NONEQUILIBRIUM DYNAMICS BETWEEN CATASTROPHIC DISTURBANCES AND OLD-GROWTH FORESTS IN PONDEROSA PINE LANDSCAPES OF THE BLACK-HILLS

An emerging goal of ecosystem management is to maintain ecosystems wit hin their range of natural variability, which requires attention to pr e-EuroAmerican landscape-scale processes and corresponding landscape s tructures (e.g., old-growth forest distribution). The prevailing ''equ ilibrium'' view of ponderosa pine forest landscapes, for example, hold s that frequent, low-intensity surface fires maintained open, park-lik e forests of large, old trees. Yet a contrasting ''nonequilibrium'' vi ew suggests that some forest ecosystems are subject to unpredictable c atastrophic disturbances that dramatically alter these ecosystems. To assess these views' relevance, we examined early historical accounts a nd records of natural disturbances in the ponderosa pine forests of th e Black Hills in South Dakota and Wyoming (U.S.A.). There is evidence of frequent, low-intensity surface fires and large, catastrophic distu rbances 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 po nderosae Hopk.) occurred. The location of these severe disturbances co incides geographically with early explorers' reports of extensive trac ts of relatively dense closed-canopy forests, including some very larg e patches (5000+ha) of dense old growth. This contrasts with sparse, o pen-canopy forests thought to be maintained by periodic, low-intensity surface fires. We suggest that the cooler, moister, central and north ern Black Hills and topographically protected areas may have been domi nated by infrequent, catastrophic disturbances that maintained large p atches of dense forests, including large, contiguous patches of old gr owth, in a relative state of nonequilibrium. The warmer and drier sout hern Black Hills, south-facing slopes, and exposed areas may have been dominated by frequent, low-intensity surface fires and other small di sturbances that maintained open-canopy forests in a relative state of equilibrium. Proposed Black Hills National Forest management plans tha t exclusively endorse the equilibrium view are misdirected and will mo ve the forest ecosystem farther outside its range of natural variabili ty.