Effects of disturbance and site factors on sapling dynamics and species diversity in northern hardwood stands

The northern hardwoods forest ecosystem of eastern North America provides a wealth of products and services to human society. These forests have been managed under partial harvesting treatments to meet wood production objectives of desired species while maintaining or increasing structural and compositional heterogeneity. Few studies have examined how disturbance and site factors interactions may influence sapling dynamics and species diversity in northern hardwood forests. We used a retrospective approach to examine the impacts of disturbance and site factors on sapling (trees > 130 cm in height and <10.0 cm dbh) dynamics and species diversity across 37 stands with time since harvest ranging from 6 to 20 years. We assessed American beech (Fagus grandifolia Ehrh), sugar maple (Acer saccharum Marsh.), red maple (Acer rubrum L.) and yellow birch (Betula alleghaniensis Britt.) saplings density as functions of disturbance intensity (% basal area cut and overstory tree mortality rate (%)) and site factors. We also examined the effects of disturbance, site factors and relative density of American beech, red maple, sugar maple and yellow birch saplings on species diversity at the sapling layer. The density of American beech saplings decreased significantly with harvest intensity. High harvest intensity significantly reduced American beech sapling density at high depth-to-water sites. However, the densities of red maple and yellow birch saplings increased significantly with increasing harvest intensity. We found that high harvest intensities had less negative effect on sugar maple sapling density compared with American beech. Our results also showed that tree species diversity (Shannon diversity index (H′)) increased with harvest intensity up to about 75% and then declined. However, depth-to-water (DTW) significantly changed the magnitude of tree species diversity response to harvest intensity. Red maple appeared to have contributed more to species diversity than the other hardwood tree species. Given that sugar maple sapling density was generally higher than American beech at high harvest intensities in this study, management regimes that promote mid-tolerant species in American beech-presence stands may eventually benefit sugar maple more than American beech in these forest types. These strategies may also increase species diversity and improve forest ecosystems functions and services.