Wood properties of Populus and Betula in long-term exposure to elevated CO2 and O3.

We studied the interactive effects of elevated concentrations of CO2 and O3 on radial growth and wood properties of four trembling aspen (Populus tremuloides Michx.) clones and paper birch (Betula papyrifera Marsh.) saplings. The material for the study was collected from the Aspen FACE (free-air CO2 enrichment) experiment in Rhinelander (WI, USA). Trees had been exposed to four treatments [control, elevated CO2 (560 ppm), elevated O3 (1.5 times ambient) and combined CO2 + O3 ] during growing seasons 1998-2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO2- and O3-exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO2, radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O3 decreased radial growth and the diameters of vessels and fibres. Clone-specific decrease in wood density and cell wall thickness was observed under elevated CO2 . In birch, the treatments had no major impacts on wood anatomy or wood density. Our study indicates that short-term impact studies conducted with young seedlings may not give a realistic view of long-term ecosystem responses.

Rising atmospheric CO2 concentration partially masks the negative effects of elevated O3 in silver birch (Betula pendula Roth).

This review summarizes the main results from a 3-year open top chamber experiment, with two silver birch (Betula pendula Roth) clones (4 and 80) where impacts of 2x ambient [CO2] (EC) and [O3] (EO) and their combination (EC + EO) were examined. Growth, physiology of the foliage and root systems, crown structure, wood properties, and biological interactions were assessed to understand the effects of a future climate on the biology of silver birch. The clones displayed great differences in their reaction to EC and EO. Growth in clone 80 increased by 40% in EC and this clone also appeared O3-tolerant, showing no growth reduction. In contrast, growth in clone 4 was not enhanced by EC, and EO reduced growth with root growth being most affected. The physiological responses of the clones to EO were smaller than expected. We found no O3 effect on net photosynthesis in either of the clones, and many parameters indicated no change compared with chamber controls, suggesting active detoxification and defense in foliage. In EO, increased rhizospheric respiration over time and accelerated leaf senescence was common in both clones. We assumed that elevated O3 offsets the positive effects of elevated CO2 when plants were exposed to combined EC + EO treatment. In contrast, the responses to EC + EO mostly resembled the ones in EC, at least partly due to stomatal closure, which thus reduced O3 flux to the leaves. However, clear cellular level symptoms of oxidative stress were observed also in EC + EO treatment. Thus, we conclude that EC masked most of the negative O3 effects during long exposure of birch to EC + EO treatment. Biotic interactions were not heavily affected. Only some early season defoliators may suffer from faster maturation of leaves due to EO.

Wood properties of trembling aspen and paper birch after 5 years of exposure to elevated concentrations of CO(2) and O(3).

We investigated the interactive effects of elevated concentrations of carbon dioxide ([CO(2)]) and ozone ([O(3)]) on radial growth, wood chemistry and structure of five 5-year-old trembling aspen (Populus tremuloides Michx.) clones and the wood chemistry of paper birch (Betula papyrifera Marsh.). Material for the study was collected from the Aspen FACE (free-air CO(2) enrichment) experiment in Rhinelander, WI, where the saplings had been exposed to four treatments: control, elevated [CO(2)] (560 ppm), elevated [O(3)] (1.5 x ambient) and their combination for five growing seasons. Wood properties of both species were altered in response to exposure to the treatments. In aspen, elevated [CO(2)] decreased uronic acids (constituents of, e.g., hemicellulose) and tended to increase stem diameter. In response to elevated [O(3)] exposure, acid-soluble lignin concentration decreased and vessel lumen diameter tended to decrease. Elevated [O(3)] increased the concentration of acetone-soluble extractives in paper birch, but tended to decrease the concentration of these compounds in aspen. In paper birch, elevated [CO(2)] decreased and elevated [O(3)] increased starch concentration. The responses of wood properties to 5 years of fumigation differed from those previously reported after 3 years of fumigation.