Diurnal physiological behavior of seedlings in the Amazon rainforest: generalist versus specialist species of shade and sun

The regenerative success of generalist and specialist species may be due to differences in their physiology. Measurements of stomatal conductance (gS) provide an efficient way to infer immediate physiological responses of plants to diurnal environment variation. Radiation, air temperature, and relative humidity were measured in the Colombian Amazon rainforest, to identify the extreme environmental conditions that limit the gS of seedlings in three site types: small gap, open forest, and closed forest. We hypothesized that the diurnal physiological performance of generalist species must be plastic in these three environments. Morphological traits, gS, and leaf temperature were evaluated in seedlings of four species: one generalist, common to all sites, and one specialist from each site. The gap site was warmer and more irradiated than the other two sites, which caused several midday physiological depressions, limited seedling survival, and facilitated the specialized strategy. Leaf and air temperatures were strong determinants of overall gS. The generalist species was physiologically plastic and, at some hours of the day, more efficient than the specialists from open forest andclosed forests. This factor interplay could allow the coexistence of both types of plants.