Tropospheric O 3 moderates responses of temperate hardwood forests to elevated CO 2 : a synthesis of molecular to ecosystem results from the Aspen FACE project

1.   The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2.   The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34–36 nL L −1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3.   After 3 years of exposure to 560 µmol mol −1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72125/1/j.1365-2435.2003.00733.x.pd

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated Co \u3c inf\u3e 2 and/or O \u3c inf\u3e 3

Leaf gas exchange parameters and the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO2 and/or O3. The plants were exposed either to ambient air (control), elevated CO2 (560 p.p.m.) elevated O3 (55 p.p.b.) or a mixture of elevated CO2 and O3 in a free air CO2 enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light-saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO2 and decreased by O3 at both control and elevated CO2. The relative stomatal limitation to photosynthesis (ls) was in both clones about 10% under control and elevated O3. Exposure to elevated CO2 + O3 in both clones and to elevated CO2 in clone 259, decreased ls even further - to about 5%. The corresponding changes in Rubisco content and the stability of Ci/Ca ratio suggest that the changes in photosynthesis in response to elevated CO2 and O3 were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O3 tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable Ci under the given treatment, that indicates close coupling between stomatal and mesophyll processes

Carbon gain and bud physiology in Populus tremuloides and Betula papyrifera grown under long-term exposure to elevated concentrations of CO2 and O3

Paper birch (Betula papyrifera Marsh.) and three trembling aspen clones (Populus tremuloides Michx.) were studied to determine if alterations in carbon gain in response to an elevated concentration of CO2 ([CO2]) or O3 ([O3]) or a com- bination of both affected bud size and carbohydrate composi- tion in autumn, and early leaf development in the following spring. The trees were measured for gas exchange, leaf size, date of leaf abscission, size and biochemical characteristics of the overwintering buds and early leaf development during the 8th–9th year of free-air CO2 and O3 exposure at the Aspen FACE site located near Rhinelander, WI. Net photosynthesis was enhanced 49–73% by elevated [CO2], and decreased 13–30% by elevated [O3]. Elevated [CO2] delayed, and ele- vated [O3] tended to accelerate, leaf abscission in autumn. Ele- vated [CO2] increased the ratio of monosaccharides to di- and oligosaccharides in aspen buds, which may indicate a lag in cold acclimation. The total carbon concentration in over- wintering buds was unaffected by the treatments, although ele- vated [O3] decreased the amount of starch by 16% in birch buds, and reduced the size of aspen buds, which may be related to the delayed leaf development in aspen during the spring. Ele- vated [CO2] generally ameliorated the effects of elevated [O3]. Our results show that both elevated [CO2] and elevated [O3] have the potential to alter carbon metabolism of overwintering buds. These changes may cause carry-over effects during the next growing season

Analysis of a Farquhar-von Caemmerer-Berry leaf-level photosynthetic rate model

journal homepage: www.elsevier.com/locate/envpo

Increasing leaf hydraulic conductance with transpiration rate minimizes the water potential drawdown from stem to leaf

Leaf hydraulic conductance (k (leaf)) is a central element in the regulation of leaf water balance but the properties of k (leaf) remain uncertain. Here, the evidence for the following two models for k (leaf) in well-hydrated plants is evaluated: (i) k (leaf) is constant or (ii) k (leaf) increases as transpiration rate (E) increases. The difference between stem and leaf water potential (ΔΨ(stem–leaf)), stomatal conductance (g (s)), k (leaf), and E over a diurnal cycle for three angiosperm and gymnosperm tree species growing in a common garden, and for Helianthus annuus plants grown under sub-ambient, ambient, and elevated atmospheric CO(2) concentration were evaluated. Results show that for well-watered plants k (leaf) is positively dependent on E. Here, this property is termed the dynamic conductance, k (leaf(E)), which incorporates the inherent k (leaf) at zero E, which is distinguished as the static conductance, k (leaf(0)). Growth under different CO(2) concentrations maintained the same relationship between k (leaf) and E, resulting in similar k (leaf(0)), while operating along different regions of the curve owing to the influence of CO(2) on g (s). The positive relationship between k (leaf) and E minimized variation in ΔΨ(stem–leaf). This enables leaves to minimize variation in Ψ(leaf) and maximize g (s) and CO(2) assimilation rate over the diurnal course of evaporative demand