Damping and lag effects of precipitation variability across trophic levels in Uruguayan rangelands

Abstract

Extensive livestock production is one of the activities that, without requiring habitat conversion, contributes to grassland conservation and human well-being. In Uruguay, approximately 60% of the land is covered by natural temperate grasslands, which are entirely devoted to extensive livestock production, mainly cattle and sheep. In this study we described the spatial and temporal variability of aboveground net primary productivity (ANPP), net secondary productivity (NSP) and trophic efficiency (TEf) of temperate grasslands in two geomorphological regions of Uruguay (Basaltic “Cuesta” and Eastern Hills), and explored their relationship with the mean and annual precipitation (MAP and PPT, respectively). Specifically, we asked whether Uruguayan grasslands are able to damp precipitation variability at the levels of vegetation and herbivores. Additionally, we searched for lag effects of precipitation on primary and secondary productivity. The study period included seven growing seasons, from 2005 to 2012. PPT, ANPP and NSP were calculated monthly for 32 sections (administrative units) of both regions, and aggregated at the growing season level. NSP-related data included not only livestock numbers, but also livestock births, mortality and transportation, as well as milk and wool production. ANPP showed a clear inter-regional difference, the Eastern Hills region was more productive (69814 MJ.ha-1.y-1) than the Basaltic “Cuesta” (60762 MJ.ha-1.y-1 44 ). NSP showed no inter-regional differences (742 vs. 754 MJ.ha-1.y-1, respectively) and TEf was higher in Basaltic “Cuesta” (0.0125) than in Eastern Hills (0.0107). As expected, ANPP was strongly determined by the amount of rainfall, both in space and time. Although we did not find a relationship between current year NSP and ANPP, secondary productivity was positively related with PPT and ANPP with a two-year lag in Basaltic “Cuesta”. Also, we found that the interannual variabilities of ANPP and NSP were, respectively, 60 and 40% lower than the variability of precipitation, but contrary to our expectations, NSP was 40% more variable than ANPP. Only 1.17% of the energy captured by grasslands in Uruguay is converted into domestic herbivore production, a value close to the average reported for other grasslands and savannas.