Betula alleganiensis) was observed at plot 7 and an anomalous
hemlock was observed at plot. Both of these occurrences contributed too little basal area
to their respective plots to be considered noteworthy.
CHART A
Distribution of tree species as observed along a slope gradient
in Heiberg Forest, Tully, NY
20.00
Basal Area (sq.ft./ac)
18.00
16.00
YB
BE
HE
RM
BC
HM
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
Plot 1
Plot 3
Plot 5
Transect Position
2
Plot 7
Plot 9
CHART B
Distribution of tree species as observed along a slope gradient
in Heiberg Forest, Tully, NY
20.00
Basal Area (sq.ft./ac)
18.00
16.00
YB
BE
HE
RM
BC
HM
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
Plot 1
Plot 3
Plot 5
Plot 7
Plot 9
Transect Position
Conclusion
With the support of the data and results of this experiment, a few conclusions can
be made. Tree species, such as hard maple and black cherry, can be found in abundance
at plots one and three. These two species require well-drained soil in order to survive.
When looking past site one up the hill, hard maple and black cherry still dominated the
landscape. At site one, one hemlock was recorded in our data set. This hemlock can be
considered an anomaly because it attributed little basal area to that plot and, therefore, not
contributing a large amount of biomass to the site. Site one also had the highest basal
area out of all other plots. As one travels further down the slope gradient, the soils
become more imperfectly drained to such a degree that standing water was found
between plots seven and nine. The hard maple and black cherry disappeared as the
transect continued down the slope. These species were replaced by American beech
initially and subsequently by hemlock and soft maple at the bottom of the slope (plot
nine). This change in species composition could be a consequence of changing levels in
soil drainage classes. For instance, species such as hemlock and soft maple do better on
wet sites than on dry sites. These species are able to outcompete hard maple and black
cherry on more moist soils. At plots five and seven an abundance of beech was also
found, it is possible that hard maple and black cherry seedlings were out competed by
beech seedlings on wetter soils. Beech is known to survive in relatively harsh conditions
and has a good tolerance to shade where as maple and cherry need much more sunlight to
regenerate. In conclusion, we reject our null hypothesis, which stated slope gradient does
not impact tree species or determine tree populations. The study found that slope
gradients do have a profound effect on tree species and populations.
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Literature Cited
Cadenasso, M.L., M.M. Traynor, and S.T.A. Pickett. 1997. Functional location of forest
edges: gradients of multiple physical factors. Canadian Journal of Forest
Research. 27, no.5: p.774-782
Drewa, P.B., W.J. Platt and E.B. Moser. 2002. Community structure along elevation
gradients in headwater regions of longleaf pine savannas. Plant Ecology. 160,
no.1: p. 61-78
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