Harvest-related disturbance effects on species richness and community composition in Lake States aspen-dominated forests

Background/Question/Methods

Understanding the effects of anthropogenic and natural disturbances on plant community structure and function is becoming increasingly important in light of projected increases in disturbance severity and frequency with global environmental change.  We examined impacts of the procurement of forest-derived bioenergy, a land-use practice that has been suggested as a climate change mitigation strategy, on the diversity and composition of aspen-dominated forest ecosystems in the Lake States region. Specifically, we tested whether and how varying levels of organic matter removal (bole only harvest, total tree harvest, total tree and forest floor removal) and compaction (light, moderate, and heavy) influence woody species richness and composition for 15 years following harvest.  This range of treatments represented a broad gradient in disturbance severities, allowing us to characterize how the interactions between plant functional traits and disturbance severity affect ecosystem recovery following disturbance.  Sampling occurred at three Long-Term Soil Productivity Study sites located within the Chippewa, Ottawa, and Huron National Forests on three different soil types (loamy, clay, and sandy, respectively).

Results/Conclusions

Results varied among soil textures.  On clay and loamy soils, compaction significantly affected species richness (F=3.97, p=0.024 and F=8.89, p=0.001, respectively) with richness decreasing as compaction increased.  The intensity of organic matter removal significantly influenced species richness at the loamy site (F=4.15, p=0.022) where the two extreme treatments (bole only harvest and total tree plus forest floor removal) both resulted in greater species richness than total tree harvest alone.  Species richness also varied significantly with time at the clay and loamy sites (F=32.75, p<0.0001; F=10.11, p=0.0002 respectively) with values being highest 15 years following harvest, contrary to expectations. No significant patterns of species richness were observed for treatments on sandy soils, but ordination of woody species suggests disturbance severity-related gradients in community composition at all sites.  Further, several species are significant indicators of harvest-related disturbance severity.  On loamy soils, Populus balsamea and Salix sp. are indicative of the most severe treatment (total tree harvest plus forest floor removal) while Acer spicatum and Cornus sericea indicate the least severe treatment (bole only harvest and minimal compaction).  These results suggest that different levels of harvest-related disturbance can impact species diversity and composition.  Moreover, these effects may vary by soil type generating important differential responses to these land-use practices across forested landscapes.