A REVISED CONCEPT OF LANDSCAPE EQUILIBRIUM - DISTURBANCE AND STABILITY ON SCALED LANDSCAPES

Temporal and spatial scales of disturbance and recovery are often conf ounded in discussions of landscape equilibrium. We developed a broad f ramework for the description of landscapes that separates the spatial and temporal scales of disturbance and recovery and predicts the resul tant dynamics of a landscape. Two key parameters representing time and space are used to describe potential disturbance dynamics. The tempor al parameter, T, is the ratio of the disturbance interval (i.e., time between successive disturbance events) to the time required for a dist urbed site to recover to a mature stage. The spatial parameter, S, is the ratio of the size of the disturbance to the size of the landscape. The use of ratios in both parameters permits the comparison of landsc apes across a range of spatial and temporal scales. A simple simulatio n model was developed to explore the implications of various combinati ons of S and T. For any single simulation, disturbances of a fixed siz e are imposed at random locations on a gridded landscape at specified intervals. Disturbed sites recover deterministically through successio n. Where disturbance interval is long relative to recovery time and a small proportion of the landscape is affected, the system is stable an d exhibits low variance over time (e.g., northeastern hardwood forests ). These are traditional ''equilibrium'' systems. Where disturbance in terval is comparable to recovery interval and a large proportion of th e landscape is affected, the system is stable but exhibits large varia nce (e.g., subalpine forests in Yellowstone Park). Where disturbance i nterval becomes much shorter than recovery time and a large proportion of the landscape is affected, the system may become unstable and shif t into a different trajectory (e.g., arid ecosystems with altered fire regimes). This framework permits the prediction of disturbance condit ions that lead to qualitatively different landscape dynamics and demon strates the scale-dependent nature of concepts of landscape equilibriu m.