Root morphological plasticity and nitrogen uptake of 59 plant species from the Great Plains grasslands, USA

We investigated 59 plant species from the Great Plains grasslands with the following objectives: (1) Determine the ability of individual plant species to: (a) redirect root surface area growth to patches with high N and P concentrations; and (b) alter their root:shoot ratio in response to a non-uniform distribution of soil N and P. (2) Determine how a patchy distribution of soil N and P within a plant's rooting zone affects biomass, total root surface area, N uptake, %N in plant tissue, and N uptake per unit of root surface area. Results showed a diversity of responses with some important common patterns. (1) Root responses (defined as total RSA, allocation of RSA, and R:S ratio) were twice as prevalent as whole plant responses (defined as total N uptake, %N in plant tissue, and plant biomass). (2) The percentage of species with the ability to allocate their RSA to nutrient rich patches was 9 times higher in forbs than in grasses, but there were proportionally more grasses than forbs that increased their R:S ratios in response to nutrient patchiness. (4) The proportions of late successional forbs that responded to nutrient patchiness was higher than that of mid successional ones, but the size of the response was substantially larger in the latter. (4) We found a very weak coupling between root plasticity and plant performance. (5) Our results tend to suggest that: (a) most plants have sufficient plasticity in root system development to track the scale of soil nutrient heterogeneity and thus show similar performance regardless of the degree of nutrient patchiness; and (b) the benefits of root plasticity may be more critical for subdominant species as a general adaptation to compete for soil nutrients in mixed plant communities regardless of the extent of soil nutrient heterogeneity.