Diagnosing Environmental Controls on Vegetation Greening and Browning Trends Over Alaska and Northwest Canada Using Complementary Satellite Observations

Abstract

Tundra and boreal forest regions have undergone extreme environmental changes in recent decades. Many studies have documented these changes and associated ecosystem impacts using a variety of methods including field measurements, remote sensing and biophysical modeling. Combined observations from satellite optical-infrared and microwave remote sensing have also been used for regional assessment and monitoring of environmental change, ecosystem processes and biogeochemical cycles in the Arctic. Remote sensing derived vegetation parameters range from relatively direct observations of vegetation greenness and chlorophyll fluorescence to higher-level vegetation productivity estimates. However, satellite remote sensing of land surface conditions is particularly challenging at high latitudes due to seasonal variations in solar illumination, snow cover, persistent cloud cover and atmospheric aerosol contamination. Here, we used satellite-derived observations of vegetation greenness (EVI), sun-induced chlorophyll fluorescence (SIF) and gross primary productivity (GPP) to clarify regional patterns and recent variations in vegetation growth over the Arctic Boreal Vulnerability Experiment (ABoVE) domain. The annual non-frozen (NF) period and volumetric soil moisture (VSM) retrieved from satellite microwave remote sensing were used as proxies for growing season length and water supply controls to investigate the impacts of climate on vegetation growth. Positive trends in regional productivity generally coincide with a longer NF season. However, the benefit of a longer NF season to vegetation growth is reduced in soil moisture constrained regions, which have become more widespread in the recent decade over almost half (48.9%) of the domain. Our results document the influence of a changing environment on regional vegetation growth and the northern terrestrial carbon sink for atmospheric CO2.