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. Key Points Longer nonfrozen periods linked to summer greenness decline across NA Drought indicator and precipitation pattern confirm summer drought signal Seasonal shift in hydrology is a leading drought factor in northern ecosystems © 2014. American Geophysical Union. All Rights Reserved.

Parida B.R.

Buermann W.

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

Geophysical Research Letters

In the northern high latitudes, alternative hypotheses with regards to how warming-related shifts in seasonality influence ecosystem productivity exist. Increased plant growth associated with a longer growing season may enhance ecosystem productivity, but shifts to earlier springs may also negatively influence soil moisture status and productivity during the peak of the growing season. Here, we analyzed nearly three decades (1982-2008) of observational records and derived products, including satellite microwave and optical imagery as well as upscaled ecosystem flux observations, to better understand how shifts in seasonality impact hydrology and productivity in the North American boreal forests. We identified a dominant adverse influence of earlier springs on peak summer forest greenness, actual evapotranspiration and productivity at interannual time scales across the drier western and central sections of the North American boreal forests. In the vast regions where this spring onset mechanism operates, ecosystem productivity gains from earlier springs during the early portion of the growing season are effectively cancelled through corresponding losses in the later portion. Our results also indicate that recent decadal shifts towards earlier springs and associated drying in the midst of the growing season over western North American boreal forests may have contributed to the reported declines in summer productivity and increases in tree mortality and fire activity. With projections of accelerated northern high-latitude warming and associated shifts to earlier springs, persistent soil moisture deficits in peak summer may be an effective mechanism for regional-scale boreal forest dieback through their strong influence on productivity, tree mortality and disturbance dynamics. © 2013 IOP Publishing Ltd.

Environmental Research Letters

Earlier springs decrease peak summer productivity in North American boreal forests

Remote Sensing

A Non-Stationary 1981–2012 AVHRR NDVI3g Time Series

International Journal of Climatology

Updated high-resolution grids of monthly climatic observations - the CRU TS3.10 Dataset

Hydrology and Earth System Sciences Discussions

Climate regime and soil storage capacity interact to effect evapotranspiration in western United States mountain catchments

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011