Long-term effects of clearcutting in the Southern Appalachians

Background/Question/Methods

Over a decade ago, a debate began over whether understory plant communities recover to their pre-disturbance states following logging.  Evidence showing reduced diversity in previously logged forests was criticized for not accounting for intersite environmental heterogeneity.  Recent studies have addressed environmental heterogeneity, but have neglected long-term recovery by using mature forests as the benchmark for diversity comparison.  Remnant old growth forests comprise a small fraction of the standing forest, but are critical for assessing the long term recovery and resilience of the diverse herb layer.  Here we investigate long-term recovery of an understory herbaceous community in terms of diversity, the role of dispersal and environmental filtering in determining species composition, and trait based assembly of species within the community.  We address intersite environmental heterogeneity by comparing paired sites of old growth forest and mature forest (logged 100 – 150 years ago). 

Results/Conclusions

We demonstrate that herb communities do not return to their pre-disturbance states after more than a century.  Species richness and abundance was greater in old growth forest than mature forest, and species composition differed significantly.  The processes determining species composition were distinct in old growth versus mature forests.  The herb layer composition in old growth forest was determined primarily by environmental variables.  Environmental filtering was not a strong explanatory variable for mature forests.  Environmental variables related to soil fertility (Mn, Mg, total exchangeable cations) explained 37% of the variation in species composition across old growth forest landscapes, and none in mature forests.   Using traits as proxies for resource use, old growth forest showed significantly more trait similarity among co-occurring species than herb assemblages in mature forest.  We conclude that succession in the understory herb layer takes much longer than previously thought, and recovery time exceeds 100 – 150 years.