Influence of gap-scale disturbance on developmental and successional pathways in Quercus-Pinus stands
Quercus-Pinus forests of the eastern USA cover millions of hectares and span a variety of ecoregions. Understanding the influence of natural disturbance on developmental and successional pathways is important for managers that wish to sustain Pinus spp. in these mixtures. Quantifying developmental and successional patterns in this forest type can help assess the need to actively manage natural processes and inform silvicultural prescriptions to achieve management goals. Little research has been conducted on natural, gap-scale disturbance processes in Quercus stands with strong components of Pinus taeda, Pinus virginiana, and Pinus echinata. We examined 60 canopy gaps in a Quercus-Pinus forest on the southern Cumberland Plateau in Alabama to document gap formation, closure, and other characteristics and to analyze the influence of localized disturbance on development and succession. The majority of gapmaker trees (56%) were Pinus individuals and 44% were hardwoods. Most gaps (58%) closed by height growth of subcanopy trees. The majority of these gap filler taxa were hardwoods: Quercus (39%), Carya (14%), Nyssa sylvatica (12%), and other hardwoods (15%), with Pinus representing 14%. The number of Pinus gapmakers and the number of gaps projected to fill by subcanopy recruitment of hardwoods indicated the forest was in the latter stages of a composition shift from Pinus to a much stronger Quercus component. Significant positive relationships existed between gap size and sapling diversity (r² = 0.15, P = 0.002), tree diversity (r² = 0.21, P = 0.0002), and total stem diversity (r² = 0.29, P < 0.0001) indicating a positive relationship may exist between gap size and diversity on xeric ridge tops where shade-tolerant species are less competitive. We speculated the ridge top positions contributed to the relatively high gap formation rates noted in this study. Pinus composition was found to be patchy, indicating a gap-based approach may be used to manage for Pinus recruitment in hardwood dominated systems.