9357274 Dawson Forests cover 37% of all terrestrial surfaces. The trees that compose these forests are a primary route by which water re-enters the hydrologic cycle. The objectives of this NSF Young Investigator (NYI) Award are to understand and evaluate the importance that trees and forests play in local and regional hydroglogic processes. This will be accomplished through an investigation that has four specific goals: (1) describe the water balance of isolated trees and of forests stands using O and H stable isotope methods, eddy correlation and aerodynamic-energy balance techniques, and the Bowen-ratio method, (2) characterize the degree to which trees control movement and loss of water from forest stands with leaf-, canopy- and stand-level transpiration estimates and isotope techniques, (3) investigate how water uptake, movement and loss respond (adjust) to experimental manipulation mimicking climate change (e.g. irrigating trees, preventing transpiration), and (4) integrate the information gathered from the observational/descriptive studies (objectives 1 and 2), the experimental studies (objective 3), and from both individuals and entire stands into a comprehensive picture of the role that trees and forests play in global water movement. Because the overall objective is to refine our understanding of the importance and role that trees and forests play in the hydrologic cycle, it is necessary to use a variety of research methods. The combination of methods chosen will provide a means to validate information, integrate different levels, and calibrate predictions. The stable isotope and eddy correlation methods are the most powerful. An analysis of the stable isotopic composition of water samples obtained from source water, plant xylem-sap, and atmospheric water vapor has been successfully used to evaluate water flux through the soil-plant-atmospheric continuum. The eddy correlation technique is a powerful way to quantify water movement through pla nt canopies and stands. Aerodynamic-energy balance methods of water flux based on the Penman-Monteith equation, and the Bowen-ratio method (sensible/latent heat flux) can provide ways to independently check the isotope and eddy correlation results. The information obtained from these studies will help us understand the impacts that climate and climate change may have on the functional and structural characteristics of forested regions of the world, refine our empirical understanding of the role that vegetated surfaces play in the hydrologic cycle and local and regional water balance, and to forge a much-needed link between atmospheric and plant biological research. ***
