Sap flux in pure aspen and mixed aspen-birch forests exposed to elevated concentrations of carbon dioxide and ozone |
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| Authors: | Uddling Johan Teclaw Ronald M Kubiske Mark E Pregitzer Kurt S Ellsworth David S |
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| Institution: | School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI 48109, USA. johan.uddling@dpes.gu.se |
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| Abstract: | Elevated concentrations of atmospheric carbon dioxide (CO2]) and tropospheric ozone (O3]) have the potential to affect tree physiology and structure and hence forest water use, which has implications for climate feedbacks. We investigated how a 40% increase above ambient values in CO2] and O3], alone and in combination, affect tree water use of pure aspen and mixed aspen-birch forests in the free air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). Measurements of sap flux and canopy leaf area index (L) were made during two growing seasons, when steady-state L had been reached after more than 6 years of exposure to elevated CO2] and O3]. Maximum stand-level sap flux was not significantly affected by elevated O3], but was increased by 18% by elevated CO2] averaged across years, communities and O(3) regimes. Treatment effects were similar in pure aspen and mixed aspen-birch communities. Increased tree water use in response to elevated CO2] was related to positive CO2 treatment effects on tree size and L (+40%). Tree water use was not reduced by elevated O3] despite strong negative O3 treatment effects on tree size and L (-22%). Elevated O3] predisposed pure aspen stands to drought-induced sap flux reductions, whereas increased tree water use in response to elevated CO2] did not result in lower soil water content in the upper soil or decreasing sap flux relative to control values during dry periods. Maintenance of soil water content in the upper soil in the elevated CO2] treatment was at least partly a function of enhanced soil water-holding capacity, probably a result of increased organic matter content from increased litter inputs. Our findings that larger trees growing in elevated CO2] used more water and that tree size, but not maximal water use, was negatively affected by elevated O3] suggest that the long-term cumulative effects on stand structure may be more important than the expected primary stomatal closure responses to elevated CO2] and O3] in determining stand-level water use under possible future atmospheric conditions. |
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