Analyzing the impact of a slope gradient on the average annual
growth of red maple (Acer rubrum) over a five year period
Group 4
FOR 332 – Forest Ecology
Lab 2
Dr. Yanai
September 9, 2003
Adam Davison
Chris Foote
Christian Schmidt
Abstract
In an attempt to better understand the growth increment dynamics of a slope
gradient, a study was performed along a transect line in Heiberg Forest in Tully, NY.
The purpose of this study was to increment core red maple (Acer rubrum) dominant and
co-dominant trees along a gradient and analyze the amount of annual growth over the
previous five growing seasons. Upon completion of the field work, the dataset was
analyzed for a positive correlation between annual growth and slope position. It was
found that such a correlation did exist as a greater amount of growth over the past 5 years
was seen at the top of the slope and steadily decreased as one sampled down the slope.
Red maple experienced better growth at the upper slope positions than at the lower slope
positions. In performing this field test, the impact of an environmental gradient, in this
case a slope gradient, could be quantitatively measured in the way of annual growth or
the ‘new wood’ of the red maple.
Introduction
As a continuation of previous fieldwork, environmental gradients will be
examined more closely to determine their impact on the existence of specific plant
communities and how changing physical factors alter the vegetative landscape along such
a gradient. The environmental gradient being examined is a slope gradient. A transect
has been laid out along a downward slope in Heiberg Forest in Tully, NY. In order to
better understand how the microenvironments of soil texture, water availability, light
penetration and temperature among others influence the presence of plant communities,
measurements will again be taken along the gradient to examine these casual
relationships.
A good measurement of site productivity is tree growth. If a site is one of optimal
growing conditions (i.e. microclimatic conditions), trees should exhibit growth in putting
on good growth rings during each growing season. This serves as an indicator of
previous conditions such as drought, flooding, variable conditions of opening and
closings in the overstory canopy, and natural fire events, which are rare in the
Northeastern U.S. Growth rates may be explained by the climatic conditions that
occurred in the previous growing season. In studies performed on sugar maple (Acer
saccharum), it was found that prior climatic conditions play a large role in the current
annual growth increment (Lane et al, 1993). The study also found that the species was
more positively impacted by temperature than by precipitation (Lane et al, 1993). This
would have interesting affects on the growth rates of a species in the same family
(Aceracea) along a slope gradient where temperatures could range from top to bottom.
Hypothesis
Reaching to back to what was discussed previously, the null hypothesis that
dominant trees at the (5) plots sampled in lab 1 will exhibit no difference along the
gradient from top to bottom in average annual growth. The alternative hypothesis is that
there will be a difference in the average annual growth of the dominant trees with better
growth found at the more optimal sites on the mid to upper slope of the gradient.
The objective of the lab is to complete sampling of the dominant trees at each of
the (5) previously sampled plots to obtain evidence to either support of refute the null
hypothesis.
Methods
Using data gathered in the previous lab session, red maple (Acer rubrum) was
identified as the tree species that was found along the slope gradient and in a large
enough diameter to be considered a dominant/co-dominant tree. After analysis of the
ascertained data set, two red maple trees per plot were identified as a dominant and codominant. Using an increment borer and a ruler with inches marked to the tenth, the two
upper overstory trees were cored at each plot. They were cored first on the uphill side of
the tree and then 90 degrees to the left of the initial intrusion. Once cored, the last five
years’ growth was observed and the physical distance measured with the ruler. The
average between the two measurements was calculated and recorded. This procedure
was followed at al the odd plots along the 10 plot transect. Once field observations were
completed, graphical representations of the dataset were formulated using MS Excel.
Note: No red maple trees were observed in the immediate vicinity of plot 5, so
attempting to maintain slope position, crewmembers walked away from the plot and
located two red maples at a distance from the plot center (~ 35 yards). These trees were
cored and their measurements recorded. Upon analysis of the data however, there was
such a large discrepancy between the data observed at plot 5 and the other observations
that the crew decided to throw out the plot 5 dataset. The red maples were observed too
far from the plot center to be considered a part of the same slope gradient that was being
studied. A graph of the observations including plot 5 has been included along with a
graph excluding the plot 5 observations. In due course, the plot 5 data will be ignored in
the Results section of the lab.
Results
The dominant red maples experienced an overall decline in average annual tree
growth progressing from the top of the slope (plot 1) to the bottom (plot 9). The
exception to this downward trend may be seen in plot 7, where the mean 5-year
increment saw a slight increase (Chart A). The co-dominant red maples also experienced
an overall decline in average annual tree growth progressing from the top of the slope
(plot 1) to the bottom (plot 9). Unlike the dominant red maples, no exception to this
decline was seen along the gradient (Chart A).
Chart A
Average annual growth over a 5-year period of
Acer rubrum along a slope gradient in Heiberg Forest, Tully, NY
5-year growth (inches)
0.7
0.6
0.5
Dominant
0.4
Co-dominant
0.3
0.2
plot 1
plot 3
plot 7
Transect position
plot 9
Chart B
Average growth over 5-year period of Acer rubrum
along a slope gradient in Heigberg Forest, Tully, NY
5-year growth
(inches)
0.7
0.6
0.5
Dominant
0.4
Co-dominant
0.3
0.2
plot 1
plot 3
plot 5
plot 7
plot 9
Transect position
Note: Chart B contains the erroneous plot 5 data. Possible reasons for the data collected
at plot 5 are addressed in the Discussion portion of this lab.
Discussion
The analysis of the data collected showed a definitive trend between the average
annual growth for the dominant and co-dominant tree species at the upper plots and the
average annual growth at the lower plots of the slope gradient. The data collected from
this lab does not support the null hypothesis, rather, it supports the alternative hypothesis,
showing the average annual growth of dominant tree species to be higher at the mid to
upper portion of the slope gradient. According to the data collected, the average annual
growth for the dominant and co-dominant tree species appears to be better at plot 1 and
plot 3, whereas the growth at plots 7 and 9 are moderately less.
After analysis of the data was conducted, it was determined that plot 5 was an
anomaly, compared to the results of the other plots. It is easy to see that the inclusion of
plot 5 data in a graphical representation of the observations seriously skewed the
emergent pattern seen at the other sample points. The questionable procedure used to
acquire two red maple sample trees and the lateral distance from the plot center where
they were found forced their disqualification as reliable observations upon analysis of the
data set. The other plots had a dominant and co-dominant species in the required vicinity
to include in the results. As the results and graph show, average growth over five years
of the dominant and co-dominant species declined from the upper portion of the slope to
the lower portion. A more likely reason for this is the increased amount of water that
collects at plot 9 could inhibit the growth of the tree species, whereas the water runoff at
plot 1 might allow for a more optimal site condition for the species. Another possible
cause is increased sunlight at plot 1 relative to plot 9.
An interesting possibility to explain the greater relative growth rates at the upper
slope position may be because of human disturbance. A study, which examined the
growth rates of red maple in two hardwood stands in West Virginia found that the species
exhibited increased height growth rates after the canopy, was disturbed (Tift and Fajvan,
1999). The upper slope of the transect was abutting a forest road and was well drained
while the bottom of the slope was rather wet and not in such close proximity to a
roadway. Not knowing the logging history of the site, it would be easy to surmise that
the greater relative growth at the upper slope positions may be due to more frequent
logging intrusions, which open the canopy and stimulate red maple growth. The upper
slope positions may be more frequented by windthrow events which would also open the
canopy and encourage an increase in annual growth of the trees.
Conclusion
Upon completion of the field work and subsequent analysis of the dataset, it was
found that the average annual growth over the previous five years along the slope
gradient in Heiberg Forest steadily declined from the uppermost slope position down to
the lowest slope position sampled. These observations were able to refute the null
hypothesis. In accepting the alternative hypothesis, the study was able to conclude that a
difference in average annual growth over the past 5 years did exist along the slope
gradient with greater growth occurring at the upper slope positions relative to the lower
slope positions.
The study could vastly be improved by perhaps sampling along a longer/steeper
gradient and by sampling a tree species with a more dramatic range in elevation. Also,
replicating the study at all four (or eight) dominant aspect positions (N, E, S,W, …NE,
SE, SW, NW) allows that variable to be accounted for. Perhaps the best way to test the
influence of slope on growth increments in overstory trees would be to sample along a
slope gradient within a homogenous stand. This would tell a more reliable story than
testing red maple in a mixed stand of northern hardwoods. Too many other variables and
possible causes exist in such a stand to make this study greatly compelling. However, if
one were to isolate those variables by testing the hypothesis in a homogenous stand along
a slope gradient, say an aspen (Populus tremuloides) stand on a mountain side in
Colorado, the results would prove more conclusive.
Understanding the impact of environmental gradients on optimal growing
conditions for certain forest types or individual tree species will allow researchers and
foresters additional knowledge in their professional fields. For instance, bio-remediation
programs rely on information that allows their efforts to be successful such as the optimal
site conditions for an endangered or endemic species. Also, foresters need to know what
tree species will be most successful in a reforestation or afforestation planting regime
when practicing forest management.
Literature Cited
Lane, C.J., D.D. Reed, G.D. Mroz and H.O. Liechty. 1993. Width of sugar maple (Acer
saccharum) tree rings as affected by climate. Canadian Journal of Forest
Research. Vol.23; no.11; pp.2370-2375
Tift, B.D., Fajvan, M.A. 1999. Red maple dynamics in Appalachian hardwood stands in
West Virginia. Canadian Journal of Forest Research. Vol.29; no.2; pp. 157-165