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HYDRAULIC REDISTRIBUTION IN A DOUGLAS-FIR FOREST: LESSONS FROM SYSTEM MANIPULATIONS
Brooks, J R., F. C. Meinzer, J. M. Warren, J. C. Domec, AND R. Coulombe. HYDRAULIC REDISTRIBUTION IN A DOUGLAS-FIR FOREST: LESSONS FROM SYSTEM MANIPULATIONS. PLANT, CELL, AND ENVIRONMENT. Blackwell Publishing, Malden, MA, 29:138-150, (2006).
To quantify seasonal variation in hydraulic redistribution, and to manipulate the soil-root system to elucidate physiological components controlling HR and the utilization of redistributed water
Hydraulic redistribution (HR) occurs in many ecosystems; however, key questions remain about its consequences at the ecosystem level. The objectives of the present study were to quantify seasonal variation in HR and its driving force, and to manipulate the soil-root system to elucidate physiological components controlling HR and the utilization of redistributed water. In the upper soil layer of a Douglas-fir forest, HR was negligible in the early summer, but increased to 0.17 mm day-1 (20-60 cm layer) by late August when soil water potential was approximately -1 Mpa. When maximum HR rates were observed, redistributed water replenished approximately 40% of the water depleted from the upper soil on a daily basis. Manipulations to the soil or to the soil/plant water potential driving force altered the raate of observed HR indicating that the rate of HR is controlled by a complex interplay between competing soil and plant water potential gradients and pathway resistants. Separating roots from the transpiring tree resulted in increased HR, and sap flow measurements on connected and disconneted roots showed reversal of water flow, a prerequisite for HR. Irrigating a small plot withdeuterated water demonstrate that redistributed water was taken up by small understorey plants as far as 5 m from the watering source, and potentially further, but the utilization pattern was patchy. HR in the upper soil layers near the watering plot was twice that of the control HR. This increase in HR also increaed the amount of water utilized by plants from the upper soil. These results indicate that the seasonal timing and magnitude of HR was strongly governed by the development of water potential differences within the soil, and the competing demand for water by the above ground portion of the tree.