Drought tolerance of tropical tree species : functional traits, trade-offs and species distribution

KEY-WORDS: Bolivia, drought tolerance, shade tolerance, functional traits, trade-offs, ecophysiology, species distribution Tropical forests occur under rainfall regimes that vary greatly in the rainfall pattern and frequency and intensity of drought. Consequently water availability is one of the most important environmental factors influencing community structure, species composition, and plant functioning across large-scale rainfall gradients and small-scale topographic gradients within forests. The relative success of tree species to establish along these gradients of water availability and their success in dealing with future changes in water availability will depend on how they are adapted to tolerate drought. In this dissertation I applied a multi-species, multi-trait approach in field studies and a controlled experiment to give detailed information on the mechanisms of drought-tolerance of a large set of tropical dry and moist forest tree species. The following research questions were addressed; 1) How do dry and moist forests differ in soil water availability? 2) How are dry and moist forest species adapted to drought and what different drought-strategies can be distinguished? 3) Is there a trade-off between drought- and shade-tolerance? and 4) How do drought- and shade-tolerance determine local and regional tree species distribution? Dry season soil water availability is clearly lower in the dry forest than in the moist forest. Especially in the dry forest there is a lot of temporal and spatial variation in soil water availability. Temporal variation depends on the annual cycle of precipitation. Spatial heterogeneity is two-dimensional; 1) water availability varies with topography of the landscape; elevated crests are dry in comparison to slopes and low valleys, and 2) soil water is vertically redistributed with soil depth; in the dry season more water is available in deep soil layers while in the wet season most water is found in the top soil. When combinin