In adjacent paper birch and white oak saplings, leaf conductance and plant-water potential were compared on selected days through a summer at a site in southern Maine. In both wet and dry soil, leaf conductance in birch was lower but water potential was higher than in oak. A model of transpiration for a homogeneous canopy and a layered soil estimated conductance and potential reasonably well when driven by measured soil-water and atmospheric variables. Simulating the major differences between birch and oak required only a difference in change of conductance with declining water potential. Birch reached a moderate stress condition sooner than oak. In moderate stress, diffusive conductance at midday is half its unstressed value and transpiration is reduced by 10 or 20 percent. Plant-water potential is not an indicator of water stress differences between species. Birch dropped its leaves in response to severe stress, but oak did not reach the severe stress stage. In terms of both greater stomatal closure and leaf abscission, birch is more of a drought avoider than oak. But ring widths were similarly affected by drought in both species. Simulated reduction of transpiration by drought implied that differences in both stand age and available water among sites cause larger differences in transpiration than differences among species on the same site. Stand composition apparently is not determined by drought survival at the sapling stage. Forest Sci. 26:313-324.
Forest Science is a peer-reviewed journal publishing fundamental and applied research that explores all aspects of natural and social sciences as they apply to the function and management of the forested ecosystems of the world. Topics include silviculture, forest management, biometrics, economics, entomology & pathology, fire & fuels management, forest ecology, genetics & tree improvement, geospatial technologies, harvesting & utilization, landscape ecology, operations research, forest policy, physiology, recreation, social sciences, soils & hydrology, and wildlife management.