CONICET | Buscador de Institutos y Recursos Humanos

Question: Do species traits explain differences in productivity in grazed and ungrazed plots? Location: Río de la Plata grasslands, Uruguay (31º54´S, 58º15´W). Methods: In a greenhouse experiment we measured the relative growth rate (RGR) of grasses with a contrasting response to grazing (increasers and decreasers). We performed six harvests weekly in order to calculate the RGR and 12 plant traits. We performed ANOVA and t-tests to compare the RGR of increaser and decreaser species. Principal Component Analysis (PCA) was used to sort species according to grazing response. Results: The RGR of decreaser grasses was higher than RGR of increasers at the second week, while at the fifth week the opposite was observed. The RGR of decreasers dropped significantly through time, being 74% lower at the end of the experiment. In contrast, the RGR of increaser species was constant through time. The PCA separated increaser from decreaser species. The attributes related with increaser species were: high specific leaf area, tillering rate, green leaf rate , total leaf number, root weight ratio and leaf weight ratio; while those associated to decreaser species were: high dead biomass, senescence rate, reproductive biomass, leaf elongation rate and total biomass. Conclusions: Traits associated to decreasers determine a reduction in light availability and an increase in reproductive investment, explaining the RGR drop. Specific differences in RGR seem to scale up to the ecosystem level and would explain the pattern in aboveground net primary production observed in the field under contrasting grazing regimes.