Stand-level structural characteristics dictate hurricane resistance and resilience more than silvicultural regime in longleaf pine woodlands

Abstract

Hurricanes frequently impact coastal forests throughout the southeastern United States and are expected to increase in frequency and intensity in the future. Given this projection, interest has grown in adopting forest management practices that can reduce the risk of hurricane damage by increasing stand resistance and resilience. Empirical evidence suggests that uneven-aged stands are more resistant and resilient to hurricanes than even-aged stands, due to their heterogeneous stand structure and multiple age classes. Nevertheless, opportunities to compare the impact of hurricanes on stands managed under different silvicultural regimes at the same location are uncommon. Moreover, no research has detailed how different stocking regulation methods influence hurricane risk in uneven-aged stands. An opportunity to conduct a natural experiment presented itself in 2004 when Hurricane Ivan passed 24 km west of the Escambia Experimental Forest in southern Alabama, USA. At the time of the storm, the experimental forest contained long-term experiments examining the performance of longleaf pine (Pinus palustris) managed under different silvicultural systems. Here, we report on the resistance and resilience of three even-aged stands compared to six uneven-aged stands that were managed under BDq (two stands), Volume Control Guiding Diameter Limit (VC-GDL) (two stands), and Diameter Limit Cutting (DLC) (two stands) stocking control methods.Average stand resistance (proportion of basal area remaining in 2005) was not statistically different among even-aged (0.866)and uneven-aged stands (all variants pooled) (0.858) p > 0.05). Among uneven-aged variants, resistance was highest in the VC-GDL stands (0.876) and lowest in the DLC stands (0.840). Across all stands, resistance was positively related to stand density (p = 0.03) and negatively associated with increases in quadratic mean diameter (p = 0.03) and size class diversity (p = 0.01). Post-hurricane (2005) departure indices found left shifts (shift in density towards smaller size classes) in the diameter distribution of 33 % of even-aged stands and 83 % of pooled uneven-aged stands. Among uneven-aged variants, significant left shifts in diameter distribution were greatest in stands managed with VC-GDL and lowest with BDq. Average post-hurricane resilience (post hurricane basal area recovery) was statistically similar in the pooled uneven-aged stands (0.42) and even-aged stands (0.37). Yet, stands with greater tree size class diversity demonstrated increased resilience. Regardless of management regime, stands carrying a high density of smaller diameter trees are more resistant to hurricane damage than stands managed at a lower density with a higher proportion of larger diameter trees. Moreover, while not statistically significant, our findings suggest that uneven-aged stands possess a greater potential for rapid hurricane recovery than even-aged stands. Therefore, land managers seeking to foster hurricane resistance and resilience should consider modifying stand prescriptions to carry a higher proportion of stocking in smaller size classes.