Article I. Drivers of tree community composition and turnover of montane forests of the Central Highlands of Chiapas, México : evidence from taxonomic and functional variation
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There is an active debate in ecology about the influence of niche and dispersal factors on species distributions and assemblages. Our overall aim was to define their relative importances in the configuration of tree species assemblages of the montane forest ecosystems of the Central Highlands, state of Chiapas, Mexico. Specifically, we sought to 1) describe and quantify floristic and functional variation amongst different forest remnants, 2) assess the degree of spatial autocorrelation of floristic and functional composition and climatic variables at a range of spatial scales, and 3) compare the relative influences of spatial variables, climatic variables, altitude above sea level, aspect and human disturbance on floristic and functional variation. We hypothesised that environmental factors and human disturbance would play the most important roles in the determination of forest variation in this very heterogeneous landscape with its long history of human disturbance. An intensive sampling of adult trees (> 5 cm DBH) was carried out in 48 plots (0.1 ha each) and an accurate determination of nine functional traits (leaf area, specific leaf area, leaf tensile strength, leaf dry matter content, and leaf nitrogen, phosphorous and carbon contents; wood density and maximum height) was done for each of 26 dominant species. Each of the sample plots was floristically, environmentally and spatially characterised, and functional characteristics of each one were determined using basal-area weighted mean trait values (CWMs). Ordination, cross-species correlations of trait values and spatial statistics were used to analyse the resulting data. The landscape was found to contain a mosaic of floristically and functionally differentiated forest stands in which three main functional groups of tree species were identified: Quercus species and Pinus species, which both reach the forest canopy or sub-canopy, and a third group of a wide taxonomic range of species that generally develop in the understorey. Variation partitioning showed that the floristic variation was more related to spatial variables derived from Principal Components of Neighbour Matrices (R2 = 0.11, P < 0.005) than to climate (R2 = 0.06, P < 0.005), whereas human disturbance was the factor most strongly linked to functional variation (R2 = 0.21, P < 0.005) although spatial variables were also important (R2 = 0.07, P < 0.005). The strong relationship of spatial variables with floristic variation suggests the need to consider the potential effect of dispersal limitation and stochastic factors, as well as climate variation and human disturbance, as drivers of the forest composition of the Central Highlands. The results also emphasise the strong effect of human disturbance on the functional variation amongst forest stands. Thus, floristic composition and functional characteristics of these forests appear to respond to different drivers. Floristic change may occur due to spatial factors without corresponding functional change, so that a major characteristic of these forests is broad patterns of functional characteristics imposed by human disturbance, to an extent independently of floristic composition.