Changes in forest structure, species diversity and spatial pattern following hurricane disturbance in a Piedmont North Carolina forest, USA

Aims Large hurricanes have profound impacts on temperate forests, but owing to their infrequent nature these effects have rarely been examined in detail. In 1996, Hurricane Fran significantly damaged many long-term tree census plots in the Duke Forest on the North Carolina Piedmont, thereby providing an exceptional opportunity to examine preand post-hurricane forest compositional trajectories. Our goal was to examine immediate, short-term (0–4 years) and longer term (;5 year) hurricane-induced structural, spatial and compositional changes in the tree population (stem d.b.h > 1 cm) in the context of our detailed, long-term knowledge of the dynamics of these forests. Methods We surveyed stem damage and tree mortality in 34 long-term permanent plots (ca. 70-year record; 404–1 012 m) and 7 large mapped tree stands (ca. 20-year record; 5 250–65 000 m) representing both transition-phase, even-aged pine stands and uneven-aged upland hardwood forests. We employed three types of damage measures to quantify stand-level damage severity: percentage of stems damaged, percentage of basal area lost and a ‘stand-level damage index’. Second-order spatial analysis (Ripley’s K-function) was used to investigate patterns in tree mortality. Important findings Our study found hurricane effects on the structural attributes of Piedmont forests to be variable and patchy. Changes in tree species composition, however, were modest. Uprooting was the major damage type for the overstory trees [diameter at breast height (d.b.h.) >10 cm] apparently due to the exposure of the crowns to high wind combined with heavy rainfall prior to and during the storm. Saplings, juvenile trees and small trees (1–10 cm d.b.h.) of the understory and midstory were mainly damaged by being pinned or bent by their damaged large neighbors. Hurricane-induced tree mortality varied weakly among species, was positively correlated with pre-hurricane tree size and remained up to 2-fold higher than pre-hurricane background mortality 5 years after the hurricane. Spatial point pattern analysis revealed a patchy distribution of tree mortality during the hurricane sampling interval. Hurricane Fran resulted in a dramatic increase in average gap size from ca. 400 m pre-hurricane to ca 1100 m after the hurricane, whereas maximum gap sizes reached 18–34 times larger than the pre-hurricane levels.