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Biological and Physical Drivers of Regional Evapotranspiration Trends in the Northeastern U.S.

M. Vadeboncoeur and H. Asbjornsen,
with Mark Green (Plymouth State University) and John Campbell (USFS)

  • Evapotranspiration (ET) is a major component of the hydrological cycle, and large changes in ET can have far-reaching consequences for streamflow and climate. Observations that global ET has increased over the past decade have generated intense scientific debate, yet the causes remain elusive. Similar declining ET trends have recently been documented at the Hubbard Brook Experimental Forest (HBEF), and preliminary analyses suggest a similar, yet undocumented, region-wide trend. We propose to conduct a collaborative cross-site, multi-scale analysis of long-term ET trends in the Northern Forest region and elucidate the underlying physical and biological mechanisms to explain observed patterns.

    Our specific objectives were to:
    • evaluate ET trends within the Northern Forest region;
    • explore the underlying physical drivers that explain observed ET trends
    • quantify historical changes in tree growth and water use efficiency in relation to climate variability and rising atmospheric [CO2]
    • analyze the combined physical and biological controls on long-term regional ET trends.

  • We hypothesized that the NE region has experienced declines in ET, and that this trend can be explained by a combination of physical and biological controls.
    • We found a similar significant 50+ year decline in ET at one additional site, but not at the sites with shorter records.  
    • Relationships between climate metrics, ET, and water use efficiency differed between the northern and southern groups of sites.
  • We are currently preparing two manuscripts based on this work.
  • Benefits of this study to the Northern Forest region include improved scientific information regarding potential region-wide trends in declining ET, leading to more informed and effective decisions about managing Northern Forests for multiple ecosystem services, particularly water quality and quantity and carbon sequestration.

This work was conducted at seven small watersheds throughout the northeast, including Hubbard Brook, Bear Brook, Sleeper's River, Huntington Forest, Biscuit Brook, Leading Ridge, and Fernow Experimental Forest.  We thank the PIs and funding agencies who made these long-term records possible.

This work was funded by USDA Forest Service through a grant from the Northeastern States Research Cooperative, Theme 2.

Project summary and final report at nsrcforest.org