Long-term recovery dynamics following hurricane-related wind disturbance in a southern Appalachian forest
Wind disturbance affects thousands of km2 annually in eastern temperate forests, yet few studies address long-term recovery. Here, I assess changes in forest structure and composition before (pre-Opal), immediately after (Y0), and 21 years after (Y21) Hurricane Opal in 0.166-1.08 ha gaps created by microbursts, and undisturbed controls. In gaps, an average of 24.0% of trees (41.1% BA) were windthrown and 1.0% of trees (1.1% BA) died standing during Opal; scarlet and black oak were disproportionately windthrown. Subsequent windthrow or standing tree mortality rates did not differ between treatments. By Y21 an average of 32.2% (49.5% BA) of trees were windthrown in gaps and 3.2% (2.4% BA) in controls; 7.2% (7.6% BA) died standing in gaps and 12.0% (11.7% BA) in controls. Pit depth and windthrown rootmass area decreased by 63% by Y21. Total snag density differed over time (23.8-38.8/ha in controls; 6.7-19.7/ha in gaps); “new” snag (e.g., died after Opal) density did not differ between treatments. Disproportionately more scarlet oak, shortleaf pine, and hickory but fewer tulip-poplar, red maple and sourwood died standing. By Y21 live tree density and (marginally) basal area recovered to pre-Opal levels in gaps but remained lower than controls. In gaps, hurricane-related mortality of trees 38.1-50.7 cm dbh was heaviest, and density of this size-class remained lower by Y21; density of trees 50.8-63.4 cm was also lower in gaps in Y21. Trees grew faster in gaps than controls but stand-level gains in average dbh were smaller in gaps due to a greater increase in small trees (ingrowth) by Y21. Initial hurricane-related mortality changed relative importance (average of relative abundance and relative BA) of species in gaps; differences in mortality and growth rates among species contributed to subsequent shifts by Y21, especially in gaps. Post-Opal gains in shade-intolerant yellow-poplar importance were small but significant by Y21; shade-tolerant generalist species such as red maple and sourwood also increased, whereas scarlet and black oak decreased. Cumulatively, small changes by multiple species resulted in decreased (-19.9%) importance of the oak-hickory group and a corresponding increase in “other”; this trend was also evident in controls at a much smaller scale (-5.6%). Results indicate that a wind-related “pulse” of heavy mortality unevenly distributed among species - in this case scarlet and black oak - can alter stand structure and relative importance of species for decades, with accelerated loss of the oak-hickory group and replacement largely by shade-tolerant generalist species.