Growth consequences of soil nutrient heterogeneity for two old-field herbs, Ambrosia artemisiifolia and Phytolacca americana, grown individually and in combination

Plant species can respond to small scale soil nutrient heterogeneity by proliferating roots or increasing nutrient uptake kinetics in nutrient-rich patches. Because root response to heterogeneity differs among species, it has been suggested that the distribution of soil resources could influence the outcome of interspecific competition. However, studies testing how plants respond to heterogeneity in the presence of neighbours are lacking. In this study, individuals of two species, Phytolacca americana L. and Ambrosia artemisiifolia L. were grown individually and in combination in soils with either a homogeneous or heterogeneous nutrient distribution. Above-ground biomass of individually grown plants of both species was greater when fertilizer was located in a single patch than when the same amount of fertilizer was distributed evenly throughout the soil. Additionally, both species proliferated roots in high-nutrient patches. A. artemisiifolia exhibited larger root:shoot ratios, increased nitrogen depletion from nutrient patches, and a higher growth rate than P. americana, suggesting A. artemisiifolia is better suited to find and rapidly exploit nutrient patches. In contrast to individually grown plants, soil nutrient distribution had no effect on final above-ground plant biomass for either species when grown with neighbours, even though roots were still concentrated in high nutrient patches. This study demonstrates that increased growth of isolated plants as a consequence of localized soil nutrients is not necessarily an indication that heterogeneity will affect interspecific encounters. In fact, despite a significant belowground response, soil nutrient heterogeneity was inconsequential to above-ground performance when plants were grown with neighbours. (C) 1999 Annals of Botany Company.