Effects of intermediate-severity disturbance on composition and structure in mixed Pinus-hardwood stands

Increasingly, forest managers intend to create or maintain mixed Pinus-hardwood stands. This stand assemblage may be driven by a variety of objectives but is often motivated by the desire to enhance native forest diversity and promote resilience to perturbations. Documenting the effects of natural disturbances on species composition and stand structure, and thus successional and developmental pathways, in stands with these mixtures is essential to achieve these goals. The specific objectives of this study were to quantify and compare the impacts of an intermediate-severity canopy disturbance on woody species composition, canopy structure, understory light regimes, and species diversity in mixed Pinus (Pinus taeda and Pinus virginiana)-hardwood stands on the Cumberland Plateau in Alabama. The natural intermediate-severity disturbance disproportionately removed large Pinus stems, promoted hardwood dominance, and effectively accelerated succession. The resultant stand structure did not resemble one of the widely recognized stages of stand development and was best characterized by the mixed-stage of development. The canopy disturbance did not significantly alter canopy-layer species diversity, but seedling- and sapling-layer diversity was significantly greater in disturbed neighborhoods. Results from this study may be used as guidelines by managers of mixed Pinus-hardwood systems. To maintain a Pinus component in stands that are succeeding to hardwood dominance, canopy disturbance alone is insufficient and must be planned in conjunction with competition reduction measures in the regeneration layer, such as fire or herbicide application. Conversely, if managers wish to promote a hardwood component in pure or near pure Pinus stands, creation of variably sized canopy openings throughout the stand may recruit hardwood reproduction to larger size classes, as the intermediate-severity disturbance documented here accelerated succession toward hardwood dominance.