Intermediate-severity disturbance impacts in a mixedwood forest: A multi-scale analysis

Mixed Quercus-Pinus stands are increasingly desired by forest managers to achieve a range of objectives, including biodiversity enhancement and resilience to global change. To create and perpetuate desired mixed Quercus-Pinus stand composition and structure, quantitative information on natural disturbance impacts on stand development and succession is necessary. Wind is the most common natural canopy disturbance in eastern North America, and impacts vary in severity and spatiotemporal scales. Intermediate-severity disturbance (ISD; events that occur along a classification gradient between frequent gap-scale and infrequent, stand-replacing events) is hypothesized to be an important driver of mixed Quercus-Pinus creation and maintenance. Our overarching goal was to quantify patterns of intermediate-severity wind disturbance in a mixed Quercus-Pinus echinata Mill. forest on the Cumberland Plateau in Tennessee, USA. Specifically, our objectives were to: 1) quantify post-disturbance species composition, stand structure, and residual tree spatial patterns, 2) infer individual-tree, neighborhood, and site-specific impacts on tree mortality, and 3) describe frequency, size, shape, and spatial distribution of ISD-created canopy openings. We inventoried plots in Pinus echinata-dominated, disturbance-impacted portions of the forest. To infer individual-tree and spatially-explicit neighborhood characteristics that influenced survival probability, we applied a random forest classification algorithm. To document changes in tree spatial distribution before and after disturbance, we used spatial point pattern analysis. To characterize forest-scale disturbance patterns, we classified canopy gaps from high-resolution orthoimagery. The most important predictors of survival were basal area, taxonomic group, and distance to nearest neighbor. Pre-disturbance trees and residuals were spatially randomly distributed. Most detected openings were 50–150 m2, and the opening size-frequency distribution exhibited a reverse J-shape. Openings were spatially clustered within the forest at distances < 200 m and complex in shape. We provide quantitative recommendations on the frequency, size, shape, and spatial distribution of silvicultural entries patterned after natural disturbance. To emulate natural disturbance patterns, we recommend patch seedtree harvests with reserves or patch clearcuts with reserves, and these harvest-created openings should be concentrated (i.e., clustered) in portions of the stand in which P. echinata is most dominant and/or most competitive. Site preparation with prescribed fire, herbicide, and/or mechanical thinning may also be necessary to prepare the seedbed and reduce hardwood competition to favor Pinus establishment in openings.