© 1998 Heron Publishing—Victoria, Canada
Whole-plant water flux in understory red maple exposed to altered precipitation regimes
Stan D. Wullschleger (1), Paul J. Hanson (1) and Tim J. Tschaplinski (1)
1. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA / Received January 29, 1997
Summary
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 H2O 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 H2O m–2 day–1) or sapwood area (288 mol H2O 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 H2O m–2 h–1 for saplings in the “wet” plot, 1.22 mol H2O m–2 h–1 for saplings in the “ambient” plot, and 0.76 mol H2O 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 (Ω)
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.
Keywords:
Acer rubrum, boundary layer conductance, decoupling coefficient, global change, sap flow, stomatal conductance, transpiration, water
stress.
