Whole-plant water flux in understory red maple exposed to altered precipitation regimes.

Sap flow gauges were used to estimate whole-plant water flux for five stem-diameter classes of red maple (Acer rubrum L.) growing in the understory of an upland oak forest and exposed to one of three large-scale (0.64 ha) manipulations of soil water content. This Throughfall Displacement Experiment (TDE) used subcanopy troughs to intercept roughly 30% of the throughfall on a "dry" plot and a series of pipes to move this collected precipitation across an "ambient" plot and onto a "wet" plot. Saplings with a stem diameter larger than 10 cm lost water at rates 50-fold greater than saplings with a stem diameter of 1 to 2 cm (326 versus 6.4 mol H(2)O tree(-1) day(-1)). These size-class differences were driven largely by differences in leaf area and cross-sectional sapwood area, because rates of water flux expressed per unit leaf area (6.90 mol H(2)O m(-2) day(-1)) or sapwood area (288 mol H(2)O dm(-2) day(-1)) were similar among saplings of the five size classes. Daily and hourly rates of transpiration expressed per unit leaf area varied throughout much of the season, as did soil matrix potentials, and treatment differences due to the TDE were observed during two of the seven sampling periods. On July 6, midday rates of transpiration averaged 1.88 mol H(2)O m(-2) h(-1) for saplings in the "wet" plot, 1.22 mol H(2)O m(-2) h(-1) for saplings in the "ambient" plot, and 0.76 mol H(2)O m(-2) h(-1) for saplings in the "dry" plot. During the early afternoon of August 28, transpiration rates were sevenfold lower for saplings in the "dry" plot compared to saplings in the "wet" plot and 2.5-fold lower compared to saplings in the "ambient" plot. Treatment differences in crown conductance followed a pattern similar to that of transpiration, with values that averaged 60% lower for saplings in the "dry" plot compared to saplings in the "wet" plot and 35% lower compared to saplings in the "ambient" plot. Stomatal and boundary layer conductances were roughly equal in magnitude. Estimates of the decoupling coefficient (Omega) ranged between 0.64 and 0.72 for saplings in the three TDE treatment plots. We conclude that red maple saplings growing in the understory of an upland oak forest are responsive to their edaphic and climatic surroundings, and because of either their small stature or their shallow root distribution, or both, are likely to be impacted by precipitation changes similar to those predicted by global climate models.

[1]  F. Meinzer,et al.  Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity , 1994 .

[2]  C. H. M. van Bavel,et al.  Measurement of mass flow of water in the stems of herbaceous plants , 1987 .

[3]  A. Arneth,et al.  Environmental regulation of xylem sap flow and total conductance of Larix gmelinii trees in eastern Siberia. , 1996, Tree physiology.

[4]  Syukuro Manabe,et al.  Equilib-rium climate change ? and its implications for the future , 1990 .

[5]  R. Ceulemans,et al.  Water flux in a hybrid poplar stand. , 1994, Tree physiology.

[6]  R. Teskey,et al.  Water use by Pinus radiata trees in a plantation. , 1996, Tree physiology.

[7]  Cynthia Rosenzweig,et al.  Potential evapotranspiration and the likelihood of future drought , 1990 .

[8]  Seasonal and diurnal trends of heat pulse velocity in Douglas-fir and ponderosa pine , 1986 .

[9]  M. Abrams,et al.  Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during a drought. , 1995, Tree physiology.

[10]  Philippe H. Martin.  The significance of radiative coupling between vegetation and the atmosphere , 1989 .

[11]  T. Gish,et al.  Soil Moisture Determination in Gravelly Soils with Time Domain Reflectometryd , 1989 .

[12]  G. Goldstein,et al.  Stomatal and environmental control of transpiration in a lowland tropical forest tree , 1993 .

[13]  M. Huston,et al.  Field performance of the Walker Branch throughfall displacement experiment , 1994 .

[14]  R. Vogel,et al.  Global warming and the hydrologic cycle , 1996 .

[15]  Terry V. Callaghan,et al.  Effects on Ecosystems , 1992, Encyclopedia of Food and Agricultural Ethics.

[16]  Y. Cohen,et al.  Water regime of a pine forest under a Mediterranean climate , 1995 .

[17]  Ramakrishna R. Nemani,et al.  Regional hydrologic and carbon balance responses of forests resulting from potential climate change , 1991 .

[18]  E. Greenwood,et al.  Evaporation from the understorey in the Jarrah (Eucalyptus marginata Don ex Sm.) forest, southwestern Australia , 1985 .

[19]  G. Jackson,et al.  Xylem cavitation in Scots pine and Sitka spruce saplings during water stress , 1995 .

[20]  J. Schultz,et al.  Ecophysiological Responses in Mesic versus Xeric Hardwood Species to an Early-Season Drought in Central Pennsylvania , 1990, Forest Science.

[21]  R. Huc,et al.  Transpiration of natural rain forest and its dependence on climatic factors , 1996 .

[22]  Philippe H. Martin.  Climate change, water stress, and fast forest response: A sensitivity study , 1996 .

[23]  P. Dye.  Response of Eucalyptus grandis trees to soil water deficits. , 1996, Tree physiology.

[24]  M. Abrams,et al.  Seasonal ecophysiology and leaf morphology of four successional Pennsylvania barrens species in open versus understory environments , 1993 .

[25]  R. Benyon,et al.  Relationships between stem diameter, sapwood area, leaf area and transpiration in a young mountain ash forest. , 1995, Tree physiology.

[26]  Mark Rounsevell,et al.  Climate Change 1995: impacts, adaptations and mitigation of climate change: scientific-technical analyses. Contribution of Working Group II to the Second Assessment Report of the Intergovernmental Panel on Climate Change , 1996 .

[27]  J. Kucera,et al.  Rapid response of large, drought-stressed beech trees to irrigation. , 1993, Tree physiology.

[28]  Todd E. Dawson,et al.  Determining water use by trees and forests from isotopic, energy balance and transpiration analyses: the roles of tree size and hydraulic lift. , 1996, Tree physiology.

[29]  Hervé Cochard,et al.  Field comparison of transpiration, stomatal conductance and vulnerability to cavitation of Quercus petraea and Quercus robur under water stress , 1993 .

[30]  K. G. McNaughton,et al.  Stomatal Control of Transpiration: Scaling Up from Leaf to Region , 1986 .

[31]  Responses in NPP and carbon stores of the northern biomes to a CO2-induced climatic change, as evaluated by the Frankfurt biosphere model (FBM) , 1995 .

[32]  John Pastor,et al.  Response of northern forests to CO2-induced climate change , 1988, Nature.