title: Transpiration and whole-tree conductance in ponderosa pine trees of different heights.
abstract: 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 ponderosa pine trees in two size classes (12 m tall and ∼40 years old, and 36 m tall and ∼ 290 years old) to determine if transpiration (E) and whole-tree conductance (gt) differed between the two sizes of trees. For both size classes, E was approximately equal to Q measured 2 m above the ground: Q was most highly correlated with current, not lagged, water 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-1for the 12-m trees and 0.57 mmol m-2(leaf area) s-1for the 36-m trees. When photosynthetically active radiation (IP) exceeded the light saturation for photosynthesis in ponderosa pine (900 µmol m-2(ground) s-1), differences in E were more pronounced: 2.4 mmol m-2(leaf area) s-1for the 12-m trees and 1.2 mmol m-2s-1for the 36-m trees, yielding gtof 140 mmol m-2(leaf area) s-1for the 12-m trees and 72 mmol m-2s-1for 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 compared to 170 mm for the 12-m trees, a difference of 15% of average annual precipitation. Lower gtin the taller trees also likely lowers photosynthesis during the growing season.
authors.affiliation: USDA Forest Service, Rocky Mountain Research Station, 240 West Prospect RD, Fort Collins, CO 80526, USA e-mail: mryan@ lamar.colostate.edu Tel.: +1-970-4981012, Fax +1-970-4981010, , , , , , US.
authors.forename: M G
authors.initials: MG
authors.lastname: Ryan
authors.affiliation: Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA, , , , , , US.
authors.forename: B J
authors.initials: BJ
authors.lastname: Bond
authors.affiliation: Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA, , , , , , US.
authors.forename: B E
authors.initials: BE
authors.lastname: Law
authors.affiliation: USDA Forest Service, Rocky Mountain Research Station, 240 West Prospect RD, Fort Collins, CO 80526, USA e-mail: mryan@ lamar.colostate.edu Tel.: +1-970-4981012, Fax +1-970-4981010, , , , , , US.
authors.forename: R M
authors.initials: RM
authors.lastname: Hubbard
authors.affiliation: Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA, , , , , , US.
authors.forename: D
authors.initials: D
authors.lastname: Woodruff
authors.affiliation: Department of Soil Sciences (Environmental Physics), Swedish University of Agricultural Sciences, PO Box 7014, 75007, Uppsala, Sweden, , , , , , SE.
authors.forename: E
authors.initials: E
authors.lastname: Cienciala
authors.affiliation: Environmental Measuring Systems, Turisticka 5, 62100 Brno, Czech Republic, , , , , , CZ.
authors.forename: J
authors.initials: J
authors.lastname: Kucera
keywords: Humidity response of stomatal conductance; Hydraulic conductance; Key words Transpiration; Whole-tree conductance