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The physiological mechanisms responsible for reduced extension growth as trees increase in height remain elusive. We evaluated biophysical constraints on leaf expansion in old-growth Douglas-fir ( Pseudotsuga menziesii  (Mirb.) Franco) trees. Needle elongation rates, plastic and elastic extensibility, bulk leaf water (Ψ L ) and osmotic (Ψ π ) potential, bulk tissue yield threshold and final needle length were characterized along a height gradient in crowns of &gt; 50-m-tall trees during the period between bud break and full expansion (May to June). Although needle length decreased with increasing height, there was no height-related trend in leaf plastic extensibility, which was highest immediately after bud break (2.9%) and declined rapidly to a stable minimum value (0.3%) over a 3-week period during which leaf expansion was completed. There was a significant positive linear relationship between needle elongation rates and plastic extensibility. Yield thresholds were consistently lower at the upper and middle crown sampling heights. The mean yield threshold across all sampling heights was 0.12 ± 0.03 MPa on June 8, rising to 0.34 ± 0.03 MPa on June 15 and 0.45 ± 0.05 MPa on June 24. Bulk leaf Ψ π  decreased linearly with increasing height at a rate of 0.004 MPa m −1  during the period of most rapid needle elongation, but the vertical osmotic gradient was not sufficient to fully compensate for the 0.015 MPa m −1  vertical gradient in Ψ L , implying that bulk leaf turgor declined at a rate of about 0.011 MPa m −1  increase in height. Although height-dependent reductions in turgor appeared to constrain leaf expansion, it is possible that the impact of reduced turgor was mitigated by delayed phenological development with increasing height, which resulted in an increase with height in the temperature during leaf expansion.

Biophysical constraints on leaf expansion in a tall conifer