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Conversion of native grasslands to cultivated pastures due to livestock intensification is a worldwide phenomenon that affects many ecosystem functions. In the temperate semiarid area of Central Argentina, a major land-use change is the conversion of native grasslands to exotic weeping lovegrass (Eragrostis curvula) pastures. We evaluated the relationship between above net primary productivity (ANNP) and annual precipitation on nine paired sites of native grasslands and tame pastures, distributed along a 300 km transect in the southern region of San Luis province, Argentina. Productivity and precipitation data for the period 2000-2010 were obtained by using MODIS/Terra and TRMM remote sensing. The annual ANPP was slightly but significantly reduced by 3% after conversion of native grasslands to weeping lovegrass pastures. In addition, the stability of ANPP, measured as the inter-annual ANPP standard deviation and coefficient of variation, was also significantly lower in introduced pastures than in more diverse grasslands. That may be explained by a differential response of these plant communities to seasonal variation on the precipitations. The ANPP of native grasslands and weeping lovegrass were both correlated with winter-spring precipitations, but under seasonal droughts, the ANPP was more severely reduced in tame pastures than in native grasslands. Thus, our results showed that negative effects of land-use change on ecosystem functions may be enhanced under climatic stresses.