Contrasting windstorm consequences in two forests, Itasca State Park, Minnesota

Thunderstorm winds (°25—35 m/s) often damage scattered trees in northern Minnesota forest. Following one moderate windstorm on 3 July 1983, I surveyed damaged trees within two Itasca Park study areas (1) to identify mortality patterns and their correlates, (2) to evaluate windstorm consequences for shade—intolerant species, and (3) to investigate formation of light—gaps. Risk of direct wind damage was predicted best by tree size (in both stands) and either tree species (in a Pinus/Acer stand) or species wood strength (in a Pinus/Abies stand). However, the overall risk or mortality, combining direct with damage and damage from falling trees, was more complex, Tree mortality in the Pinus/Acer study area was related to tree size, species, species wood strength, and incidence of species—specific fungal pathogens. Mortality in the Pinus/Abies study area, where many understory trees were killed, was not predictable on the basis of tree size, but mortality risk was affected by tree species and species wood strength (data on fungal pathogens were unavailable for this stand). Within most populations, the size distribution off windstruck trees differed from the population size distribution. However, mortality risk, including that from falling trees, increased with tree size only for Populus tremuloides. In one study area, smaller trees were most vulnerable within the Picea and Pinus strobus populations. The difficulty of generalizing about windstorm consequences is illustrated by large intraspecific variation in amount of damage between different populations of Betula papyrifera. Pinus resinosa, and Pinus strobus. For shade—intolerant species expected to benefit from disturbance, windstorm consequences differed in the two stands, despite shared postfire origins and several tree species in common. In the Pinus/Acer forest, heavy mortality to shade—intolerant Populus tremuloides, Pinus resinosa, and Pinus strobus combined with a well—developed, windfirm understory and infrequent light—gap formation to benefit existing shade—tolerant, windfirm Acer saccharum and tall shrubs. In the nearby Pinus/Abies forest, the windstorm produced more regeneration opportunities for shade—intolerant species (Betula papyrifera, Populus tremuloides, Fraxinus pennsylvanica), because the shade—tolerant understory species of this stand (Abies balsamea, Picea glauca) were more heavily damaged, and because more light—gaps formed, although such light—gaps were small relative to published gap size thresholds for tree establishment. Differences between the two communities in their responses to the same storm event appear to result from differences in forest structure and differences in the windfirmness of shade—tolerant understory taxa