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Abstract:
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.
