We review and compare well-studied examples of five large, infrequent disturbances (LIDs)-fire, hurricanes, tornadoes, volcanic eruptions, and floods-in terms of the physical processes involved, the damage patterns they create in forested landscapes, and the potential impacts of those patterns on subsequent forest development. Our examples include the 1988 Yellowstone fires, the 1938 New England hurricane, the 1985 Tionesta tornado, the 1980 eruption of Mount St. Helens, and the 1993 Mississippi floods. The resulting landscape patterns are strongly controlled by interactions between the specific disturbance, the abiotic environment (especially topography), and the composition and structure of the vegetation at the time of the disturbance. The very different natures of these interactions yield distinctive temporal and spatial patterns and demand that ecologists increase their knowledge of the physical characteristics of disturbance processes. Floods and fires can occur over a long period, whereas volcanic eruptions and wind-driven events often last for no more than a few hours or days. Tornadoes and floods produce linear patterns with sharp edges, but fires, volcanic eruptions, and hurricanes can affect broader areas, often with gradual transitions of disturbance intensity. In all cases, the evidence suggests that LIDs produce enduring legacies of physical and biological structure that influence ecosystem processes for decades or centuries.

Data on tree species composition and population structure are used as a source of ecological information on the species—rich forest of Barro Colorado Island, Panama Canal Zone (BCI). Thirteen stands, representing both the young and old forest on the Island, were sampled using 10 m ° 20 m quadrats for all individuals °2.5 cm dbh. Over 300 species were encountered and most were identified. The data are evaluated for interpreting late secondary succession (>60 yr), detecting soil—vegetation patterns, and for yielding autecological information. Successional status was estimated by placing species with similar population structure patterns into groups, and then calculating the abundance of species groups that seemed indicative of successional status. Five population structure patterns were recognized. The results suggest that the older BCI forest is not climax after at least 130 yr of succession. Both principal components analysis and a Bray—Curtis type ordination were used to determine whether ecological patterns on BCI are reflected by species composition. Forest age was best reflected by an ordination of the larger tree species. Species found only in the young forest or only in the older forest are identified. Stands with a distinctive gley soil were not segregated on the ordination. Species diversity increases most rapidly during the first 15 yr of succession, but continues to increase slowly after 65 yr. The Shannon—Wiener diversity measure averaged 4.8 (log base 2). Some data suggest that wind—caused canopy gaps are important for the persistence of several species in the older forest, e.g., Cecropia sp.

Differences in fire frequency and the rate of secondary succession following fire have had a major effect on the present composition of forest vegetation in a 4500-ha_ undisturbed "':ate_rshed in the subalpine zone of the Medicine Bow Mountains, southeastern_ Wyommg, USA. Penodtc fi_re coupled with slow secondary succession has perpetuated lodg~pole pme fore~t on the upland, whtle mature Engelmann spruce-subalpine fir forests have developed m she~tered ravmes and valley botto~s where fire is less frequent and succession following fire is more_ raptd_ ~nd/or more ~trect. A graphtc model is presented showing the relationship between topographic positiOn, fire-free mterval, and the occurrence of mature forests dominated by spruce and fir.

No abstract

No abstract