Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis

The extent to which stomatal conductance ( g s ) was capable of responding to reduced hydraulic conductance ( k )and preventing cavitation‐inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air‐injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), g s , and xylem pressure (Ψ p‐xylem ) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and g s . The k measurement was expressed per unit leaf area ( k l , leaf‐specific conductance). Leaves measured within 2 d of shoot injection showed reduced g s and E relative to non‐injected controls, and both parameters were strongly correlated with k l At this time, there was no difference in leaf Ψ p‐xylem between injected shoots and controls, and leaf Ψ p‐xylem was not significantly different from the highest cavitation‐inducing pressure (Ψ p‐cav ) in the branch xylem (‐1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of g s and E values to the control range. This was associated with a decrease in leaf Ψ p‐xylem below Ψ p‐cav and loss of foliage. The results suggest the stomata were incapable of long‐term regulation of E below control values and that reversion to higher E caused dieback via cavitation.