Abstract:
Tropical dry forests are among the most threatened ecosystems worldwide, yet the mechanisms controlling tree growth at local scales remain poorly understood. The aim of this study was to combine available information on environmental conditions and tree functional traits to better understand their relevance for tree growth in seasonal tropical dry forests. We used data from forest dynamics of six permanent one-ha plots at two elevations (lower elevation deciduous forest: ~600 m a.s.l.; upper elevation semi-deciduous forest: ~1,200 m a.s.l.) in southern Ecuador to calculate relative growth rate (RGR) based on stem diameter for all living stems over a two-year census interval. We combined functional traits of leaves (i.e., foliar nitrogen and phosphorus concentrations, leaf thickness, leaf toughness, specific leaf area) and stems (i.e., bark thickness, wood specific gravity, vessel density, vessel diameter, hydraulic conductivity) with soil chemistry data (phosphorus availability, carbon: nitrogen ratio and others) to build predictive models of tree growth. At both elevations, RGR declined strongly with increasing tree size, indicating a consistent size-dependent constraint on RGR. Functional traits emerged as significant predictors of RGR, with growth increasing in association with acquisitive traits and decreasing with conservative traits. In particular, high wood specific gravity (WSG) was negatively associated with RGR, consistent with species adopting conservative strategies characterized by greater investment in structural integrity and hydraulic safety at the expense of rapid growth. Soil properties showed only weak and inconsistent associations with RGR. Model comparisons confirmed that tree size and functional traits, notably WSG, explained substantially more variation in RGR than soil variables, while models including additional predictors did not improve model performance. Our findings highlight the dominant role of species functional strategies and tree size in explaining variation in tree growth in tropical dry forests.
