Old-growth forests of the Central Hardwood Region: Two decades of structural and compositional change across a productivity gradient

Background/Question/Methods

The Central Hardwood Region (CHR) was historically dominated by Quercus forests, woodlands, and savannas. Over the past century, changes in structure and species composition in CHR forests have been attributed to changes in the natural disturbance regime, with a pointed focus on reduced fire frequency. The primary objective of this research was to determine how mortality rates, species composition, and stand structure have varied in old-growth forests of the CHR over two decades. A network of permanent plots was established in 1992-93 and resampled in 2011-12 to quantify changes in the structure and composition of nine old-growth forests (five in Indiana and four in Missouri) within the CHR. These forests are on a general gradient of decreasing site productivity, starting in eastern Indiana and continuing to southwest Missouri. We hypothesize: (1) structure in these forests has remained relatively stable, however (2) forest composition is shifting from a Quercus spp. and Carya spp. dominated overstory to an Acer spp. and Fagus grandifolia dominated overstory on the mesic sites, while (3) Quercus spp. and Carya spp. continue to dominate forests on xeric sites.

Results/Conclusions

In Indiana, our results indicate that over the past two decades the structure of the five old-growth forests has remained relatively stable. Despite this structural stability, these forests are experiencing a compositional shift. As the older, large diameter canopy trees senesce (1.6-3.7% annual mortality), shade tolerant, self-replacing species are being recruited into gaps in the canopy. Acer saccharum is one of the most commonly recruited species due to its abundance across strata in these forests. With continued mortality of large diameter, dominant canopy trees, A. saccharum will likely continue to recruit into the overstory and could dominate all strata of these forests in the future. Unlike Indiana, our results indicate that both the structure and composition of the four Missouri old-growth forests have remained relatively stable. On these xeric sites, species such as Quercus and Carya are present in all strata, suggesting that these species are self-replacing following canopy senescence. Our findings support our hypotheses that forest structure has remained relatively stable, while species composition in old-growth forests is shifting on the mesic sites and remaining static on the xeric sites. These results suggest that forests across an east-west gradient will continue to become less similar through time.