An emerging goal of ecosystem management is to maintain ecosystems within their range of natural variability, which requires attention to pre-EuroAmerican landscape-scale processes and corresponding landscape structures (e.g., old-growth forest distribution). The prevailing "equilibrium" view of ponderosa pine forest landscapes, for example, holds that frequent, low-intensity surface fires maintained open, park-like forests of large, old trees. Yet a contrasting "nonequilibrium" view suggests that some forest ecosystems are subject to unpredictable catastrophic disturbances that dramatically alter these ecosystems. To assess these views' relevance, we examined early historical accounts and records of natural disturbances in the ponderosa pine forests of the Black Hills in South Dakota and Wyoming (U.S.A.). There is evidence of frequent, lowintensity surface fires and large, catastrophic disturbances before EuroAmerican influence. Several large, stand-replacing fires occurred between 1730 and 1852, and, shortly after EuroAmerican settlement, a major outbreak of mountain pine beetles (Dendroctonus ponderosae Hopk.) occurred. The location of these severe disturbances coincides geographically with early explorers' reports of extensive tracts of relatively dense closed-canopy forests, including some very large patches (5000 ϩ ha) of dense old growth. This contrasts with sparse, open-canopy forests thought to be maintained by periodic, low-intensity surface fires. We suggest that the cooler, moister, central and northern Black Hills and topographically protected areas may have been dominated by infrequent, catastrophic disturbances that maintained large patches of dense forests, including large, contiguous patches of old growth, in a relative state of nonequilibrium. The warmer and drier southern Black Hills, south-facing slopes, and exposed areas may have been dominated by frequent, low-intensity surface fires and other small disturbances that maintained open-canopy forests in a relative state of equilibrium. Proposed Black Hills National Forest management plans that exclusively endorse the equilibrium view are misdirected and will move the forest ecosystem farther outside its range of natural variability.
