Questions Does the magnitude of grazing‐induced changes in species composition vary with habitat productivity? How does the sign and magnitude of grazing effects on species richness and beta‐diversity change with increasing productivity? Do major life forms exhibit consistent responses to grazing along productivity gradients? Location Steppes and grasslands of southern South America in Argentina and Uruguay. Methods We evaluated grazing effects on plant composition, species richness, beta‐diversity and life‐form abundances along a ten‐fold, regional productivity gradient and within subregions of contrasting productivity, using a common sampling protocol for 23 paired grazed vs ungrazed plots. The annual integral of the normalized difference vegetation index was used as a surrogate for above‐ground net primary productivity. Results Compositional dissimilarity between grazed and ungrazed plots, as well as grazing‐induced differences in plant richness and beta‐diversity all increased with habitat productivity. Grazing decreased species richness in low‐productive steppes but enhanced the richness of high‐productive grasslands. On average, grazing reduced beta‐diversity in high‐productive sites but not in low‐productive sites. Dominant species were more strongly suppressed by grazing towards productive grasslands. Grazing generally decreased shrub species cover, whereas graminoid and forb cover did not consistently change with grazing through the productivity gradient. Conclusions Our results indicate that the overall grazing effects on vegetation structure increased along a regional productivity gradient. Yet the sign of grazing impacts on species richness and beta‐diversity shifted with habitat productivity, in agreement with models of herbivore‐mediated co‐existence and species colonization in productive systems. Further, we found that narrowing the spatial extent of analysis to the subregion generally obscured grazing–productivity relationships. Biodiversity conservation programmes should carefully weigh the varied impacts of livestock grazing across productivity gradients.

Latin America in general and the Rio de la Plata Grasslands (RPG) in particular, are one of the regions in the world with the highest rates of change in land use/land cover (LULC) in recent times. Despite the magnitude of this change process, LULC descriptions in the RPG are far from being complete, even more those that evaluate LULC change through time. In this work we described LULC and its changes over time for the first 14 years of the 21st century and for the entire grassland biome of the Rio de la Plata, one of the most extensive grassland regions in the world. We performed simple but exhaustive classifications at regional level based on vegetation phenology, using extensive LULC field database, time series of MODIS NDVI satellite images and decision trees classifiers, generating an annual map for all RPG. The used technique achieved very good levels of accuracy at the regional (94.3%–95.5%) and sub-regional (78.2%–97.6%) scales, with commission and omission errors generally low (Min = 0.6, Max = 10.3, Median = 5.7, and Min = 0, Max = 41.8, Median = 6.8 for regional and sub regional classification respectively) and evenly distributed, but fails when LULC classifications are generated in years when the climate is very different from those used to generate spectral signatures and train decision trees, or when the NDVI time series accumulates large volumes of lost data. Our results show that the RPG are immersed in a strong process of land use change, mainly due to the advance of the agricultural frontier and at the expense of loss of grassland areas. The agricultural area increased 23% in the analyzed period, adding over than 50,000 Km2 of new crops. Most agricultural expansion, and therefore the greatest losses of grassland, concentrates on both sides of Uruguay river (Mesopotamic Pampa and the western portion of Southern and Northern Campos) and the western portion of Inland Pampa. The generated maps open the door for more detailed and spatially explicit modeling of many important aspects of ecosystem functioning, for quantification in the provision of ecosystem services and for more efficient management of natural resources.

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 Río 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 Los pastizales son uno de los ecosistemas terrestres que más modificaciones han sufrido. Los cambios en el uso del suelo han tenido un gran impacto sobre los depósitos y flujos de C. Comprender el impacto de los cambios en uso y cobertura del suelo es crítico a la hora de evaluar el potencial de los pastizales de retener C. En este trabajo analizamos los flujos y depósitos de C a lo largo de los gradientes ambientales de una de las áreas de pastizales templados más extendidas: los Pastizales del Río de la Plata en Sudamérica. El análisis resume la información provista por estudios de campo, teledetección y simulaciones con modelos. La Productividad Primaria Neta Aérea (PPNA) varió entre 240 y 316 g C ? m 22 ? añ o 21. La PPN subterránea fue más variable que la PPNA, cubriendo un rango de valores entre 264 y 568 g C ? m 22 ? añ o 21. El C total varió entre 5 004 y 15 008 g C ? m 22. En promedio, un 13% de ese C correspondió a biomasa vegetal. La contribución de la biomasa vegetal varió entre 9.5% y 27%. El depósito de C vegetal más importante fue el subterráneo. La biomasa verde aérea representó menos de un 7% del C vegetal. El C orgánico del suelo (COS) se concentró en los compartimentos pasivos y de dinámica lenta. El depó...

Carbon gains are a key aspect of ecosystem functioning since they represent the energy available for upper trophic levels. Carbon gains (or primary production) are strongly correlated with other ecosystem attributes such as secondary production and they are also the support for the provision of many ecosystem services. Given the documented dependency of primary production on precipitation, we expect that altered precipitation regimes, such as those projected by climate models, will have a significant impact on carbon gains. Land use and land cover changes are also expected to have a significant impact on the dynamics of carbon gains. We generated a spectral database of the fraction of photosynthetically active radiation intercepted by vegetation (fPAR), in order to study long‐term trends (i.e., decades) in carbon gains and its spatial and temporal relationships with precipitation and land cover patterns in Uruguay, which is part of the Rio de la Plata Grasslands, one of the largest temperate grasslands biome of the world. We found that carbon gains of native forests and grassland afforestation exhibited the strongest positive spatial response to precipitation, whereas crops and rangelands the weakest. In addition, we found that the temporal response of carbon gains to precipitation was strong and positive for all land uses. Although there were not clear trends in precipitation, we found strong negative trends in carbon gains through time, particularly in rangelands of the “Northern Campos” of Uruguay, where these trends represent a decrease between 10% and 25% of the annual aboveground net primary production. On the other hand, positive trends in carbon gains through time were associated to grassland afforestation and native forests. Therefore, during the period analyzed, land cover had a stronger influence on the observed trends in carbon gains than precipitation. These patterns emerged as a consequence of the interaction among precipitation, temperature, edaphic factors and management. Present trends in the controlling factors of C gains would exacerbate the observed patterns with serious consequences for the provision of ecosystems services.

Question: How does above-ground net primary production (ANPP) differ (estimated from remotely sensed data) among vegetation units in sub-humid temperate grasslands? Location: Centre-north Uruguay.

El ser humano modifica el territorio para llevar a cabo actividades productivas o construir viviendas. Estas modificaciones producen importantes cambios en la estructura y el funcionamiento de los ecosistemas, afectando en última instancia la propia calidad de vida de las personas. En este artículo en primer término describimos algunos de los cambios ocurridos en el uso de la tierra en buena parte de Argentina y Uruguay. Utilizamos conjuntamente información aportada por estadísticas oficiales de ambos países y datos resultantes del procesamiento y clasificación de imágenes satelitales. Nuestros análisis muestran que tanto el área agrícola como forestal se han expandido en los últimos años. En Argentina, la mayor expansión la tuvieron los cultivos anuales (desde un 4% a un 14.3%, entre 1988 y 2002) y particularmente la soja, tanto en la provincia de Buenos Aires como en las provincias del Norte Argentino. En Uruguay la mayor expansión entre los censos de 1990 y 2000, estuvo dada por los cultivos forestales, los cuales llegaron a ocupar más del 35 % del área de algunas secciones censales. Luego del 2000, la expansión de la soja también afectó el litoral uruguayo, donde varias secciones censales aumentaron su área de soja a tasas cercanas al 5% anual. En segundo término, presentamos el marco conceptual a los efectos de entender los procesos que determinan estos cambios y examinar su dinámica espacial y temporal. En base a este marco conceptual es posible modelar los cambios en el uso de la tierra a partiendo de la probabilidad de transición entre usos. Los controles de estas transiciones pueden ser ambientales (por ej. tipos de suelos, clima, etc.), económicos (por ej. margen bruto, precios internacionales, etc), sociales (por ej. disponibilidad de mano de obra, tenencia de la tierra, etc.), o políticos (por ej. la ley forestal, política impositiva, líneas de créditos, etc.). Finalmente usando la idea de servicios ecosistémicos presentamos un marco conceptual para la planificación del uso de la tierra considerando sus impactos ambientales, sociales, económicos y políticos.

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