Comparação entre métodos de amostragem de luz e o efeito dos fatores bióticos e abióticos sobre sua disponibilidade em uma floresta tropical na Amazônia Central

Abstract

Several studies indicate that light is the main feature linked to the number of species, growth, regeneration and reproduction of trees in the understory of tropical old-growth forests. However, empirical studies show contrasting results, both in relation to the distribution of light in the landscape and its effects on plant assemblages, which hinders the formation of a robust theory on this subject. The differences between the studies are probably related to natural factors, such as the complex architecture of tropical forest and the ontogenetic variation of species, and technical factors, such as the inefficiency of indirect and subjective measurements of light and the lack of control for environmental gradients. This study has two main objectives. (1) Compare methods of light measurement and (2) test for some possible environmental gradients related to variations of light. For the first goal, we compared direct and indirect methods for measure light, sample sizes and sampling efforts in direct measurements. For the second goal, we tested whether there is any pattern of light distribution related to topography, number of trees occupying the understory and woody biomass. We collected the light data on 23 permanent plots of 250 meters (in length) and 1 km apart. No significant relationships were found between the photographs and direct measurements and sample sizes, but there is a strong correlation between the measurements obtained with low and with high sampling effort. Light was not related to topography, nor to the number of trees in the understory. The amount of light reaching the understory of the forest is not associated with topography, which was measured as a continuous variable. Our approach was different fron what was suggested in previous studies which used topographic categories and a simplified categorization of light in gaps and non-gaps. Unlike the topography, variation in light was related to biomass. Variation in light decrease from areas with less biomass to areas with higher biomass.