Remotely Sensed Variations of Pan-Arctic Terrestrial Vegetation Productivity from 1982- 2005

We applied the MOD17A2 production efficiency model (PEM) using AVHRR Pathfinder (PAL) and MODIS LAI/FPAR time series, and corrected NCEP reanalysis daily surface meteorology to assess terrestrial NPP for the pan-Arctic basin and Alaska from 1982 to 2005. We applied a "moving window" correction of the NCEP reanalysis time series using observed daily minimum and mean surface air temperatures and dew points from 5,874 surface weather stations distributed across the region. We conducted a pixel-wise integration of the AVHRR and MODIS time series by empirical adjustment of the AVHRR record using land cover specific regressions of overlapping NPP results for 2000. Annual NPP for the domain showed a significant, positive productivity trend of 0.2% per year (P=0.01) from 1982 to 1997 followed by a significant productivity decline of 0.8% per year (P=0.035) after 1997. The annual onset of the growing season defined by the timing of the spring drawdown of atmospheric CO2 from NOAA CMDL high latitude (>50 ° N) stations was highly correlated (r = -0.510; P = 0.013) with annual anomalies of regional average NPP for the study domain before 2000, while this correlation was reduced (r = -0.148, P = 0.29) after 2000. The satellite derived NPP anomalies were also highly correlated (r = 0.62, P = 0.01) with stand-level observations of boreal Aspen growth anomalies for 72 stands across west-central Canada. These results indicate that low temperature constraints on vegetation productivity are decreasing (P < 0.001), particularly for Eurasia, whereas increasing moisture stress (P = 0.069), especially after 2000, is offsetting the potential benefits of longer growing seasons and resulting in a recent, widespread productivity decline.