Abstract
Forest canopy free space, element density (clutter), and the distribution and relative abundance of coarse canopy elements were measured at six sites: two boreal (south-central Alaska), two temperate (Washington State and Victoria, Australia), and two tropical (Pacific Costa Rica and Malaysian Borneo). Great clutter, high relative abundance of dead elements, and shallow understory characterized boreal canopies. Presence of dead elements and deep understory characterized temperate sites, sharing clutter profiles that differed in magnitude but not shape from each other. Few dead elements, little obvious zonation, and constant, mid-canopy clutter characterized tropical sites. The neotropical site showed a clear overstory, while the paleotropical site did not. The results motivate three hypotheses: (1) forest canopies of similar latitude and climate have similar structure; (2) canopy zones form cohesive, repeatable structures as “understory” (near-ground nondominant vegetation bounding small free-spaces), “overstory” (high, dominant foliage bounding small free-spaces), “bole zone” (mid-canopy boles bounding the largest free spaces), and “dead zone” (unique to conifer sites and typified by dead elements and upwardly narrowing free-spaces above bole zone); and (3) Australian and paleotropical canopies, regions rich in vertebrate gliders, are an order of magnitude more open (less cluttered) than neotropical and North American canopies, regions poor in gliders. FOR. SCI. 50(3):312–325.