Print PageShade tolerance is an ontogenetically variable life history trait evolved as a response of long-lived trees to prevailing disturbance regimes (canopy turnover rate). Understanding shade tolerance patterns is critical to the success of forestry methods that seek to enhance stand regeneration after logging. Post-logging open sites and naturally generated gaps are rapidly filled by the development of high tree densities that produce deeply shaded understories. We test the general hypothesis that forest succession is driven by the replacement of shade intolerant (SI) pioneers by increasingly shade-tolerant (ST) species. We review data on successional patterns and light environments for evergreen trees in South American temperate forests and use Leaf Mass Area (LMA) as an indirect measure of inter-specific differences in shade tolerance. We also test the relationship between tolerance to low light and disturbance regimes by comparing the ranges of shade tolerance for Chilean trees with those from other evergreen temperate forests in Australia, New Zealand, and western North America.
Results/Conclusions
Field-based classifications of Chilean evergreen trees identify ST and SI guilds, plus a small group of species with intermediate tolerances. However, when light environments are considered, physiological and anatomical data indicate a continuous, non-linear gradient of shade tolerances between the ST/SI extremes. Few species can be classified as true SI-pioneers that regenerate in full sunlight. Data on photosynthetic capacity and LMA for Chilean trees suggest that the number of species belonging to the intermediate tolerance guild has been grossly underestimated. This broad group of species with intermediate tolerance is attributed to a disturbance regime dominated by individual tree falls, especially in lowland and coastal forests. The dominance of species with intermediate shade tolerance questions simple management protocols that assume that trees can regenerate within large openings or under sparse shelterwood cover. SI-pioneers do, however, dominate the canopies of Andean forests historically subjected to catastrophic disturbances. Because of the extended longevity of SI species, these are never completely replaced by ST in most stands. This evidence contradicts the general succession model based on shade tolerances. Shade-tolerance spectrum of Chilean trees approaches that of New Zealand forests, rather than that of Australian and western North American temperate forests where frequent, large-scale fire disturbance prevails, and SI, fast-growing trees dominate over a limited number of ST species.
