Clonal plants in a spatially heterogeneous environment: effects of integration on Serengeti grassland response to defoliation and urine-hits from grazing mammals

It has been hypothesized that clonal integration between ramets in environments with spatially variable rates of herbivory and nutrient availabilities leads to increased growth and fitness in the genet. An increase in genet growth could potentially influence ecosystem processes such as primary productivity and nutrient cycling. I tested the idea that clonal integration would lead to greater aboveground productivity, compensatory response to defoliation, and N re-distribution in a factorial experiment in field plots in the Serengeti Ecosystem, Tanzania. Each plot had either all stolons severed or left intact, had repeated defoliation (to simulate grazing by the African buffalo) or none, and was either located next to a plot that received urea (to simulate a urine-hit) or next to a plot that remained untreated. Plots that received stolon severing treatments had 32% less peak biomass than did connected control plots, and this suggests that plants grew better when ramets remained connected. However, compensatory response by plots to repeated defoliation was inconsistent with the hypothesized benefits of ramet integration at the ecosystem level: productivity response to defoliation was similar between plots with connected vs. severed ramets (i.e. no stolon severing x defoliation interaction was found). When averaged across other treatments, defoliated plots had increased productivity compared to nondefoliated plots during the growing season. Thus, ramet connection and defoliation did increase productivity when they were considered alone, but productivity response to defoliation was unaffected by ramet connections. Urea additions, which led to a 78% increase in productivity in adjacent "urine-hits", had no consistent effect on productivity but did increase leaf percent N in adjoining study plots. Thus, in the Serengeti, urine hits probably have very localized effects on productivity during the initial growing season.