Vegetation productivity patterns at high northern latitudes: a multi-sensor satellite data assessment

Satellite-derived indices of photosynthetic activity are the primary data source used to study changes in global vegetation productivity over recent decades. Creating coherent long-term records of vegetation activity from legacy satellite data sets requires addressing many factors that introduce uncertainties in vegetation index time series. We compared long-term changes in vegetation productivity over high northern latitudes (>50°N), estimated as trends in growing season NDVI derived from the most widely used global NDVI data sets. The comparison included the AVHRR based GIMMS-NDVI version G (GIMMSg) series, and its recent successor version 3g (GIMMS3g), as well as the shorter NDVI records generated from more modern sensors such as SeaWiFS, SPOT-VGT and MODIS. The data sets from the latter two sensors were provided in a from that reduces the effects of surface reflectance associated with solar and view angles. Our analysis revealed both large geographic areas, totaling 40% of the region, where all data sets indicate similar changes in vegetation productivity over their common temporal record, as well as areas where data sets show conflicting patterns. The newer, GIMMS3g dataset showed statistically significant (α = 0.05) increases in vegetation productivity, not seen in its predecessor (GIMMSg), in >15% of the study area, whereas the reverse was rare (<3%). The latter has implications for earlier reports on vegetation activity changes based on GIMMSg, particularly in Eurasia where 'greening' is particularly pronounced in the GIMMS3g data. Our findings highlight both critical uncertainties and areas of confidence in the assessment of ecosystem-response to climate change using satellite-derived indices of photosynthetic activity. Broader efforts are required to evaluate NDVI time series against field measurements of vegetation growth, primary productivity, recruitment, mortality and other biological processes to better understand ecosystem responses to environmental change over large areas.

JRC.H.3 - Forest Resources and Climate