Publication

We studied the impacts of land use in temperate Argentina on the normalized difference vegetation index (NDVI), which has often been used to estimate the interception of photosynthetically active radiation and carbon uptake by terrestrial vegetation. The NDVI was derived from the National Oceanic and Atmospheric Administration (NOAA) satellites. We incorporated land use, climate, soil, and NDVI data into a geographic information system with a county-level spatial resolution. Land use was characterized in terms of the proportion of different crop types at the county level. Three attributes were derived from the seasonal dynamics of the NDVI: the annual integral (NDVI-I), the difference between the maximum and minimum NDVI divided by the integral (RREL), and the date of maximum NDVI (DMAX). The environmental controls of the NDVI attributes for the counties showing the lowest proportion of croplands ("low-impacted vegetation" areas, LIV) were analyzed using stepwise multiple regressions with the three NDVI attributes as dependent variables and the climatic and soil variables as independent variables. Mean annual precipitation, evapotranspiration, and the seasonality of precipitation were the main determinants of the spatial distribution of the NDVI attributes in the LIV areas. We applied these models to counties with a high proportion of croplands, and we analyzed the magnitude of the difference between the observed and predicted values and their correlation with land use variables. The LIV model for NDVI-I did not differ by more than 15% from the observed values in the counties with a high proportion of crops. This indicates a low impact of land use on the NDVI-I. In contrast, the LIV models for RREL and DMAX, two indices that describe the seasonality of NDVI, did not predict accurately these attributes in the highly cropped areas. Annual crops increased RREL up to 80%. Winter crops advanced DMAX and summer crops delayed DMAX, in both cases up to 150 d compared to the LIV areas. The interannual variability of NDVI-I was negatively correlated with the mean NDVI-I. This agrees with previous observations on unmodified areas of the Northern Hemisphere. The interannual variability of the three attributes of the NDVI curve decreased as the proportion of the cropped area of the county devoted to summer crops increased. Our results are relevant to the impacts of land use on the timing and magnitude of energy and carbon exchange in temperate agricultural systems and clarify how agricultural land use can be better represented in regional and global models of energy and carbon exchange.