Estabilidade de comunidades vegetais diante das mudanças climáticas: o efeito da diversidade funcional e das fontes alternativas de água na resposta das vegetações campestres à seca

Abstract

Unveiling the factors underlying differential stability across biological systems remains one of the most fundamental questions in Ecology. Considering the worldwide increase in drought intensity, duration and frequency, the main goal of this study was to investigate the role of two factors, functional diversity (diversity on species response to disturbances and/or stresses and on their effects on the ecosystem functioning) and use of alternative water sources (fog) on plants response to drought. Since grasslands cover a large proportion of the terrestrial land surface and their productivity is controlled by precipitation, those factors were evaluated, particularly, on grassland stability. Firstly, a meta-analysis of manipulation rainfall experiments performed on 101 grasslands locations distributed worldwide was conducted to evaluate the general stability on biomass production; and to quantify the relative importance of drought features and prevailing conditions in explaining stability variation. On average, grasslands were resilient to drought, they kept biomass unchanged after drought; but significantly differed on their ability to resist, to keep biomass unchanged during drought, and to recover, to compensate damages incurred during drought, leading to a negative relationship between the last two properties. Those differences were not explained by either experimental drought features, nor by prevailing conditions. This suggests that soil and/or intrinsic biotic features, such as the degree of functional diversity, could be more important to determine differential stability across grasslands. A new conceptual framework, integrating eco-physiological strategies and functional originality indices was then proposed, in order to assess the role of functional diversity and to predict community stability under distinct scenarios of species loss. The framework was applied, using the Campos de Altitude, a Brazilian tropical mountain grassland vegetation, as a model system. The general stability of Campos de Altitude would be ensured under most of the evaluated scenarios, due to the presence of a large number of resistant species. However, instability was observed for scenarios where a small group of non-resistant species, which rely on foliar water uptake to deal with drought were lost (i.e. absorption of fog through leaves, FWU). Those species were one of the group most sensitive to drought, and thus conservation priorities should be assigned on them. Finally, the interspecific variation in FWU was investigated, and the ecological implications of this variation under climate change scenario was assessed. It was shown that species foliar traits and ecological strategies could explain this variation, with ruderal species showing greater FWU than stress-tolerant species. In conclusion, although grasslands were resilient to drought in terms of above-ground biomass, their functional diversity migh be unstable. In a future with more droughts and less fog, resistant species could outperform non-resistant ones, leading to a functional homogenisation. Future experiments should subject vegetation to more extreme droughts, in order to test those predictions and to develop a better understanding about the mechanisms underlying differential stability across biological systems