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Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (D) and soil water interact to control transpiration in the dominant canopy species of a mixed hardwood forest in northern Lower Michigan. An improved understanding of how these environmental factors affect whole-tree water use in unmanaged ecosystems is necessary in assessing the consequences of climate change on the terrestrial water cycle. We used continuously heated sap flow sensors to measure transpiration in mature trees of four species during two successive drought events. The measurements were scaled to the stand level for comparison with eddy covariance estimates of ecosystem water flux (Fw). Photosynthetically active radiation and D together explained 82% of the daytime hourly variation in plot-level transpiration, and low soil water content generally resulted in increased stomatal sensitivity to increasing D. There were also species-specific responses to drought. Quercus rubra L. showed low water use during both dry and wet conditions, and during periods of high D. Among the study species, Acer rubrum L. showed the greatest degree of stomatal closure in response to low soil water availability. Moderate increases in stomatal sensitivity to D during dry periods were observed in Populus grandidentata Michx. and Betula papyrifera Marsh. Sap flow scaled to the plot level and Fw demonstrated similar temporal patterns of water loss suggesting that the mechanisms controlling sap flow of an individual tree also control ecosystem evapotranspiration. However, the absolute magnitude of scaled sap flow estimates was consistently lower than Fw. We conclude that species-specific responses to PAR, D and soil water content are key elements to understanding current and future water fluxes in this ecosystem.

[1] We report results from the first 3 years (1999)(2000)(2001) of long-term measurements of net ecosystem exchange (NEE) at an AmeriFlux site over a mixed hardwood forest in northern lower Michigan. The primary measurement methodology uses eddy covariance systems with closed-path infrared gas analyzers at two heights (46 and 34 m) above the forest (canopy height is $22 m). One objective is to contribute to a more firmly established methodology of estimating annual net ecosystem production (NEP), by systematically examining the consequences of several variations of criteria to identify periods of unreliable measurements, and to fill gaps in the data. We compared two methods to fill data gaps (about 30% of time in 1999) due to missing observations or rejected data after quality control; one using short-term ensemble averages of the daily course and the other by semiempirical parametric models based on relationships between ecosystem respiration and soil temperature and between gross ecosystem photosynthetic uptake and photosynthetically active radiation. The modeled estimates were also used to replace eddy covariance fluxes during periods of weak and/or poorly developed turbulence when eddy covariance measurements cannot be expected to represent the ecosystem exchange. Examination of the fractions of eddy covariance fluxes and storage change relative to the expected ecosystem respiration suggested a friction velocity (u * ) of 0.35 m s À1 as the lower limit for the acceptance of micrometeorologically determined NEE for this site. The differences in estimated annual NEP due to different criteria of data acceptance, measurement height, or gap-filling strategies turned out to be at least as large as the interannual variations over the 3 years. After discussing various analysis strategies we conclude that the best estimate of annual NEP at our site is achieved by replacing data gaps and measurements in low-u * conditions at all times with site and period-specific parametric models, using the upper measurement level (about 2.1 canopy heights). These ''best estimates '' of annual NEP for 1999'' of annual NEP for -2001'' of annual NEP for amounted to 170 (1999'' of annual NEP for ), 160 (2000, and 80 (2001)