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HYDRAULIC REDISTRIBUTION OF SOIL WATER BY ROOTS IN FORESTS OF THE PACIFIC NORTHWEST
Citation:
Brooks, J R., F. C. Meinzer, J W. Gregg, T. M. Hinckley, AND R. A. Coulombe. HYDRAULIC REDISTRIBUTION OF SOIL WATER BY ROOTS IN FORESTS OF THE PACIFIC NORTHWEST. Presented at Linking the Complexity of Forest Canopies to Ecosystem and Landscape Function, IUFRO Canopy Processes, Corvallis, OR, July 16-17, 2001.
Description:
One aspect of structural complexity of forest canopies is the root system structure belowground, which influences patterns of soil water utilization by trees. Deeply rooted trees and other plants can hydraulically lift water via their roots from several m below the soil surface to within 1 m of the surface where it is released and may be available for reabsorption by the same individual and by neighboring individuals of the same or other species. Hydraulic redistribution may lead to enhanced exploitation of soil water resources by vegetation, especially in areas that experience yearly drought cycles. It may also be important for maintaining nutrient uptake by fine roots located in the relatively nutrient rich upper portion of the soil profile that normally undergoes pronounced desiccation during the dry season.
In this study, our objective was to determine whether hydraulic redistribution of soil water by roots plays a role in the water economy of two Pacific Northwest forest ecosystems: a ponderosa pine ecosystem receiving 500 mm of precipitation, and a Douglas fir ecosystem receiving 2500 mm of precipitation. We measured soil water content, soil water potential and root sap flow in young and old stands in both ecosystems through the summer drought season. Multiple lines of evidence indicated that hydraulic redistribution occurred at all four study sites, suggesting that it could be an important process in both wet and dry forests of the Pacific Northwest. Some roots exhibited reverse sap flow at night, with water flowing toward the soil rather than toward the stem. Consistent with this, soil water content increased transiently at night, partially compensating for depletion during the previous day. In an old-growth ponderosa pine stand, hydraulic redistribution contributed about 38% of the water utilized per day in the upper 2 m of the soil profile during July. Diel fluctuations in soil water potential at this site were also consistent with hydraulic redistribution. By late September, partial nighttime recharge of the soil profile was no longer evident and the net daily soil water depletion was greater than that observed earlier in the season. In a 20-year-old Douglas fir stand, partial nighttime recharge resulting from hydraulic redistribution contributed about 25% of the water taken up from the upper 2 m of the soil profile in mid-August. Applications of 700 liters of deuterated water to 1x1m plots to create strong horizontal soil water potential gradients provided further evidence for hydraulic redistribution. Both the magnitude of reverse sap flow in roots and the nightly recharge in soil water content increased following these localized irrigations. On the opposite side of the tree, soil water content continued to cycle, but temporarily ceased declining. Taken together, these results indicate that hydraulically redistributed water constitutes a significant fraction of the total daily evapotranspiration during the dry summer months and retards the rate of water depletion in the upper portion of the soil profile.