The ephemeral nature of barrier islands accents a concern for the continued integrity of island upland communities as island geomorphic evolution is increasingly forced by sea level rise. Several easily measured and mapped biophysical variables are identified here for the monitoring of changes in Pinus taeda (loblolly pine) forests subject to sea level forcing. A dendrochronological analysis builds site-based growth trends. This ecological investigation connects forest growth and ecohydrology, initiates baselines, and characterizes vulnerabilities among different classifications. The relationships developed allow for the forecasting of changes to island forest structure with rising sea level.
A principal components analysis is performed to group forest stands by biophysical data. A biophysical variable, plant area index, measured with the electronic LAI-2000 optical instrument, can represent differences in photosynthetically active plant area across gradients as well as change within sites over monitoring periods. Another descriptive biophysical metric, height of peak canopy density (HPCD), is derived from merged crown ratio data and acts to weight plot-level foliar area distribution. Evidence of differential site effects on long-term barrier island P. taeda development is found using dendrochronological techniques.
Results/Conclusions
Site environmental differences, generally related to soil moisture dynamics, explain 66% of the variation in HPCD. Of the biophysical metrics measured, the most reliable site indicator is the maximum canopy height (MCH), a finding consistent with studies in other forests. In this case, 76% of variation in MCH is explained by site conditions.
Growth at lower site index (SI) stands declines at a significantly faster rate than on higher SI sites. The 10-year average radial growth rate of trees declines over sampled periods of 20 years following stand initiation, and is up to 27% less in lower SI stands. This correlates with more shallow water tables in low SI sites. Root competition should be greater at these low SI sites when compared to better sites that may access more stable water tables at greater depths. In tree-ring chronology correlations with climate, the differences in correlation among site types can be explained by this soil profile/rooting zone and water source theory.
The site gradient analyses here are the basis of a space-for-time scale substitution in outlining possible effects to barrier island forests with rising sea levels. Also, average growth rates found in early stand development can be extrapolated to predict growth and developmental reactions by biophysical types to fresh-water table and saline mixing zone rise associated with sea level rise.