Transpiration and whole-tree conductance in ponderosa pine trees of different heights

Changes in leaf physiology with tree age and size could alter forest growth , water yield, and carbon fluxes. We measured tree water flux (Q) for 14 po nderosa pine trees in two size classes (12 m tall and similar to 40 years o ld, and 36 m tall and similar to 290 years old) to determine if transpirati on (E) and whole-tree conductance (g(t)) differed between the two sizes of trees. For both size classes, E was approximately equal to Q measured 2 m a bove the ground: Q was most highly correlated with current, not lagged, wat er vapor pressure deficit, and night Q was <12% of total daily flux. E for days 165-195 and 240-260 averaged 0.97 mmol m(-2) (leaf area, projected) s( -1) for the 12-m trees and 0.57 mmol m(-2) (leaf area) s(-1) for the 36-m t rees. When photosynthetically active radiation (I-P) exceeded the light sat uration for photosynthesis in ponderosa pine (900 mu mol m(-2) (ground) s(- 1)), differences in E were more pronounced: 2.4 mmol m(-2) (leaf area) s(-1 ) for the 12-m trees and 1.2 mmol m(-2) s(-1) for the 36-m trees, yielding g(t) of 140 mmol m(-2) (leaf area) s(-1) for the 12-m trees and 72 mmol m(- 2) s(-1) for the 36-m trees. Extrapolated to forests with leaf area index = 1, the 36-m trees would transpire 117 mm between 1 June and 31 August comp ared to 170 mm for the 12-m trees, a difference of 15% of average annual pr ecipitation. Lower g(t) in the taller trees also likely lowers photosynthes is during the growing season.