Control of transpiration in a 220-year-old Abies amabilis forest

We measured sap flow at the branch and tree levels, and calculated tree transpiration at the stand level, in a 220-year-old Abies amabilis (Dougl.) Forbes forest. Temporal and spatial patterns of branch sap flow rate per unit leaf area reflected differences in canopy position and diurnal variation in radiation exposure. Average leaf area normalized branch conductance of upper canopy branches ranged from 0.50 to 1.01 mm s(-1). Maximum leaf area normalized tree sap flow rates were similar to those previously measured in a younger A. amabilis forest (about 80 g m(-2) leaf area h(-1)), but dominant trees in the old growth stand transpired approximately three times more per day (up to 281 kg H2O per day) than dominant trees in the younger forest. This difference was attributed primarily to leaf area: dominant trees in the old growth stand had approximately three times more leaf area than those in the younger Abies stand. Crown conductance on a ground area basis varied by an order of magnitude between the smallest and largest trees measured (maximum 4.84 mm. s(-1) and mean 1.44 mm s(-1) versus maximum 37.54 mm s(-1) and average 12.16 mm s(-1), respectively). There was considerable spatial and temporal variation in stomatal versus boundary layer control of transpiration as expressed by the Omega decoupling coefficient. Omega for the largest measured tree ranged from less than 0.1 to greater than 0.6, and remained above 0.3 for more than 9% of the daylight hours monitored. The degree of decoupling decreased with tree size, with Omega of the smallest tree sampled never exceeding 0.3. Daily stand transpiration ranged from less than 0.4 mm to greater than 3.3 mm. depending on radiation and vapor pressure deficit. (C) 2001 Elsevier Science B.V. All rights reserved.