Sap flux in pure aspen and mixed aspen–birch forests exposed to elevated concentrations of carbon dioxide and ozone
Johan Uddling (1, 2, 3), Ronald M. Teclaw (4), Mark E. Kubiske (4), Kurt S. Pregitzer (5) and David S. Ellsworth (6)
1. School of Natural Resources and Environment, University of Michigan, 440 Church Street, Ann Arbor, MI 48109, USA / 2. Present address: Department of Plant and Environmental Sciences, University of Gothenburg, P.O. Box 461, SE-405 30 Göteborg,
Sweden / 3. Corresponding author () / 4. USDA Forest Service, Northern Research Station, 5985 Hwy K, Rhinelander, WI 54501, USA / 5. Ecosystem Science Center, School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend
Ave., Houghton, MI 49931, USA / 6. Centre for Plant and Food Science, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia / Received October 17, 2007; accepted February 28, 2008; published online June 2, 2008
Summary
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 O3 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.
Keywords:
climate, FACE, leaf area index, soil water, transpiration, tree community.
