Phenotypic plasticity and the scale of environmental heterogeneity: Studies of tropical pioneer trees in variable light environments

This thesis addresses phenotypic plasticity of canopy structure, leaf dynamics and allocation of tropical pioneer tree seedlings in response to fine scale spatial and temporal variation in light environments. Experimental studies focused on four species from lowland Mexican rain forest, Heliocarpus appendiculatus, Cecropia obtusifolia, Piper auritum and Trema micrantha, examining three aspects of fine scale heterogeneity. (1) Spatial heterogeneity arises within the canopy due to self-shading among leaves. In Heliocarpus, the number and longevity of leaves were reduced by high ambient light levels or a steeper self-shading gradient. A model of shoot-level carbon gain predicts that the optimal number of leaves on a shoot is dependent upon patterns of self-shading, while leaf longevity is inversely correlated with mean carbon assimilation rates. (2) In a temporally variable environment, the patterns and functional significance of plasticity will depend upon the response time of different traits in interaction with the temporal scale of heterogeneity. In Cecropia and Heliocarpus, responses of whole plant traits following a transfer between low and high light were rapid and asymptotic, with time constants for different traits of four to ten days. In fluctuating light environments, canopy display and allocation track oscillations of six to ten days or longer, but integrate fluctuations of one to three days, as predicted from the dynamics following the transfer. Thus, the temporal scale and pattern of resource heterogeneity influence plant growth and development independently of total resource availability. (3) The responses of leaf area display to spatial asymmetry of incident radiation were assessed in natural forest gaps. In all four species, there was a highly significant correlation between the mean direction (azimuth and elevation angle) of diffuse radiation and the orientation of the canopy; relative growth rate was also correlated with diffuse, but not direct, radiation. Phenotypic responses to the environment are expressed at a range of temporal and organizational scales within the individual plant; the contribution of phenotypic plasticity to individual performance must therefore be considered in relation to environmental heterogeneity experienced within the life cycle of individual organisms, as well as among individuals in a population.