Sugar Maple Trees at the Hubbard Brook Experimental Forest
|Jeffrey S. Amthor1 & James W. Hornbeck2
1 Environmental Sciences Division, Oak Ridge National Laboratory, Mail Stop 6422, PO Box 2008, Oak Ridge TN 37831-6422. Email: AmthorJS@ORNL.gov
2 US Department of Agriculture Forest Service, Northeastern Research Station, PO Box 640, Durham, NH 03824. Email: Hornbeck_Jimfirstname.lastname@example.org
ATMOSPHERIC CO2 concentration increased more than 30% during the past 250 years, due in the past mainly to land use changes (deforestation and cultivation of soils) and now driven chiefly by fossil fuel use. Because elevated CO2 often (but not always) causes partial closure of stomatal pores in tree leaves, and because stomatal aperture can regulate (at least partially) transpiration, rising atmospheric CO2 might be altering ecosystem water use. In particular, rising CO2 might slow evapotranspiration and as a result enhance streamflow. Indeed, several published claims of increasing water yield due to effects of rising CO2 on transpiration exist, although long-term field data have not been used to evaluate such claims. The goal of this work is to use long-term Hubbard Brook hydrologic data to evaluate the response of whole-ecosystem water use to rising CO2. Annual atmospheric CO2 increase has been monotonic during the Hubbard Brook era.
Precipitation and streamflow data from Hubbard Brook watersheds 1, 3, 6, 7, and 8 (i.e., the undisturbed gaged watersheds with long-term records) are being used to estimate both water-year (June to May) and "growing season" (mid May to mid November) whole-forest evapotranspiration (i.e., transpiration plus interception losses plus soil/snow-surface evaporation and sublimation). The growing season estimates are meant to reflect water use when it is regulated by tree physiology and canopy display. They are also used to avoid the winter period because measurements of snow input are the weakest part of the Hubbard Brook hydrologic dataset.
Water Year Results
In a two-variable analysis, water-year precipitation was positively related to CO2 concentration (and therefore time) in all five watersheds, but not statistically significantly. Water-year streamflow was also positively related to water-year precipitation in all watersheds (in a two-variable analysis), and the relationship was highly significant. In a multiple-linear regression analysis, evapotranspiration is positively related to precipitation (P < 0.1 in W7, but insignificant in all other watersheds) and negatively related to CO2 concentration (P < 0.001 in W3, but insignificant in all other watersheds). The analysis of water-year hydrology in W3 is strongly influenced by early and late years in the data record. A general, consistent negative relationship between rising CO2 and water-year evapotranspiration is not obvious from the water-year hydrologic data.
Results from the growing-season analyses differ in important respects from the water-year analyses. Trends toward greater growing-season streamflow in south-facing watersheds (W1, W3, W6) seem to be explained entirely by increases in growing-season precipitation, with little or no effect of rising CO2 on evapotranspiration. On the other hand, increasing growing-season precipitation on north-facing watersheds (W7, W8) resulted in smaller (and statistically insignificant) increases in growing-season streamflow, apparently because growing-season evapotranspiration has been increasing as atmospheric CO2 has risen in recent decades. This result, though still preliminary (and subject to significant data scatter), is counter to the notion that rising CO2 will reduce evapotranspiration from terrestrial ecosystems.
Planned Future Work
The analyses of Hubbard Brook hydrology in the context of rising atmospheric CO2 concentration and other environmental changes (such as warming and increasing precipitation) will continue. Obviously, as the data record becomes even longer, the power of the analyses will increase. In addition, more emphasis will be placed on assessing the influence of specific definitions of the growing season (i.e., the dates used to define the beginning and the end of the growing season) on relating hydrology to rising CO2. Finally, similar analyses will be conducted for other gaged watersheds in forest ecosystems (analysis of Fernow data has begun) to enhance the generality of the results.
Amthor JS (1998) Searching for a relationship between forest water use and increasing atmospheric CO2 concentration with long-term hydrologic data from the Hubbard Brook Experimental Forest. ORNL/TM-13708. Oak Ridge National Laboratory, Oak Ridge, TN, 25 p.
Amthor JS, Hornbeck JW (1999) Rising CO2 and forest water use: long-term data from Hubbard brook Experimental Forest, New Hampshire. In: DB Adams (ed) Potential consequences of climate variability and change to water resources of the United States. American Water Resources Association, Herdon, VA, TPS-99-1, p 399-402.
This research is supported through the DOE/NSF/NASA/USDA/EPA Interagency Program on Terrestrial Ecology and Global Change (TECO) by the U.S. Department of Energy's Office of Biological and Environmental Research under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation.