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Question
How is the magnitude and seasonality of carbon uptake affected by the replacement of native grasslands by eucalyptus plantations?

Location
Río de la Plata Grasslands in Argentina and Uruguay.

Methods
A total of 115 paired sites of fast-growing Eucalyptus grandis plantations and adjacent grasslands were used to characterize the magnitude and seasonality of (1) radiation interception by canopies and (2) above-ground net primary productivity based on a time series of MODIS-derived normalized difference vegetation index (NDVI). The response of NDVI to precipitation was explored across temporal scales.

Results
NDVI in afforested vs. grassland plots presented higher annual averages (1.3-fold), lower seasonal ranges (average relative range of 0.11 vs. 0.29) and delayed growing seasons (2-month shift). Temporally, NDVI was positively associated with precipitation input, showing a correlation with longer periods of precipitation accumulation in tree plantations compared to grasslands (> 7 vs. 2–3 months). Estimated average annual above-ground net primary productivity (ANPP) almost quadrupled as a consequence of replacing grasslands by tree plantations (?4 vs. ?17 Mg dry matter. ha?1·yr?1), and this difference was evidenced throughout the whole study period.

Conclusions
Afforested grasslands intercept more radiation and have higher and more stable ANPP throughout the year, probably as a result of major changes in leaf phenology and root distribution patterns, which in turn allowed better access to water. Changes in carbon uptake can influence climate/biosphere feedbacks and should be considered in land-use planning, especially when grassland afforestation is recommended as a tool to mitigate global warming.