Did disturbance diminish vegetation-environment relationships in forests of the Great Smoky Mountains (USA)?

Background/Question/Methods

Although extensive logging had occurred throughout the eastern United States by the early 20th century, the nature of long-term logging legacies is poorly understood at the landscape scale. This is due to both the scarcity of historical land-use data and the lack of forests undisturbed by human land-use. Consequently, it is not clear if and how this widespread past disturbance changed how forests are structured across the landscape. Steep climatic and topographic factors have a strong influence on forest communities in the southern Appalachian region, but it is not clear if logging may have accentuated or masked the effects of these environmental gradients on forest composition. We explored these relationships using land-use history data from Great Smoky Mountains National Park, which maintains some of the largest tracts of old growth forest in the eastern US. We used Generalized Dissimilarity Modelling and a vegetation dataset compiled from multiple studies to address questions about the relationship between beta diversity, including homogenization and community turnover along environmental gradients, and logging history. Biotic homogenization, which can occur following human disturbance in regional plant communities, is a particularly sensitive issue in the southern Appalachians—one of most biodiverse forest ecosystems in North America.

Results/Conclusions

We found that early 20th century clear-cutting has caused long-term homogenization of forest communities on a landscape scale in addition to decreasing local (plot-level) richness. This homogenization effect was present even after controlling for variation in local richness, as clear-cut plots exhibited a 25% decrease in heterogeneity (average distance to median; based on Raup-Crick values). Past clear-cutting also changed the overall and relative long-term importance of drivers of community turnover—most notably by increasing the importance of distance-mediated processes, such as dispersal limitation, and decreasing the importance of topographic position. In other words, our results suggest that past clear-cutting changed how spatial and environmental gradients structure communities in this diverse region. These findings provide information on the specific differences in drivers of variation between old growth and clear-cut forest communities, which is invaluable for informing forest management, conservation, and remediation. In summary, our results suggest two main conclusions: compared to old-growth forest plant communities, (i) clear-cutting caused forest plant communities to be more similar to each other even decades after the initial disturbance, and (ii) clear-cutting did weaken some, but not all, of the long-term relationships between plant community composition and environmental gradients.