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Hydraulic redistribution of soil water by roots in forests of the Pacific Northwest

Hydraulic redistribution of soil water by roots in forests of the Pacific Northwest. J. R. Brooks 1 , F. Meinzer 2 , J. Gregg 1 , T. Hinckley 3 R. Coulombe 4 1 Western Ecology Division U.S. EPA, 2 USDA Forest Service PNW Laboratory, 3 University of Washington, & 4 Dynamac Corp.

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Hydraulic redistribution of soil water by roots in forests of the Pacific Northwest

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  1. Hydraulic redistribution of soil water by roots in forests of the Pacific Northwest J. R. Brooks1, F. Meinzer2, J. Gregg1, T. Hinckley3 R. Coulombe4 1Western Ecology Division U.S. EPA, 2USDA Forest Service PNW Laboratory, 3University of Washington, & 4Dynamac Corp.

  2. Hydraulic redistribution of soil water by roots

  3. Consequences • Enhanced exploitation of soil water? • Maintenance of nutrient uptake? • Maintenance of soil microbial/mycorrhizal activity?

  4. General Objective Assess prevalence and magnitude in key PNW forest types

  5. Specific Objectives Patterns related to tree size and age Magnitude and patterns along precip. gradient Seasonal patterns Species-specific differences? Utilization of water by other plants?

  6. Methods • Study Sites: Young and old stands in a ponderosa pine ecosystem receiving 500 mm of precipitation, and a Douglas fir ecosystem receiving 2500 mm of precipitation. • Measurements: soil water content, soil water potential and root sap flow. • Experimental Manipulations: Applications of 700 liters of deuterated water to 1x1m plots to create strong horizontal soil water potential gradients.

  7. Ponderosa Pine Ecosystems (500 mm Precipitation) Young (15 yr) Logged old-growth Old (50/250 yr) Never logged

  8. Douglas-Fir Ecosystem (2500 mm precipitation) Young (20 yr) Logged old-growth Old (450 yr) Never logged

  9. Soil Water Potential using soil psychrometers

  10. Deuterated water (D2O) Horizontal redistribution? Redistributed water available to other plants?

  11. Introducing a Thermal Dissipation Probe system for measuring bi-directional root water flux

  12. Designed to work in stems with one flow direction. • Cannot indicate flow direction • Zero flow is determined by a max temperature difference between the probes.

  13. Directional probe

  14. Lab test conclusions • Directional probe • Quick response time • Accurate • Flux magnitude probe • Slower Response time • Linear increase with flow rate • Scaling a problem due to variation in sapwood flow rate

  15. Hydraulic Redistribution Field DataPreliminary Results

  16. Evidence of Redistribution No Evidence of Redistribution Source of Redistributed Water

  17. Young Douglas-Fir • Net loss on a daily basis is 2/3 of the daily drawdown • Redistribution activity occurs both in the upper and lower soil layers

  18. Seasonal Changes in Redistribution Old P. Ponderosa

  19. D2O pulse

  20. Applied 700 l of 1500 ‰ D2O • Sampled understory plants,soils and target trees.

  21. Use of Redistributed Water by Other Plants?

  22. Preliminary Findings Evidence points to hydraulic redistribution Significant contribution to transpiration Retards rate of depletion in upper profile Most of water extracted from depth > 50 cm Water can be taken up by other plants

  23. Where do we go from here?

  24. Flux data from P. Ponderosa sites • Young and old trees: different in flux rates over the season • Soil or tree size? • Does redistribution of water play a role in these differences?

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