Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream
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Although drought in temperate deciduous forests decreases transpiration rates of many species, stand-level transpiration and total evapotranspiration is often reported to exhibit only minor interannual variability with precipitation. This apparent contradiction was investigated using four years of transpiration estimates from sap flux, interception–evaporation estimates from precipitation and throughfall gauges, modeled soil evaporation and drainage estimates, and eddy covariance data in a mature oak-hickory forest in North Carolina, USA. The study period included one severe drought year and one year of well above-average precipitation. Normalized for atmospheric conditions, transpiration rates of some species were lower in drought than in wet periods whereas others did not respond to drought. However, atmospheric conditions during drought periods are unlike conditions during typical growing season periods. The rainy days that are required to maintain drought-free periods are characterized by low atmospheric vapor pressure deficit, leading to very low transpiration. In contrast, days with low air vapor pressure deficit were practically absent during drought and moderate levels of transpiration were maintained throughout despite the drying soil. Thus, integrated over the growing season, canopy transpiration was not reduced by drought. In addition, high vapor pressure deficit during drought periods sustained appreciable soil evaporation rates. As a result, despite the large interannual variation in precipitation (ranging from 934 to 1346 mm), annual evapotranspiration varied little (610–668 mm), increasing only slightly with precipitation, due to increased canopy rainfall interception. Because forest evapotranspiration shows only modest changes with annual precipitation, lower precipitation translates to decreased replenishment of groundwater and outflow, and thus the supply of water to downstream ecosystems and water bodies.
Keywordsbroadleaf deciduous drainage drought precipitation transpiration water yield
This Research was supported by the Office of Science (BER) U.S. Department of Energy, Grant No. DE-FG02_00ER63015. We would like to thank P. Stoy, H. McCarthy, H.-S. Kim, B. Poulter, and K. Schäfer for assistance with data collection and analysis, and K. Johnsen and D. D. Richter for unpublished observations on fine roots.
- Emerman SH, Dawson TE. 1996. Hydraulic lift and its influence on the water content of the rhizosphere: an example from sugar maple, Acer saccharum. Oecologia 108:273–8.Google Scholar
- Hanson PJ, Amthor JS, Wullschleger SD, Wilson KB, Grant RF, Hartley A, Hui D, Hunt ER Jr, Johnson DW, Kimball JS, King AW, Luo Y, McNulty SG, Sun G, Thornton PE, Wang S, Williams M, Baldocchi DD, Cushman RM. 2004. Oak forest carbon and water simulations: model intercomparisons and evaluations against independent data. Ecol Monogr 74:443–89.CrossRefGoogle Scholar
- Hanson PJ, Huston MA, Todd DE. 2003. Walker branch throughfall displacement experiment. In: Hanson PJ, Wullschleger SD, Eds. North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes. New York: Springer-Verlag. p 8–31.Google Scholar
- Juang JY, Katul GG, Porporato A, Stoy PC, Siqueira MS, Detto M, Kim HS, Oren R. 2007. Eco-hydrological controls on summertime convective rainfall triggers. Glob Change Biol 13:887–96.Google Scholar
- Kim H-S. 2009. Measurement and Modeling of Radiation and Water Fluxes in Plantation Forests. Nicholas School of the Environment and Earth Sciences. Durham, NC: Duke University.Google Scholar
- Köecher P, Gebauer T, Horna V, Leuschner C. 2009. Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies. Ann For Sci 66, Article No.: 101.Google Scholar
- Siqueira M, Katul G, Porporato A. 2008. Onset of water stress, hysteresis in plant conductance, and hydraulic lift: scaling soil water dynamics from millimeters to meters. Water Resour Res 44.Google Scholar
- Wear DN, Greis JG. 2002. The southern forest resource assessment summary report. In: Southern Forest Resource Assessment. Gen. Tech. Rep. SRS-53. Wear DN, Greis JG, Eds. US Department of Agriculture, Forest Service, Southern Research Station, Asheville, NC.Google Scholar
- Wullschleger SD, Hanson PJ. 2003. Sensitivity of sapling and mature-tree water use to altered precipitation regimes. In: Hanson PJ, Wullschleger SD, Eds. North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes. New York: Springer-Verlag.Google Scholar