© 2005 Heron Publishing—Victoria, Canada
Environmental controls on sap flow in a northern hardwood forest
B. D. Bovard (1, 2, 3), P. S. Curtis (1), C. S. Vogel (4), H.-B. Su (5, 6) and H. P. Schmid (6)
1. Department of Evolution, Ecology and Organismal Biology, 318 W. 12th Ave., Ohio State University, Columbus, OH 43210, USA / 2. Department of Environmental Studies, Florida International University, Miami, FL 33199, USA / 3. Corresponding author (bovardb@fiu.edu) / 4. University of Michigan Biological Station, 9008 Biological Road, Pellston, MI 49769, USA / 5. Department of Geography, East Carolina University, Greenville, NC 27858, USA / 6. Atmospheric Science Program, Department of Geography, 701 E. Kirkwood Avenue, Indiana University, Bloomington, IN 47405, USA / Received June 20, 2003; accepted May 29, 2004; published online November 1, 2004
Summary
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.
Keywords:
Acer rubrum, Betula papyrifera, drought, eddy covariance, PAR, Populus grandidentata, Quercus rubra, soil water, transpiration, vapor pressure deficit.
