Gap formation and dieback in Fuego-Patagonian Nothofagus forests

This paper reviews gap formation and dieback processes in Nothofagus forests of Patagonia and Tierra del Fuego, Argentina and Chile between 40°S and 55°S. Stand history, site factors (e.g., soil depth), and the wind disturbance regime strongly influence the spatial and temporal formation of gaps. We demonstrate that for Nothofagus species, gaps may occur on fine spatial scales, where each gap involves only one or a few treefalls in a matrix of closed forest (e.g., old-growth N. pumilio in deeper soils), or coarser scales, where blowdowns may affect many hectares (e.g., N. pumilio on very shallow soils). Temporally, gap formation in Nothofagus may be synchronized throughout the stand, as in the canopy dieback of even-aged stands, or formation may be more continuous or episodic in all-aged stands subjected to periodic windstorms. Preliminary evidence for various forms of dieback in South American Nothofagus are given, including canopy dieback and wave mortality in N. betuloides, partial wave mortality in N. pumilio and N. betuloides, and persistent partial crown dieback in N. pumilio and N. antarctica. Partial wave mortality is a localized, progressive expansion of gaps which superficially resembles the true, cyclical wave pattern in Abies of North America, but partial waves appear to be formed by episodic windstorms rather than stress-related senescence and mortality, which is characteristic of cyclical waves. Partial crown dieback is believed to represent incomplete recovery from defoliating and woodboring insects, although climatic factors and other biotic agents (e.g., the mistletoe-like Misodendrum) may also be involved. The cohort senescence theory of dieback may apply in other cases of presumed dieback, but wind, acting both as stress and disturbance, often blurs the distinction between autogenie and allogenic processes. Two examples are given which demonstrate how the regeneration response of Nothofagus spp. in fine-scale gaps varies over gradients of precipitation, latitude, and elevation, as the vegetation associated with Nothofagus forests changes. Preliminary results suggest that coarse-scale gaps associated with dieback usually favor self-replacement by Nothofagus.