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Using standard rope techniques, we accessed the crowns of trees ranging in height from 95 m to 112.3 m with ecophysiological equipment (Figure 2). We measured xylem pressure potential with a Scholander-Hammel type pressure chamber (see Web Topic 3.5) on stems cut approximately 20 cm from the shoot tip. Photosynthesis, stomatal conductance, and transpiration were measured in ambient conditions of light, temperature, and vapor pressure with a LI6400 portable photosynthesis system (LiCor Inc. Lincoln, NE). Foliar carbon isotope composition (?13C, ‰) of dried and ground foliage was analyzed by isotope ratio mass spectrometry at the Colorado Plateau Stable Isotope Laboratory and used as an indicator of the time-integrated ratio of leaf internal to ambient CO2 concentration (Ci/Ca; see textbook Chapter 9 and Web Topic 9.5; Farquhar et al., 1982). Xylem vulnerability to cavitation was measured by the centrifuge method (Pockman et al., 1995) on branch segments (8 to 9 mm diam.) collected in the upper crown (avg. ht. = 110 m) and lower crown (avg. ht. = 57 m) of 6 trees.
Xylem pressure was strongly correlated with height, both at predawn and at midday when the leaf-to-air vapor pressure difference was highest (Figure 3, Part A). The slopes of the predawn gradients were not significantly different from that necessary to hold water against gravity (ca. –0.01 MPa m–1). Slopes of midday gradients did not differ from predawn gradients during the wet season, but during the dry season were significantly higher for some trees, up to –0.014 MPa m–1, indicative of the increased driving force necessary to move water rapidly through the xylem. Minimum xylem pressures were always above –2.0 MPa in the upper crown, similar to that of other conifers (Pinol and Sala, 2000).
Stomatal conductance and photosynthesis measured at saturating light in the upper crown (ca. 105 m) during the dry season decreased by an average of 43% and 33%, respectively, from mid-morning when air vapor pressure deficit (VPD) and xylem tension were low to mid afternoon when VPD and xylem tension reached their daily maxima. Values of Ci/Ca calculated from gas exchange measurements spanned a range that included the time-integrated estimates based on stable carbon isotope analysis (see below). Thus, stomatal conductance near maximum tree height responds to conditions associated with increasing water stress and is strongly limiting to photosynthesis at midday during the dry season.
Stable carbon isotope composition (?13C) of foliage was strongly and positively correlated with height in all study trees (Figure 3, Part B). Maximum values of nearly –21‰ at the tops of the tallest trees indicate low Ci/Ca ratios similar to those for perennial plants from extremely arid environments. Minimum δ13C within the crown ranged from –27‰ to –31‰ (see textbook, pp. 216–218), values indicating minor stomatal limitation of photosynthesis as occurs in plants from mesic environments.
Mean water potential for 50% cavitation (Ψ50%) was estimated to be –6.1 MPa for all branches measured. In paired branches from upper and lower crown positions in each of 6 trees, lower crown branches were always more susceptible to water-stress-caused-embolism than top branches (Ψ50% lower = –5.6 MPa, upper = –6.6 MPa, paired t-test: df = 5, t = –3.47, p = 0.018). The observed difference in Ψ50% between upper and lower branches was not significantly different from the difference in the hydrostatic component of water potential between these heights (paired t-test: df = 5, t = –1.69, p = 0.15).
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