Document: NAT-3-32-30

Relationships between tree size, soil water content and predawn plant water potential in an oak woodland in Oregon .

GEHRES, N.T.* ¹, N.PHILLIPS ¹, A.SCHAUER ¹, N.G.MCDOWELL ¹, B.J.BOND ¹ and M.G.RYAN ²

Oregon State University, Corvallis, OR USA ¹
US Forest Service, Ft. Collins, CO USA ²

Abstract:
We investigated the relationships between tree height, seasonal patterns of soil water content () and predawn leaf water potential (_pd) in 10m- and 25m-tall woodlands of Oregon white oak near Corvallis, Oregon. Measurements of to 1m depth were taken weekly through a drying period from June 17 to October 15, 1999 using a time domain reflectometry technique. Additionally, _pd was measured on 4 occasions during this same period with a Scholander-type leaf pressure chamber. These measurements were not corrected for differences in gravity for the two tree heights. _pd of the two stands was indistinguishable during the early season (average = -0.4 MPa; p > 0.05) but diverged steadily through the summer with 25 m trees showing progressively lower leaf water potentials (p < 0.05). The 10 m trees did not show a decrease in _pd over the season (p > 0.05). By the season-end, _pd in 25 m trees was more than two times lower (more stressed) than that in 10 m trees (-0.9 MPa versus -0.4 MPa, respectively; p < 0.0001). On the other hand, soil moisture decreased steadily over the season but there was no difference in soil moisture drawdown between the two sites (p > 0.05). These results show that seasonal change in pre-dawn water potentials in the trees was not related to change in water availability near the soil surface. Larger trees may not have been able to equilibrate overnight with soil water potential in the late season.

Keywords: stand age; transpiration; soil water; pre-dawn water potential; Oregon white oak

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS