Determinantes locales del desempeño y la distribución espacial de estrategias funcionales en plantas de un bosque seco colombiano

Abstract

Environmental conditions are determinants of the assembly and coexistence of species in plant communities because they act as ecological filters that impose restrictions on their establishment and performance. Changes in water availability and variability in soil resources govern ecological spaces that can be occupied by species through the selection of particular traits. Moreover, they influence the functioning of the ecosystem (e.g. biomass dynamics) through the effects of functional traits on the performance of species and their primary productivity. On a regional scale, climate and soil fertility have been shown to generate substantial changes in plant communities, but at a local scale factors such as topography can play an important role in microenvironmental conditions that also influence the functional composition of plant communities and the way in which they generate effects on the ecosystem. In this research temporal and spatial data of plants and soil resources (water and nutrients) were analyzed from three permanent 1-ha plots located in a Colombian tropical dry forest to explore how topography can exert changes in soil conditions and how these changes influence plant functional traits and biomass dynamics at the community and species level. Two main objectives were addressed: (a) to evaluate changes in soil resources (water and nutrient content) in three 1-ha permanent plots with contrasting topographic roughness (flat, wavy and hilly), and their effects on the functional composition of plants and demographic changes of tree biomass at the community level; (b) to evaluate how local spatial variations in soil resources (water content and nutrients) and functional traits influence the spatial variation of the standing biomass of the species, as well as their demographic changes (survival, recruitment, mortality and net changes in biomass). Changes in soil water content and nutrients and functional composition of plant communities were associated with differences in the topographic roughness. Soil water content decreased from flat to hilly sites while soil nutrients increased possibly because the low soil water content of hilly sites decreases leaching and increases the concentration of nutrients. This led to a higher dominance of conservative species with dense tissues in flat sites due to nutrients limitation and in hilly sites due to soil water scarcity. The water content and nutrients of the soil had intermediate values in undulated sites which led to a greater dominance of acquisitive species. Despite these differences, the demographic changes of biomass at the community level did not differ between plots, suggesting that the effects of each species may not be differentiated at the community level. In this way, at local scales the topography generates changes in soil water and nutrients modifying plant functional configuration. The spatial variation of soil resources and functional traits had coordinated effects on standing biomass. The highest standing biomass was concentrated in sites with low soil water content and high soil nutrients where conservative species dominated, possibly because their dense tissues avoid cavitation and stem breakage, increasing biomass accumulation. Most of the remaining biomass was concentrated in sites with high soil water content and low soil nutrients where acquisitive species were associated, possibly because in these sites there are more competitive in supplying their rapid turnover rates. However, survival, mortality and net biomass changes increased in sites with low soil water content and high concentration of soil nutrients where there was a higher standing biomass. Thus, the spatial variation of soil resources and functional traits determine the distribution of standing biomass, but sites with low soil water content, high soil nutrients and conservative species promote greater storage and dynamics of biomass in the dry forest.