## 2 Materials and Methods
### Remote Sensing Data and Field Measurements  
In this study, we used FAPAR as the primary source of information about vegetation status, because it directly measures the amount of incident radiation absorbed by the plant canopy, one of the key drivers of plant development and GPP [57]. The results described in this paper were generated using time series of maximum value composited dekadal (defined as a 10-day period) FAPAR of the JRC-MARS archive (Joint Research Centre of the European Commission, Monitoring Agricultural Resources Unit). In this product, FAPAR is estimated using the CYCLight algorithm [58] applied to calibrated, cloud-screened and atmospherically corrected (SMAC algorithm [59]) SPOT-VEGFATION (VGT) imagery at a 1/112 \({}^{\circ}\)(about 1 km) spatial resolution for the period of April 1998-January 2013 (nearly 15 years).  
Field measurements of above-ground biomass at the end of the growing season were performed by the Centre de Suivi Ecologique (CSE, Dakar, Senegal) over pastoral sites in the Matam region (Figure 1a,b), an area of transition between cropland and herbaceous land cover according to GLC2000. It is worth noting that a large fraction of the sampling sites is classified as cropland in GLC2000 albeit agriculture is negligible at these sites (crops are present only in few sites, with less than 1% of total area cropped). Herbaceous dry matter biomass was estimated along 1 km \(\times\) 1 m transects with the stratified sampling method described in Diouf and Lambin [60]. The transect is  
Figure 1: (**a**) The study area encompassing the main Sahel eco-regions. The herbaceous and cropland land covers are the union of GLC2000 (Global Land Cover 2000) herbaceous cover dominated classes (Classes 13 to 15) and cropland classes (Classes 16 to 18), respectively. Blue points in Senegal refer to the location of field measurements sites (zoom in (**b**)).  
stratified into four strata according to vegetation type and estimated herbaceous biomass. Then, between 35 and 100 plots (depending on the spatial heterogeneity of the site) of 1 m\({}^{2}\) are randomly distributed in each stratum. The aboveground herbaceous biomass of each plot is clipped, and the fresh weight is determined _in situ_. A sub-sample of the clipped biomass is dried in an oven to measure the dry weight and to estimate the dry to fresh weight ratio. Mean site dry biomass is finally extrapolated as the average of the biomass of each stratum weighted by its relative contribution (proportion of transect occupied by the stratum). A total number of 33 sites were available in an area covering roughly 100 km \(\times\) 100 km. Sampling sites were revisited yearly in late October from 2005 to 2011. Missing data in the database are due to logistic difficulties and bush fires that occurred before planned field missions. Furthermore, the data of five sites were not used in the present analysis, as they reported anomalous inter-annual variability of biomass production compared to neighboring sites. As a result, a total of 167 data points for 28 sites were available for the validation exercise. A 3 \(\times\) 3-pixel window centered on the coordinates of the transect midpoint was used to extract the average of the remote-sensing indicator. Visual inspection of all sites on Google Earth imagery did not reveal the presence of a spatial heterogeneity in vegetation cover that was incompatible with the extraction window used.