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Remotely-sensed trends in vegetation productivity and phenology during population decline of the Bathurst caribou herd

The Bathurst caribou herd declined from ~349,000 animals in 1996 to ~8,200 in 2018. Climate-driven changes to tundra and boreal vegetation is one hypothesis for the decline. We modeled and mapped annual productivity and phenology across the herd’s range using enhanced vegetation index (EVI) data derived from a MODIS time series spanning 2000-2017. Maximum annual EVI, growing season length, and time-integrated EVI increased significantly on 16%, 18%, and 49% of the core annual range, respectively. Trends towards longer growing seasons were driven entirely by earlier spring green-up and, along with time-integrated EVI, were most prevalent in tundra regions. Trends in forested regions were overwhelmingly related to the influence of forest fires, which burned more than half of the range below the forest-tundra ecotone since 1965. These trends suggest that climate-driven changes in production and phenology have occurred in the tundra and forest-tundra portions of the range and could have contributed to the recent herd decline. However, the trends may also be a result of herd decline itself, given the loss of this large herbivore from the landscape. Elucidating cause and effect will require explicit analysis of interactions between climatic variables, herd dynamics, and vegetation change, complemented by comprehensive field investigations.