Increasing summer drying in North American ecosystems in response to longer nonfrozen periods

In snow-dominated northern ecosystems, spring warming is predicted to decrease water availability later in the season and recent findings suggest that corresponding negative impacts on plant productivity and wildfire frequency are already observable. Here we estimate the overall vulnerability of North American ecosystems to warming-related seasonal shifts in hydrology through identifying robust interannual linkages between nonfrozen periods, peak-to-late summer vegetation greenness, and an indicator of drought for 1982-2010. Our results show that longer nonfrozen periods earlier in the year are persistently associated with declines in peak-to-late summer greenness and moisture availability across large portions of North America. Hereby, vulnerabilities increase markedly across the dominant land covers with decreasing annual precipitation rates, lowering contributions of summer rainfall, and increasing altitude. The implications are that in a warmer world, seasonal hydrological shifts may emerge as a leading factor for summer drought in relatively dry temperate-forested ecosystems and across the northern high latitudes.