COMPENSATORY MECHANISMS OF CENTRAL US HARDWOOD FOREST COMMUNITIES IN A CHANGING ENVIRONMENT

Although it is known that forest productivity is highly correlated wit h canopy leaf area, the influence of environmental gradients within a forest on leaf area-productivity relationships has not been well-docum ented, particularly for mixed-species forests. The present study was c onducted to define the mechanisms by which Central Hardwood forest com munities respond to changes in environment. Ecological Land Type Phase s (ELTPs) of the Central Hardwood region, delineated by soils, topogra phy, and indicator vegetation, provided a framework to examine the imp acts of environmental gradients on leaf area-productivity relationship s at a landscape level. Three ELTPs(1) Quercus prinus / Carex picta-Va ccinium, Dry slope, (2) Quercus alba-Acer saccharum / Parthenocissus, Dry-Mesic Slope, and (3) Fagus-Acer saccharum / Arisaema, Mesic Slope- were selected and stratified into three slope positions viz. the uppe r edge of the community, the middle or center of the community on the slope, and the lower edge of the community. Circular plots of 0.02 ha in size were established for sampling. Standing stock (basal area, vol ume, and biomass), annual production, leaf area index, and growth effi ciency were determined for each slope position and examined to determi ne the mechanisms and trends for community change across slope positio ns. Among the three communities studied, dry slopes compensated for dr ier conditions through both a shift in species composition and reduced annual production. Dry-mesic slopes exhibited adaptation only through changing species composition whereas mesic slopes compensated for dri er conditions primarily through reduction in annual production. These results indicate that mixed-species forest communities adjust to envir onmental gradients through several mechanisms and that the relative im portance of these mechanisms changes with community type.