O ver the past two decades, understanding of the dynamics of forests in tropical and temperate regions has developed primarily through the study of canopy gaps formed when mature trees die. The functional significance of gap-phase dynamics (Watt 1947) in mesic (moist) forests rests on two widely documented observations. First, the surrounding matrix of closed forest canopy is largely impenetrable to juveniles of most canopy-tree species, either because the saplings cannot tolerate the shade cast by canopy trees, or because young shoots are abraded as they grow into the tree crowns above them (Kelty 1986). As a consequence, most of the species that dominate old-growth forests appear to require gaps in the canopy to reach maturity (e.g., Canham 1988). Second, many species, both canopy and understory, possess a wide diversity of specialized traits that allow them to colonize the resource-rich gaps and then compete for these resources (e.g., Canham and Marks 1985, Collins et al. 1985). Researchers have documented remarkable similarities in the processes of canopy-gap formation and closure across a wide range of forest ecosystems (see reviews in Brokaw