Canopy stomatal conductance in Pinus monticola and Tsuga heterophylla trees growing in stands of differing height and age in an Inland Pacific Northwest mixed conifer forest.
Pangle, Robert*,1, Kavanagh, Kathleen*,1, Schotzko, Alisa*,1, 1 University of Idaho, Moscow, Idaho, United States
ABSTRACT- According to the hydraulic limits theory, tall trees experience greater water stress and deceased stomatal conductance as path-length from the soil to leaf increases. We tested this theory using sap-flux measurements of canopy stomatal conductance (Gs) in even-aged mixed conifer stands growing in the Priest River Experimental Forest of northern Idaho. The study included six dominant Pinus monticola (PIMO) trees varying in height from 10.7 to 37.9 m, and eleven dominant Tsuga heterophylla (TSHE) trees from 6.1 to 30.5 m. Tree leaf area ranged from 26.96 to 163.9 m2 for PIMO trees, and from 31.5 to 350.0 m2 for TSHE. We calculated mean daily Gs on Julian days 167, 176, and 254, days when soil water content was not overly limiting (16.0 to 28.4 %), vapor pressure deficit (VPD) was above a 0.6 kPa threshold, cloudless conditions prevailed, and wind (greater than 0.5 m s-1) was present. Daily mean VPD for periods which Gs was reported ranged from 1.34 to 2.48 kPa. PIMO mean canopy Gs (per unit leaf area) on day 167 for each height class was 84.96 (SE = 0.29), 54.04 (SE = 3.95), and 23.48 mmol m-2 s-1 (SE = 1.32) respectively, for short, intermediate, and tall PIMO trees. TSHE mean canopy Gs on day 176 was 20.80 (SE = 0.36), 15.54 (SE = 0.47), and 13.37 mmol m-2 s-1 (SE = 1.70) respectively, for short, intermediate and tall TSHE trees. Day 254 Gs values for PIMO was 72.25 (SE = 5.95), 45.40 (SE = 2.00), and 26.85 mmol m-2 s-1 (SE = 0.75) respectively, for short, intermediate, and tall PIMO trees. Finally, Day 254 TSHE mean Gs was 35.77 (SE = 7.56), 29.37 (SE = 1.37), and 24.79 mmol m-2 s-1 (SE = 3.40) respectively, for short, intermediate, and tall TSHE trees. Overall, Gs in the tall height class was approximately 32% and 67% of that observed in the short height class in PIMO and TSHE respectively. Consistent with the hydraulic limits theory, Gs declined with height in PIMO. In contrast, a strong decline gradient with height was not observed in TSHE, where the relative difference in mean daily Gs was much less pronounced between height classes. These findings are consistent with observed trends in stable carbon isotopes of bulk foliage, foliar carbohydrates and soil respired CO2 at these sites.
Key words: canopy conductance, tree height, hydraulic limits, stomatal conductance
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