Wood properties of P opulus and B etula in long‐term exposure to elevated CO 2 and O 3

We studied the interactive effects of elevated concentrations of CO 2 and O 3 on radial growth and wood properties of four trembling aspen ( P opulus tremuloides   M ichx.) clones and paper birch ( B etula papyrifera   M arsh.) saplings. The material for the study was collected from the A spen FACE (free‐air CO 2 enrichment) experiment in R hinelander ( WI , USA ). Trees had been exposed to four treatments [control, elevated CO 2 (560 ppm), elevated O 3 (1.5 times ambient) and combined CO 2  +  O 3 ] during growing seasons 1998–2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO 2 ‐ and O 3 ‐exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO 2 , radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O 3 decreased radial growth and the diameters of vessels and fibres. Clone‐specific decrease in wood density and cell wall thickness was observed under elevated CO 2 . 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.