Damage and recovery of tree species after two different tornadoes in the same old growth forest: a comparison of infrequent wind disturbances

Catastrophic wind disturbances in natural forests remain poorly studied, and the literature allows few comparisons of damage and recovery in the same forests disturbed by distinct wind events. Thus, it is still uncertain whether published findings are idiosyncratic or representative. Here I describe damage and recovery patterns in a large, primary hemlock–hardwoods forest in Pennsylvania, USA, which was struck by two separate tornadoes in 1985 and 1994, roughly 3 km apart. The damage from the 1985 event was nearly complete, while the 1994 event left nearly two-thirds of the trees standing. The relationship of risk of damage to size of tree was very different between events, rising very steeply and achieving 100% around 40 cm DBH in the intense 1985 event, and more linearly in the 1994 event. Interspecific differences in amount of damage within a given size class were significant in the less intense 1994 event, but were not detectable in the 1985 event. In both events, uprooted trees created root pits proportional to the size of the tree, and the pit size–tree size relationship did not differ between events. Although amount of damage differed between events, both sites showed a pattern of greater uprooting in intermediate size classes, considering only fallen trees. Sprouting by damaged trees was not abundant after either event, with 25% of snapped trees sprouting after the 1985 event, and 15.5% of damaged or down trees sprouting after the 1994 event. Survival of sprouted trees appeared similar between events: 68% were alive 4 years after the 1985 event, and 78% were alive 2 years after the 1994 event. Regeneration in the more severe 1985 blowdown was dominated by mid-successional Betula species, while recruitment of new seedlings after the 1994 event was more modest. Future forest composition will shift dramatically after the very severe 1985 blowdown, while the forest area affected by the 1994 event will be a mixture of surviving canopy trees, sprouts and seedlings that survived in the understory, and some newly-established individuals, which will slightly increase the representation of Betula in the canopy.