OOS 32-5 - Forest structure and windstorm resistance and resilience

Windstorms frequently affect coastal forests in the Southeast, and their frequency and intensity are expected to increase. Given this projection, there has been an increase in interest in adopting forest management practices that enhance stand resistance and resilience, thus reducing the risk of hurricane damage. Our multiple studies, conducted on Southern forests following Hurricanes Ivan and Michael, examined the relationship between stand structure and windstorm resistance and resilience. Using long-term pre- and post-hurricane stand structural data, we found that tree and stand characteristics like tree size (dbh and height), basal area, stand density, above ground biomass, and size class distribution are significantly related to stand resistance and resilience to hurricanes. In even-aged stands, resistance and resilience varies with the stand age and stage of development, while multiple stand and stocking regulation factors interact to determine resistance and resilience in uneven-aged stands. For example, we found longleaf pine (Pinus palustris) resistance (post-storm proportion of basal area remaining) ranged from 0.840-0.876 depending on stocking regulation method. Resistance was positively related to stand density (p = 0.03) and negatively associated with increases in quadratic mean diameter (p = 0.03), above ground biomass (p =0.01), and size class diversity (p = 0.01). As a result of hurricane damage, we found left shifts in the diameter distribution of about one-third of our even-aged stands and more than four-fifth of our uneven-aged stands after the storm. Average post-hurricane resilience (post hurricane basal area recovery) was higher in stands with greater tree size class diversity. A stand with a high proportion of small diameter trees is more resistant to hurricane damage than one with a lower density and a higher proportion of large diameter trees. In general, our findings suggest that structurally diverse stands, such as uneven-aged stands, possess a greater potential for rapid hurricane recovery than even-aged stands. As a result, management that focuses on creating stands with a high proportion of stocking in small size classes should enhance windstorm resistance and resilience.