Societal challenges in understanding and responding to regime shifts in forest landscapes.

M any natural landscapes have undergone dramatic permanent alterations as a result of human activities, including conversion to cultural landscapes; such changes are readily observed and understood. However, extensive ecological change can also occur in regional landscapes that are maintained in a seminatural state, changes that go largely unrecognized because the regional landscape retains an approximation of its dominant physiognomic cover, such as forest or grassland. In PNAS, Lindenmayer et al. (1) describe the concept of regime shifts in forest landscapes that represent landscape traps in that “entire landscapes are shifted into a state in which major functional and ecological attributes are compromised [and] lead to feedback processes that either maintain an ecosystem in a compromised state or push it into a further regime shift in which an entirely new type of vegetation cover develops.” Such state changes can result in dramatic reductions in functionality (e.g., carbon sequestration, water yields) and biodiversity, as with their primary example of mountain ash forests (Eucalyptus regnans) in southeastern Australia. The degradation of seminatural landscapes at regional scales, whereby essential functional capabilities and biotic elements are permanently lost as a result of altered disturbance regimes, is a widespread phenomenon. An outstanding example of regional scale simplification of landscapes is the permanent replacement of diverse native steppe in North America’s Great Basin with grasslands dominated by annuals, such as cheatgrass (Bromus tectorum), and an associated change in fire regime (2). A comparable forest example is the massive shift from open pine-dominated forests to dense fuel-loaded stands highly vulnerable to unnaturally intense and large wildfires in western North America as a result of fire suppression, logging, and grazing (3). Many more examples of “trapped” landscapes can be expected to occur as a result of climate change and human activities, as suggested for the Greater Yellowstone region (4).