Efeito do gradiente de estresse no processo de facilitação em dunas arenosas tropicais

Abstract

The presence of adult plants often increases the performance of woody seedlings under stressful environmental conditions, but this facilitation relationship may become neutral or competitive, depending on the intensity of the environmental severity. Early stress gradient hypothesis predicts that facilitation should increase continuously along with environmental severity. However, a recent theoretical refinement of the stress gradient hypothesis proposes that facilitation is more intense under moderate stress, becoming less intense in extremely severe environments. Field experiments across a gradient of environmental severity in the northeast coast of Brazil were carried out to verify how Byrsonima crassifolia acts as a facilitation plant, and to test the effect of stress on the intensity and importance of ecological interactions. First, we compared variations in temperature, humidity, salt and organic matter concentrations underneath B. crassifolia canopy and in the open area to verify whether this plant improves abiotic conditions in the environment that facilitates the development of other species. We also analyzed variations in these physical-chemical parameters and wind speed at different distances from the sea in order to verify the existence of a gradient of environmental severity. Pilosocereus catingicola seedlings were transplanted to the area underneath B. crassifolia canopy and to the open area to confirm the existence and intensity of facilitation along the gradient of environmental severity. The diversity of species located under the canopy of B. crassifolia was determined in order to verify the importance of the species in the structuring of the local plant community. In general, our results showed that B. crassifolia acts as a facilitating plant providing better conditions as associated species, mainly through shading, improving as moisture and temperature rates, and organic matter production. We also verified the existence of the gradient of environmental severity along the coast and that facilitation varies along the gradient, reducing the intensity and importance at the extreme of the gradient severity.