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HYDRAULIC REDISTRIBUTION OF SOIL WATER DURING SUMMER DROUGHT IN TWO CONTRASTING PACIFIC NORTHWEST CONIFEROUS FORESTS
Brooks, J R., F. C. Meinzer, R. Coulombe, AND J W. Gregg. HYDRAULIC REDISTRIBUTION OF SOIL WATER DURING SUMMER DROUGHT IN TWO CONTRASTING PACIFIC NORTHWEST CONIFEROUS FORESTS. TREE PHYSIOLOGY. Heron Publishing, Victoria, B.C, Canada, 22:1107-1117, (2002).
The magnitude of hydraulic redistribution of soil water by roots and its impact on soil water balance were estimated by monitoring time courses of soil water status at multiple depths and root sap flow during droughted conditions in a dry ponderosa pine ecosystem and a moist Douglas-fir ecosystem. . Localized applications of deuterated water to create a strong horizontal soil water potential gradient was also monitored to assess the potential for horizontal redistribution of water and utilization of redistributed water by co-occurring shallow-rooted plants. We observed large diel changes in soil water storage in the upper 2 m of soil in both forests. In a 20-yr-old Douglas-fir stand, approximately 28% of the water removed each day was supplied by notcturnal hydraulic redistribution during late August. In an old-growth ponderosa pine stand, approximately 35% of the total water utilized appeared to be attributable to hydraulic redistribution during July and August. By late September, hydraulic redistribution in the ponderosa pine stand was no longer apparent, even though total water use from the upper 2 m of soil was nearly identical to that observed earlier. Based on these results, hydraulic redistribution would allow the equivalent of 21 and 16 additional days of stored water to remain in the upper soil horizons in the ponderosa pine stand Douglas-fir stand, respectively, after a 60-d drought. At both sites, localized applications of deuterated water induced strong reversal of root sap flow and caused soil water content to cease declining or even temporarily increase at locations too distant from the site of water application to have been influenced by movement of water through the soil without facilitation by roots. Xylem dD values of ponderosa pine seedlings suggested utilization of redistributed water. Therefore, hydraulic redistribution may enhance seedling survival and maintain overstory transpiration during summer drought.
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Organization:U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL HEALTH AND ENVIRONMENTAL EFFECTS RESEARCH LABORATORY
WESTERN ECOLOGY DIVISION
TERRESTRIAL ECOLOGY BRANCH