Gap Characteristics and Vegetation Response in Coniferous Forests of the Pacific Northwest

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The role of gaps of various sizes and origins in the regeneration of Nothofagus dombeyi varies markedly along the transandean gradient. This variation chiefly results from site-to-site differences in associated tree species and understory bamboos, as well as from differences in the abiotic environment. Other wide-ranging tree species, such as Pseudotsuga menziesii in the Pacific Northwest (Spies and Franklin 1989), are also known to respond differently to gaps in different forest types. In assessing the response of a given species to gaps, ecologists should consider that the response may vary geographically. Gap response may be predicated on the particular physical and biotic characteristics of the study site and might well be different in other parts of the species range. In South American Nothofagus forests, gap-creating disturbances of various types clearly have a major influence on forest structure and composition, and knowledge of disturbance regimes is essential to explaining vegetation patterns. Despite the striking influence of coarse-scale allogenic disturbance on the composition of these forests, the influence of fine-scale autogenic processes must also be considered. For example, microsite modification by both understory bamboos (Veblen 1982) and canopy trees (Veblen et al. 1979) may influence the response patterns of Nothofagus spp. to gaps. The recent emphasis in forest ecology on the pervasive influences of allogenic disturbance should not divert attention from autogenic processes, since most vegetation change reflects both types of processes.