Characterizing the Size Dependence of Resource Acquisition Within Crowded Plant Populations

We present and apply a method for analyzing size-dependent patterns of resource acquisition by individuals within stands of competing plants. Our approach relies on curve-fitting to relate empirical measures of resource acquisition with various aspects of plant size and form. These relationships are referred to as size–uptake relationships (SURs). We demonstrate the use of this approach by characterizing the size dependence of light and nitrogen acquisition within developing stands of birch seedlings. Results of a number of phenomenological studies have predicted that competition for light is expected to be size asymmetric and that competition for belowground resources is expected to be size symmetric. Our study provides the first direct empirical test for these hypotheses. Application of the SUR approach largely confirms predictions regarding the size-dependent nature of light and belowground resource (nitrogen) acquisition. The uptake of nitrogen by birch seedlings was size symmetric, regardless of which aspect of size we considered (i.e., root mass, root length, root surface area, number of root tips, number of mycorrhizal root tips). In contrast, light interception was size asymmetric in relation to height and leaf area, but size symmetric in relation to total plant biomass. Differences in the degree of size-asymmetry of light acquisition between different measures of size were driven by the nature of the allometric relationships between plant height, biomass, and leaf area. SURs offer a tool for exploring some of the mechanistic processes related to resource-based interactions. Using this approach, we demonstrate that it is possible to characterize simultaneously the size-dependent patterns of multiple resources within competing plant populations. We suggest that this approach may prove valuable for identifying how size-dependent patterns of resource acquisition vary through ontogeny. We conclude with a discussion of the need for a more mechanistic understanding of plant–plant interactions and the role that the SUR approach can play towards this goal.