Silver birch and climate change: variable growth and carbon allocation responses to elevated concentrations of carbon dioxide and ozone

We studied the effects of elevated concentrations of carbon dioxide (CO2]) and ozone (O3]) on growth, biomass allocation and leaf area of field-grown O3-tolerant (Clone 4) and O3-sensitive clones (Clone 80) of European silver birch (Betula pendula Roth) trees during 1999-2001. Seven-year-old trees of Clones 4 and 80 growing outside in open-top chambers were exposed for 3 years to the following treatments: outside control (OC); chamber control (CC); 2 x ambient CO2] (EC); 2 x ambient O3] (EO); and 2 x ambient CO2] + 2 x ambient O3] (EC+EO). When the results for the two clones were analyzed together, elevated CO2] increased tree growth and biomass, but had no effect on biomass allocation. Total leaf area increased and leaf abscission was delayed in response to elevated CO2]. Elevated O3] decreased dry mass of roots and branches and mean leaf size and induced earlier leaf abscission in the autumn; otherwise, the effects of elevated O3] were small across the clones. However, there were significant interactions between elevated CO2] and elevated O3]. When results for the clones were analyzed separately, stem diameter, volume growth and total biomass of Clone 80 were increased by elevated CO2] and the stimulatory effects of elevated CO2] on stem volume growth and total leaf area increased during the 3-year study. Clone 80 was unaffected by elevated O3]. In Clone 4, elevated O3] decreased root and branch biomass by 38 and 29%, respectively, whereas this clone showed few responses to elevated CO2]. Elevated CO2] significantly increased total leaf area in Clone 80 only, which may partly explain the smaller growth responses to elevated CO2] of Clone 4 compared with Clone 80. Although we observed responses to elevated O3], the responses to the EC+EO and EC treatments were similar, indicating that the trees only responded to elevated O3] under ambient CO2] conditions, perhaps reflecting a greater quantity of carbohydrates available for detoxification and repair in elevated CO2].