Greening vs browning? Surface water cover mediates how tundra and boreal ecosystems respond to climate warming

Climate warming in northern high latitudes has progressed twice as fast as the global average, leading to prominent but puzzling changes in vegetation structure and functioning of tundra and boreal ecosystems. While some regions are becoming greener, others have lost or shifted vegetation condition as indicated by a browning signal. The mechanisms underlying this ‘greening or browning enigma’ remain poorly understood. Here we use multi-sourced time-series of satellite-derived vegetation indices to reveal that spectral greening is associated with reductions in surface water cover (i.e. fraction of surface water bodies), whereas spectral browning is linked to increases in surface water cover. These patterns are consistently observed from both 30 m resolution Landsat data and 250 m resolution MODIS data on the basis of grid cells sized of 1, 2 and 4 km. Our study provides, to our knowledge, the first biome-scale demonstration that interactions between vegetation condition and water cover change can explain the contrasting trajectories of ecosystem dynamics across the northern high latitudes in response to climate warming. These divergent trajectories we identified have major implications for ecosystem functioning, carbon sequestration and feedbacks to the climate system. Further unraveling the interaction between vegetation and surface water will be essential if we are to understand the fate of tundra and boreal biomes in a warming climate.