The Gap Study
In our long-term studies we have followed the natural experiments occurring as
a result of the 1986-88 drought in which the average precipitation was only
60% of the ten-year mean precipitation. One response to drought has been a
long-term decrease in pitch pine and oaks in the Coweeta basing (Figure 4). To
study forest succession in canopy gaps, we have girdled trees in sites a low
and high elevation an with a without rhododendron understory (gap plots in
Figure 1). We are currently measuring microclimatic variables, N mineralization,
and seedling physiology in the gaps that complement long-term studies of seed
rain and seedling demography for assessing recruitment.
A numerical model of seed source strength and dispersal was developed and fitted
against seed trap results in mapped plots (Figure 5). Source strength
parameters (fecundities) and density functions (seed shadows) have been
developed for all species and used to determine the extent to which seed
availability regulates recruitment. The residuals of seedlings occurrence
from the model predictions of seed rain are analyzed to determine the additional
environmental filters that govern recruitment.
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