Wood properties of trembling aspen and paper birch after 5 years of exposure to elevated concentrations of CO₂ and O₃

We investigated the interactive effects of elevated concentrations of carbon dioxide ([CO₂]) and ozone ([O₃]) 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 CO₂ enrichment) experiment in Rhinelander, WI, where the saplings had been exposed to four treatments: control, elevated [CO₂] (560 ppm), elevated [O₃] (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 [CO₂] decreased uronic acids (constituents of, e.g., hemicellulose) and tended to increase stem diameter. In response to elevated [O₃] exposure, acid-soluble lignin concentration decreased and vessel lumen diameter tended to decrease. Elevated [O₃] 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 [CO₂] 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.