Cooperative Institute for Research in Environmental Sciences

Satellite-based model detection of recent climate driven changes in northern high latitude vegetation productivity.

We applied a satellite remote sensing based production efficiency model (PEM) using AVHRR Pathfinder (PAL) and MODIS LAI/FPAR time series with corrected NCEP/NCAR reanalysis daily surface meteorology to assess annual terrestrial net primary productivity (NPP) for the panArctic basin and Alaska from 1982 to 2005. We applied a “moving window” correction of the NCEP/NCAR 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 also 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. Mean annual NPP for the domain showed a positive productivity trend of 0.14% per year ( 0.043=P ) during the 1980’s and 1990’s, followed by a recent, severe NPP decline after 2000. Our results also show that low temperature constraints on boreal-Arctic NPP are decreasing ( 001.0 < P ), whereas increasing moisture stress ( 09.0=P ) is offsetting the potential benefits of longer growing seasons and driving the recent productivity decline. The satellite remote sensing derived productivity record compares favorably with stand inventory network measurements of boreal aspen stem growth ( 614.0=r , 002.0=P ), and atmospheric CO2 measurement based estimates of the timing of growing season onset ( 788.0=r , 001.0 < P ). Our results indicate that the NPP decline is widespread, and is unprecedented for at least the past 50 years across central and western Canada, and the past 24 years for the larger pan-Arctic basin and Alaska domain. This condition also appears to be a biospheric response to a declining water balance under a warming climate.