Evidence that hydraulic conductance limits photosynthesis in old Pinus ponderosa trees

We tested the hypotheses that hydraulic conductance is lower in old (about 250 years old and 30 m tall) compared to young (about 40 years old and 10 m tall) Pinus ponderosa Dougl. ex Laws. trees and that lower hydraulic condu ctance of old trees limits their photosynthesis. Hydraulic conductance at t he end of summer 1995, calculated from leaf water potential and leaf gas ex change measurements on one-year-old needles, was 44% lower in old trees com pared to young trees growing in a mixed age-class stand on the east slope o f the Oregon Cascades. Whole-tree sapflow per unit leaf area averaged 53% l ower in old trees compared to young trees and mean hydraulic conductance ca lculated from sapflow and water potential data was 63% lower in old trees t han in young trees. For the entire summer, stomatal conductance (g(s)) and assimilation (A) declined more steeply with air saturation deficit (D) in o ld trees than in young trees. For both old and young trees, mean g(s) and A were approximately 32 and 21% lower, respectively, at typical midday D val ues (2.5-3.0 kPa). We hypothesized that if hydraulic conductance limits g(s ), and A, then increasing or decreasing the leaf specific conductance of a branch will result in proportional changes in the responses of g(s), and A with D. Removal of 50% of the foliage from a set of experimental branches o n old trees caused g(s), and A to decline less steeply with D in early summ er, but values were not significantly different from control values in late summer. Cutting transverse notches in branches on young trees had no effec t on the responses of g(s), and A with D. Leaf nitrogen content and photosy nthetic capacity were similar suggesting that differences in g(s), and A be tween old and young trees were not caused by differences in photosynthetic capacity.