Bimodal root diameter distributions in Serengeti grasses exhibit plasticity in response to defoliation and soil texture: implications for nitrogen uptake

1. The effects of soil texture (low or high sand) and defoliation (clipped or unclipped) on root diameter distributions and nitrogen uptake were studied in the Serengeti grasses Digitaria macroblephara (Hack.) Stapf and Themeda triandra Forssk. in glasshouse cultures. 2. At the end of the experiment, plants were harvested and images of the washed roots were digitized using a scanner. Root size distributions were distinctly bimodal and were described with pairs of lognormal distributions with six parameters used as variables in subsequent analyses. To investigate N uptake, a nitrate solution containing N-15 was added to the soil 5 days before final harvest. 3. In both grasses, root diameter distributions were more responsive to defoliation than to soil texture. However, the two species diverged strongly in their investment in fine roots in the more sandy soils. 4. For each species, linear relationships were found among root biomass, root surface area and specific root length, which suggests that variation in root architecture is genetically constrained in each species. Plasticity emerged because the soil texture and defoliation experienced by an individual determined the specific architecture that it expressed within these linear relationships. 5. Root parameters obtained from the experiment and previously published root biomass and enzyme activities were used to predict N-15 uptake of the two grasses. Root diameter distribution and enzyme activities were important predictors of N uptake in the fast-growing grazing tolerant D. macroblephara, whereas root system size and root diameter distribution predicted N uptake in the slower growing, taller-statured T. triandra. 6. The differential responses of these grasses to variation in soils and defoliation are consistent with the hypothesis that habitat heterogeneity and herbivory interact to influence species coexistence in plant communities of African savannas.