Document: NAT-3-32-36

Evidence of hydraulic limitation and compensation in large, old Douglas-fir trees.

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

Oregon State University, Corvallis, OR 97331 USA ¹
USDA Forest Service, Ft. Collins, CO 80526 USA ²

Abstract:
As trees grow older and larger, increased hydraulic resistance due to pathlength could reduce stomatal conductance (g_s) and net photosynthesis (A). However, reduced leaf area:sapwood area ratios and/or increased capacitance could partially or wholly compensate for this increased resistance. To investigate these possibilities, we measured gas-exchange and hydraulic properties in 20y, 40y, and 450y stands of Douglas-fir trees in Wind River, Washington during 1998 and 1999. Hydraulic limitation was assessed through measurement of A, g_s, leaf water potential, and foliar cellulose dC¹³. Hydraulic compensation was assessed via whole-tree estimates of leaf area:sapwood area ratios and sapflow estimates of capacitance. Evidence of a hydraulic limitation was observed in 450y trees. Mid-day leaf water potential averaged 0.6 MPa more negative for the 450y compared to the 20y and 40y. g_sand A were consistently higher in the 20y than the 40y or 450y. Leaf-specific hydraulic conductance was significantly lower in 450y than the 40y or 20y trees. Leaf cellulose dC¹³ averaged -26.2 (20y), -25.1 (40y), and -23.4 (450y) per mil, indicating decreased g_s with tree height. Compensatory mechanisms were also observed. The leaf area:sapwood area ratio of the 450y (0.46m²cm^-2;) was significantly lower than for the 40y (0.58) or 20y (0.56). Capacitance, a third compensatory mechanism, was greater in 450y than 20y trees. Despite compensatory mechanisms, hydraulic limitations to photosynthesis remain manifested in large, old Douglas-fir trees.

Keywords: carbon isotopes, age-related growth decline

This abstract is being presented at: 5:00 PM in session:
Oral Session #34: Water Relations in Trees.