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We tested for reductions in water transport with increasing tree size, a key component in determining whether gas exchange and growth are hydraulically limited in tall trees. During the summers of 1998 and 1999, we measured water transport with Granier-type, constant-heat sap flow probes, vapor pressure deficit, and leaf and soil water potentials in overstory  Pseudotsuga menziesii  (Mirb.) Franco trees in three stands differing in size and age (15, 32 and 60 m in height and about 20, 40 and 450 years in age, respectively) in a  P. menziesii -dominated forest in the Pacific Northwest, USA. A total of 24 trees were equipped with sap flow sensors—six 60-m trees, nine 32-m trees and nine 15-m trees. Based on the sap flow measurements and leaf area information estimated from leaf area–sapwood area relationships, we estimated crown-averaged stomatal conductance ( G  S ) and leaf-specific hydraulic conductance ( K  L ). We tested the hypothesis that  G  S  and  K  L  vary inversely with tree height (15 &gt; 32 &gt; 60 m). Analysis of variance of  G  S  ranked as 15 = 60 &gt; 32 m during the early summer and 15 &gt; 60 &gt; 32 m during late season drought. Over the growing season, mean daily  G  S  (± SE) was 29.2 ± 4.4, 24.0 ± 6.8 and 17.7 ± 7.2 mmol m −2  s −1  for the 15-, 60- and 32-m trees, respectively. The value of  K  L  differed among tree heights only during late season drought and ranked 15 &gt; 32 = 60 m. A hydraulic mass balance suggests that greater sapwood conductivity in 60-m trees compared with 32- and 15-m trees is a likely cause for the departure of the above rankings from those predicted by height and leaf-to-sapwood area ratio.

Canopy and hydraulic conductance in young, mature and old Douglas-fir trees