Intermediate disturbance in a late-successional hemlock-northern hardwood forest

1. An old-growth mesic forest in northern Michigan, USA, experienced an unusually intense storm in July 2002. Permanent inventory plots and a 2.9-ha mapped stand allow comparison of effects of this rare disturbance with patterns of 'baseline' mortality. 2. Tree mortality attributable to immediate effects of the storm was of similar magnitude to mortality over the previous decade, amounting to about 9% of basal area and 7% of stem density (mean of 88 inventory plots). 3. Storm mortality differed from baseline mortality in patterns related to species and size. Tsuga canadensis suffered little mortality of either type. Betula alleghaniensis had high baseline mortality in all size classes, but very low storm mortality. Acer spp. and Fagus grandifolia showed similar overall levels of baseline and storm mortality, but storm mortality was higher for larger stems. 4. Spatial patterns of mortality differed between baseline and storm-caused disturbance, with storm mortality patterns related to composition and to substrate variation over the stand. Baseline canopy mortality was hyperdispersed at local scales, but mortality due to the storm was strongly clustered at distances up to 30 m. 5. Properties of rare, intermediate disturbances cannot be predicted by simply scaling up patterns due to frequent, less intense, events. They may have distinctive influences on community dynamics, countering trends towards dominance by shade-tolerant species, generating demographic and spatial structure in the canopy, and distinctively affecting understorey environment. 6. These results suggest that climate change may induce changes in forest ecosystems by changing disturbance patterns even when species are not close to limits of physiological tolerance. Forest management for natural regeneration may need to take the effects of rare, intermediate disturbance into account.