Scaling plant size to below-ground zone of influence in annuals under contrasting competitive environments

Abstract

1. Understanding the development of plants' below-ground zone of influence, the region over which a plant acquires soil resources, is essential for understanding and predicting interplant competition. 2. The objective of this experiment was to characterize root system development of four weed species (Abutilon theophrasti Medic., Amaranthus retroflexus L., Chenopodium album L. and Setaria faberi Herm.) when grown in monoculture and in mixture to assess the influence of maize competition on root system volume and shape. 3. Using a herbicide-injection technique, root development was assessed at two field sites with contrasting soil textures. Lateral root growth (LRG) was measured over the growing season at 15, 30 and 60 cm depths. Similarly, vertical root growth (VRG) was assessed when roots reached 15, 30 and 60 cm depths. The technique proved useful for quantifying VRG and LRG over time for all four weed species. From vertical and lateral root system expansion, the volume of soil exploited by the root system (V) was calculated and used as an estimator of the below-ground zone of influence. 4. For A. theophrasti and A. retroflexus, VRG was less influenced than LRG by interspecific competition, and was independent of plant size. In contrast, LRG was strongly related to plant size. Differences in LRG were primarily responsible for the sizeable reductions in below-ground zone of influence for plants of similar ages but of lower above-ground biomass (AGB) due to competition with maize. 5. Above-ground biomass of A. theophrasti and A. retroflexus was exponentially related to below-ground zone of influence with a scaling exponent of 0.71 (AGB=84.14V0.71, R2=0.74). Soil type (sandy loam vs silt loam), plant species, or interspecific competition (monoculture vs mixture with maize) did not substantially influence this relationship. 6. The scaling factor reported in this study could be used to improve dynamic models of plant competition in annual agroecosystems. This work also provides insights into the likelihood of below-ground competition between neighbouring plants during root system development. © 2006 The Authors.