Biomass allocation and growth of tropical dry forest tree seedlings across light and soil gradients.

摘要

This dissertation examines the effect of abiotic factors on the growth of tropical dry forest tree seedlings, and the developmental plasticity that allows seedlings to respond to those factors. Different species of tropical dry forest tree seedling are demonstrated to reverse ranks in response on different combinations of light and soil environments that occur within the tropical dry forest. This is interpreted to indicate that tropical dry forest species may be niche-differentiated with respect to soil. This is found to conflict with the assumption of ecological equivalence central to neutral models of community structure.; Neutral models are examined further and it is determined that fitness manifolds cannot reconcile the niche-differentiation and neutral perspectives without restrictive assumptions about the distribution of environmental heterogeneity in nature.; Two experiments designed to test whether or not dry-forest tree species demonstrate adaptive plasticity in biomass allocation that is consistent with the balanced growth hypothesis have conflicting results. In the first study, individuals of the dry forest tree Gliricidia sepium are shown to shift their developmental allometries so that a larger fraction of biomass is allocated to leaves in high nutrient environments and a larger fraction is allocated to lateral roots in low nutrient environments, although the effect is observed only at small sizes for lateral roots. It is demonstrated that failing to account for allometry in analyses of adaptive plasticity will often lead to spurious results. It is further demonstrated that when taproot biomass is large, root: shoot ratio is a poor indicator of allocation to belowground foraging.; In a second experiment that includes the effect of both light and soil on G. sepium and Hymenaea coubaril, two species that differ greatly in life-history characteristics, soil is shown to have a negligible effect on morphology and biomass allocation while the effect of light is uniformly strong. Morphology is measured in terms of root and leaf surface area rather than as mass in individual biomass components. The magnitude of the plastic response is shown to be greater for G. sepium than for H. coubaril.