Montagem funcional de comunidades vegetais sob a perspectiva da filtragem abiótica

Abstract

The assembly of communities is influenced by historical, biotic and abiotic processes that act as filters that restrict local communities in time and space. Based on this, the hypothesis of this work is that the variation in the water availability along the altitudinal gradient acts as a factor of the plant communities. The following predictions were then made: 1) in the lower altitudes, the plants will display leaves with smaller specific area, higher concentrations of nitrogen and phosphorus, denser wood, lower amount of water saturation of the stem and lower heights than in the higher altitude; 2) there will be fewer taxonomic diversity in the area of lower altitude, the reverse occurring in the area of higher altitude; 3.1) there will be greater richness and functional dispersion in the higher altitude area, the inverse occurring in the lower altitude area and 3.2) there will be functional convergence in the areas of lower altitude and divergence in the higher altitude. In order to test these predictions we estimated climatic variables, determined soil chemical and physical properties and measured seven functional traits (leaf, trunk and whole plant) in all the species that occurred in the four areas, along an altitudinal gradient, in a tropical semiarid, Pernambuco, Brazil. A higher mean annual water deficit was observed at the lower level (509 m), in most of the months of the year, and was null or almost null at the two upper levels (888 and 1004 m). The areas of 1004 m and 632 m showed greater soil nutrient availability. The specific leaf area (SLA), nitrogen concentration (LNC) and phosphorus (LPC), and maximum height (MH) changed along the altitudinal gradient, with the highest SLA values found at 888 m, LNC at 509 m, LPC at 632 m, and MH at 1004, the other functional characteristics did not change along the gradient. Most of the characteristics in the different areas did not show a phylogenetic signal, indicating that the degree of kinship did not influence the values of the functional characteristics. Copper (Cu), total base saturation (V%), cation exchange capacity (CEC) and hydrogen and aluminium (H + Al) showed to influence the values of the characteristics in the different plots evaluated in the gradient. The results demonstrate that the characteristics are influenced in a differentiated way by the change of processes in the assembly of the communities. In relation to the taxonomic diversity it is observed an increase of the richness, diversity and equability, following the elevation in the gradient. Already the functional diversity, showed that the functional richness underwent modifications along the gradient, being affected by the abiotic variables, and showing correlation with the taxonomic richness, indicating that this functional component was affected by the taxonomic diversity. The functional dispersion did not change and did not show any relation with the abiotic variables, indicating functional similarity between the areas. There was a convergence of characteristics in the area of 632 m and divergence in 1004 m, indicating that abiotic processes are more important in the assembly of communities in areas with less water availability and that biotic processes become more important when abiotic factors become less limiting . These results indicate that the processes that act to promote the coexistence of species may not be the same as those that cause functional differentiation.