The compound effects of wind and salvage logging change forest trajectories Callie A. Oldfield and Chris J. Peterson Forests are dynamic systems, requiring disturbance to maintain species diversity. The well-known Intermediate Disturbance Hypothesis (IDH)1 predicts that maximum species diversity will be reached at a moderate level of disturbance (Fig. 1). Moderate severity disturbances can support both species that require disturbance to propagate and species that persist through disturbance, while a high severity disturbance will favor only disturbance-loving species. Conversely, low-severity evens will allow only climax species to persist. This IDH is a cornerstone ecological concept that informs thinking about effects of disturbances. Fig. 1. A depiction of the Intermediate disturbance hypothesis, which predicts species diversity based on disturbance severity level. While the trajectories of forests after natural disturbances such as fire and wind have been well documented, forest response to compound disturbances is an area of ongoing research. Thus we often know what to expect from a fire or windstorm alone, but how a fire plus windstorm, or windstorm plus logging will alter diversity and composition, is an area that is often full of surprises. Based on the IDH, combined disturbances would result in a disturbance of a greater severity, and therefore the IDH would predict that diversity after the compound disturbance would be lower than after a natural disturbance alone. This is a strong argument by conservationists that forests impacted by natural disturbances should not have subsequent salvage logging. In the journal Forests, Oldfield and Peterson2 find that forest composition but not diversity is affected by the combination of salvage logging after wind disturbance. Wind disturbed and wind + salvage logged forests showed similar patterns in species diversity and richness, basal area, and number of stems over time, all metrics relating to forest recovery. However, the species composition of the saplings – young trees that may make up the future forest canopy – differed in parts of the forest that had been salvage logged (Fig. 2). Oldfield and Peterson examined these metrics by comparing measurements of species compositions in a Southern Appalachian forest in 2012 and 2017. Undisturbed forest served as a control for species composition and diversity, which they compared to parts of the forest that were damaged by a moderate severity tornado, and parts that were both damaged by the tornado and then subsequently salvage logged. This work, taking place over 6 years, shows evidence that forest diversity is not impacted by salvage logging, but that species composition is altered. Fig. 2. The study site post-wind disturbance (a.) compared to wind + salvage logging (b.) Connell, J. H. (1978). Diversity in tropical rain forests and coral reefs. Science, 199(4335), 1302-1310. Oldfield, C. A., & Peterson, C. J. (2019). Woody Species Composition, Diversity, and Recovery Six Years after Wind Disturbance and Salvage Logging of a Southern Appalachian Forest. Forests, 10(2), 129.
