Two decades of change in the structure and composition of old-growth forests in the Central Hardwood Region, U.S.A.

摘要

Although the existence of old-growth as a structural stage is widely accepted, a universal definition of old-growth has been elusive. A common thread across definitions is the prevalence of small-scale gap disturbances that create structural stability at the stand scale through time. To evaluate stability in old-growth forests, long-term changes in both structure and composition must be evaluated. Long-term data provide a critical tool for the evaluation of change in these forests.;The Central Hardwood Region (CHR) of the central United States contains 89 million hectares (ha) of deciduous hardwood forest, the majority of which is classified as Quercus and Quercus-Carya dominated. Over the past two centuries, human impacts, such as logging and clearing of land for agriculture and infrastructure, have made old-growth forests within the CHR rare, accounting for approximately 100,000 ha of forested land.;We studied two decades of change in nine old-growth hardwood forests of the CHR. These forests are located in Indiana and Missouri across a gradient of decreasing productivity, from eastern Indiana to southwest Missouri. We used data from these forests to evaluate mortality rates and changes in structure and composition across this productivity gradient.;We found that all nine forests have remained relatively stable over the past two decades, though the Missouri forests exhibited greater stability than the Indiana forests. All of the forests displayed diameter distributions that indicated these old-growth forests contain all-aged populations of trees with many small diameter stems and fewer large diameter stems. Between sampling events, there were no significant changes in the distribution of stems amongst size classes in any of these forests.;Despite high structural stability, compositional changes are taking place in these forests. During the two decade study period, overstory (≥10.0 cm dbh) mortality in these forests ranged from 25-49% (or 1.5-3.7% annually). In addition, the majority of forests experienced a decrease in overstory density and basal area of Quercus spp. and an increase in overstory basal area of Acer spp. This indicates that Acer spp. increased in diameter following mortality of the original Quercus spp. cohort.;While compositional changes are taking place, these changes were not uniform across the productivity gradient sampled. We found that forests on more mesic site conditions are shifting from dominance by Quercus and Carya species to dominance by A. saccharum, A. rubrum, and Fagus grandifolia, while forests on more xeric site conditions are retaining Quercus spp. and Carya spp. dominance. These results suggest that forests across an east-west gradient in the CHR will continue to become more dissimilar through time.