Disturbances can change the structure and function of forest stands. How forestsrespond to disturbance depends on species differences and spatially varying environmental factors. In this
study, I analyzed the spatial variation in the growth of two forest stands in Hubbard Brook Experimental
Forest. A calcium addition was introduced to one forest stand, and a clear
-cut disturbance was introduced
to the other. There are hundreds of perm
anent plots in each forest stand.
Basal area of each species in each plot was used as an indicator of forest growth. To analyze the
relationship between forest growth and environmental factors, geographic variables (elevation, slope,
aspect, distance to stream) and soil chemistry variables (carbon concentration, nitrogen concentration)
were considered potentially influential in this study.
A forest stand level analysis was first conducted to measure the overall forest change during the time
terest. The calcium addition disturbance was not found to have changed the forest growth trend. The
-cut disturbance decreased the basal area, followed by recovery at a nearly constant rate over the
following 25 years. A stand level species-
specific analysis revealed that there were large interspecific
differences in basal area, and in basal area change. The interspecific differences in physiological
properties influenced recovery.
To focus on the influence of environmental factors, I removed the
interspecific differences by using a
“Forest Change Index” (FCI) to indicate the forest growth condition. The FCI is calculated by summing
up the standardized basal area increment of all species in a plot. The FCI measures the combined effect of
abundance, and mean individual tree growth. Comparing the inter
-plot differences in FCI before
and after disturbance, I found that spatial variation patterns to be highly persistent through time, both for
calcium addition and clearcutting. After the clear
-cut, the spatial variation pattern in forest growth was
changed during the first two decades following disturbance, but subsequently reemerged. This result
suggests that spatial reorganization with disturbance was temporary.
A regression analysis o
n the environmental variables shows that geographic variables control the
spatial variation in forest growth. Soil carbon concentration was only significant in the clear
while nitrogen concentration was not influential in either stand. Furthermo
re, a generalized joint attribute
model (GJAM) analysis showed species differences in response to environmental variables, which
explains the persistence in spatial variation.
The persistent spatial pattern likely resulted from nearly constant restric
tions on resources. Although
species composition and growth was altered by disturbance in the short term, these responses caused a
convergence over time. Further research will be needed to test this hypothesis and analyze the
mechanism behind the persistent spatial variation in forest growth.