When humans cut a forest, they hardly ever cut it all. We tend to cut in patches leaving isolated remnant stands of varying sizes and shapes scattered across the landscape. An important consequence is the creation of great lengths of stand edge relative to the area of surviving forest. Because patchy cutting is practised globally, forest edges have proliferated in every forest ecosystem.

We define a forest edge as an abrupt transition between two relatively homogeneous ecosystems, at least one of which is a forest. Natural forests often include recognizable edges, usually corresponding to physical gradients in topography, hydrology, or substrate (Whittaker 1956, Roman et al. 1985) or marking the borders of large disturbances such as fires or hurricanes (Bormann and Likens 1979). In clearcuts, humans have created a novel form of edge that differs from natural edge in several important ways. First, human-generated edges are abrupt. Harvesting operations tend to stop at arbitrary linear boundaries leaving a vertical wall of surviving trees. In northern Europe, where woodlands have been intensively managed for centuries, edges may be defined by ditches and banks with managed hedges. By contrast, natural disturbances generate ragged edges, with individual trees varying in crown structure and degree of damage (Trimble and Tryon 1966, Runkle and Yetter 1987). Human-generated edges dier from natural edges in the size and character of the adjacent clearings. As a consequence of larger size, for example, human-generated clearings tend to experience more severe wind turbulence than natural canopy gaps, a condition that may damage trees in adjacent forests (Geiger 1965, Liu et al. 1996).