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We report state-and-transition models for Uruguayan grasslands built on a methodological approach that objectively defined states/phases associated, a priori, with rangeland management. Such approach was based on randomly sampled areas corresponding to mapped grassland communities. Each sampled area matched a MODIS pixel. Vegetation structural indicators were recorded in every pixel. After a multivariate analysis, field observations were grouped according to similarities in terms of structure, and different “states” and “phases” were identified. Ecosystem functioning and the supply of regulating ecosystem services were estimated for each grassland state/phase using the normalized difference vegetation index derived from the MODIS sensor. Finally, workshops were held in order to detect local stakeholders’ perceptions and discuss the management practices to promote the desired transitions among phases. Results were presented for two vegetation units of the Basaltic “Cuesta” region. The “inductive approach” applied led to not only the description of “states” but also the identification of more subtle changes in vegetation ("phases"). Our approach minimized biases due to personal experience, as well as differences derived from using different observation protocols. The two vegetation units presented an internal heterogeneity associated with changes in basal stratum height, total cover, stratification, frequency of decreasing species due to grazing, and proportion of plant life forms. The ecosystem functioning descriptors of each phase responded to extreme climatic events differently. On the basis of stakeholder’s opinions and experiences, stocking rate, sheep/cattle ratio, and grazing method were the main management practices promoting the transition among phases.

In native campos of Uruguay, overseeding legumes coupled with phosphorus (P) fertilization is a technology used to increase animal production. Short-term improvements in both forage productivity and quality are repeatedly reported. However, some evidence suggests that this management may at times lead to the collapse of the native community and invasions by exotic species. Indeed, it is yet unclear to what extent overseeding legumes into native grasslands affects its long-term integrity. This study uses data from a long-term experiment to assess whether increased P fertilizer rates—typically used to encourage legume establishment and growth—are associated with reduced species diversity. In 1996 a grazed native grassland in eastern Uruguay was either left untouched (control) or overseeded with a mix of Trifolium repens and Lotus corniculatus and then fertilized at either a moderate or high rate of P (197 or 394 kg · ha-1 over 13 years, respectively). The three treatments were arranged in a randomi...

Grasslands are one of the most modified biomes on Earth. Land use changes had a large impact on carbon (C) stocks of grasslands. Understanding the impact of land use/land cover changes on C stocks and fluxes is critical to evaluate the potential of rangeland ecosystem as C sinks. In this article we analyze C stocks and fluxes across the environmental gradients of one of the most extensive temperate rangeland areas: the Rio de la Plata Grasslands (RPG) in South America. The analysis summarizes information provided by field studies, remote sensing estimates, and modeling exercises. Average estimates of aboveground net primary production (ANPP) ranged from 240 to 316 g C ? m 22 ? yr 21 . Estimates of belowground NPP (BNPP) were more variable than ANPP and ranged from 264 to 568 g C ? m 22 ? yr 21 . Total Carbon ranged from 5 004 to 15 008 g C ? m 22 . Plant biomass contribution to Total Carbon averaged 13% and varied from 9.5% to 27% among sites. The largest plant C stock corresponded to belowground biomass. Aboveground green biomass represented less than 7% of the plant C. Soil organic carbon (SOC) was concentrated in the slow and passive compartments of the organic matter. Active soil pool represented only 6.7% of the SOC. The understanding of C dynamics and stocks in the RPG grasslands is still partial and incomplete. Field estimates of ANPP and BNPP are scarce, and they are not based on a common measurement protocol. Remotely sensed techniques have the potential to generate a coherent and spatially explicit database on ANPP. However, more work is needed to improve estimates of the spatial and temporal variability of radiation use efficiency. The absence of a flux tower network restricts the ability to track seasonal changes in C uptake and to understand fine-scale controls of C dynamics. Resumen