Article II. Factors affecting the distribution and abundance of saplings of tree species in forests of the Central Highlands of Chiapas, México
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Variation of the characteristics of tree species assemblages in forests is defined at multiple scales by spatial and biophysical factors, as well as disturbance regimes, which affect not only the composition and structure of adults but also their regeneration. We sought to determine the factors affecting the distribution and abundance of saplings of tree species of tropical montane forests of the Central Highlands (state of Chiapas, Mexico). First, it was hypothesised that climate factors and human disturbance would play the most important roles in the determination of composition and abundance of saplings at the landscape level. Second, it was hypothesised that canopy openness would be a major factor at the local scale. Finally, we asked what were the relationships between the functional traits of the dominant tree species and their sapling abundances, hypothesising that tree species with more acquisitive trait values would have more abundant saplings in this highly human disturbed landscape. For this, an intensive sampling of the sapling assemblages (individuals < 5 cm DB , ≥ 30 cm height) was carried out in 96 plots e ually divided between open and closed canopies. Each sample plot was floristically, spatially and environmentally characterised and, additionally, for 26 previously defined dominant species of large tree assemblages (> 10 cm DBH), eight functional traits were measured (leaf area, specific leaf area, leaf dry matter content, leaf tensile strength, leaf nitrogen content, leaf phosphorous content, wood density, and maximum height). Results indicate that the distribution of the sapling species and their abundances was not uniform across the study area, with floristic changes occurring over very short distances. Anthropogenic disturbance together with altitude explained differences in species richness and abundances between the saplings of pine-dominated and broad-leaved forests, whereas canopy openness had an effect for only nine of the 84 species analysed. Variation partitioning indicated that climate is the main factor explaining floristic variation in the sapling assemblages although spatial variables (PCNM eigenfunctions) were also related to sapling composition, suggesting a complementary effect of dispersal limitation. Leaf area was the only functional trait related to sapling abundance, having a positive correlation suggesting that regeneration of the dominant species in the study area is greater for those species that have big leaves, which may be associated with acquisitiveness. Overall, these results suggest that as hypothesised, distribution and abundance of sapling assemblages at the landscape scale are controlled by climatic variation and human disturbance, with a small contribution of spatial factors. The local-scale effect of canopy openness was smaller than expected, suggesting that due to generalised human disturbance in forests, light availability is not a major limiting factor on sapling regeneration. The positive relationship between species leaf area and sapling abundance, however, indicates that this point requires further research.