Can small-scale altitudinal gradients predict spatial and temporal patterns in tropical forests?

Abstract

In tropical montane forests, compositional and structural changes are commonly driven by broad-scale altitudinal variation. Here, given the lack of knowledge on small-scale vegetation changes and temporal dynamics, we address the effects of small-scale variations in soil and altitude on tree community structure, temporal dynamics and phylogenetic diversity in a semi-deciduous tropical forest of the Atlantic Forest Domain, southeastern Brazil. In 2010 and 2015 we sampled thirty plots of 400 m2, set up along an altitudinal gradient between 1000 and 1500 m a.s.l.. In each plot, we collected soil samples for chemical and textural analyses. We fitted linear models to test the effects of altitude and soil on community dynamics and phylogenetic parameters. Altitude and soil explained the spatial variation in number of individuals and phylogenetic diversity metrics. From lower to higher altitudes, we found decreasing fertility, increasing tree density and decreasing phylogenetic diversity. Altitude significantly influenced the increases in total biomass (from 240.9 to 255.4 t ha−1) and individual biomass (from 0.15 to 0.17 t) recorded in the interval. And while community temporal dynamics had rates of 1.96% for mortality, 1.02% for recruitment, 1.61% for biomass loss and 2.81% for biomass gain, none of them were explained by altitude or soil. Temporal species substitution averaged 0.1 in the interval. Altogether, these results suggest that the small-scale variations in altitude and soil likely determine the conditions and resources that drive community assembly and structure, which are expressed by spatial variations along the altitudinal gradient. At the same time, temporal patterns were not influenced by altitude-related environmental variation, resulting in a similar dynamic behaviour across the gradient, suggesting that broad-scale factors may play a more important role than local ones.