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 und erstory of an upland oak forest and exposed to one of three large-scal e (0.64 ha) manipulations of soil water content. This Throughfall Disp lacement 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 1120 tree(-1) day(-1)). These size-class diffe rences were driven largely by differences in leaf area and cross-secti onal 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 trea tment 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 i n the ''dry'' plot compared to saplings in the ''wet'' plot and 2.5-fo ld lower compared to saplings in the ''ambient'' plot. Treatment diffe rences in crown conductance followed a pattern similar to that of tran spiration, with values that averaged 60% lower for saplings in the ''d ry'' plot compared to saplings in the ''wet'' plot and 35% lower compa red to saplings in the ''ambient'' plot. Stomatal and boundary layer c onductances were roughly equal in magnitude. Estimates of the decoupli ng coefficient (Omega) ranged between 0.64 and 0.72 for saplings in th e three TDE treatment plots. We conclude that red maple saplings growi ng in the understory of an upland oak forest are responsive to their e daphic and climatic surroundings, and because of either their small st ature or their shallow root distribution, or both, are likely to be im pacted by precipitation changes similar to those predicted by global c limate models.