Two decades of compositional and structural change in deciduous old-growth forests of Indiana, USA

Abstract

Aims Using a network of permanent plots, we determined how multiple old-growth forests changed over an 18–19-year period at a statewide scale. This examination of change allowed us to assess how the compositional and structural stability of each forest varied with site characteristics (topography, physiography and productivity) and stochastic disturbance. Methods In 2011, we resampled 150 plots distributed across five oldgrowth hardwood forests in Indiana, USA that were originally sampled in 1992–1993. Within each plot, we relocated and remeasured the diameter at breast height (dbh) of all trees (≥10.0 cm) present during the 1992–1993 sample, w hich allowed us to track their individual fates through time for growth and mortality calculations. Trees that grew to ≥10.0 cm dbh since plot establishment were designated as ingrowth. The dbh and species of all saplings (stems ≥2.0 cm but <10.0 cm dbh) were also recorded. For each forest, we calculated density (stems ha −1 ), basal area (BA; m 2 ha −1 ) and importance value (relative density + relative BA)/2) of trees by species. For saplings, density per ha was calculated by species for each forest. We also calculated annual mortality rate (AMR) for three diameter classes (10–29.9, 30–59.9 and ≥60 cm) and species richness (S), evenness (E) and Shannon–Weiner diversity (Hʹ) for the tree and sapling layers. Differences between years were compared for each forest using paired t-tests and Wilcoxon signed rank tests. Important Findings Although we observed commonality in changes across some sites, our results suggest that these forests differ in their rates and trajectories of change. Changes in total stand BA and density varied across sites and were influenced by past disturbance and mortality rates. We observed a general increase in the overstory dominance of Acer saccharum coupled with a general decrease in the dominance of Quercus section Lobatae (red oak group) species. Mortality of overstory trees present in 1992–1993 ranged from 27 to 49% over the study period (mean AMR 1.6–3.7%). Most sites experienced greater mortality of early and mid-successional species, but one site experienced heavy mortality of Fagus grandifolia, a shade-tolerant late-successional species. Shade tolerant species, A. saccharum in particular, dominated the sapling layer at most sites. However, recruitment of this species into larger size classes did not occur uniformly across all sites and the species was comparatively uncommon at one site. Overall, our results suggest that oldgrowth remnants, even within a single state, cannot be viewed as equivalent units with regard to research or management. Stochastic disturbance events and surrounding land use may have amplified effects on small scattered remnants. Therefore, continued monitoring of these rare, but biologically important forests is critical to their long-term management and protection.