Plant Architecture and Allocation in Different Neighborhoods: Implications for Competitive Success

Abstract

A plant's competitive ability is determined by its capacity to acquire resources from resource pools shared with neighbors. Resource acquisition abilities depend on plant part placement in relation to available resources. Using a model system of annuals, we examined how the architecture of and allocation to plant parts was affected by different neighbor species. Species having different morphologies were grown as "targets" (target plants) in continuous canopies of each of four neighbor species in the greenhouse. Architectural and allocational traits of Abutilon theophrasti did not vary significantly with neighbor identity. Several architectural and allocation traits of Dutura stramonium and Polygonum penslvanicum plants did vary significantly among neighbor identities; however, in all but two cases, analyses of covariance revealed that these differences were due to differences in shoot biomass among neighbors. We could therefore not relate changes in architecture and allocation to the effects of specific neighbors, and therefore to competitive ability under specific conditions. In all species part masses were more variable than part lengths or areas, suggesting that allocational flexibility in these species allowed them to maximize part size at the expense of part mass to some extent. When we examined within plant biomass allocation patterns among plants of different size classes, regardless of neighbor, we found differences among species in how these patterns changed with plant size. Abutilon and Datura plants in the smallest size class showed a marked reduction in allocation to stems in the upper canopy, while producing similar numbers of nodes, when compared to plants in the largest size class. By contrast, Polygonum plants in the smallest size classes produced many fewer nodes than, but had a relative allocation pattern similar to, plants in the largest size class. These differences had consequences for plant shape, and so suggest differences among species in architectural response to shading. Any such response that allowed a plant to increase its resource—gathering ability would represent an important mechanism of competition.