Growth response of young birch trees (Betula pendula Roth.) after four and a half years of CO2 exposure

A field experiment consisting of 18 birch trees grown in open top chambers in ambient and elevated CO2 concentrations was set up with the aim of testing whether the positive growth response observed in many short-term studies is maintained after several growing seasons. We present the results of growth and biomass after 4.5 years of CO2 exposure, one of the longest studies so far on deciduous tree species. We found that elevated CO2 led to a 58% increase in biomass at the end of the experiment. However, estimation of stem mass during the growing season showed that elevated CO2 did not affect relative growth rate during the fourth growing season, and therefore, that the large accumulation of biomass was the result of an early effect on relative growth rate in previous years. Trees grown in elevated CO2 invested more carbon into fine roots and had relatively less leaf area than trees grown in ambient CO2, In contrast with previous studies, acceleration of growth did not involve a significant decline in nutrient concentrations of any plant tissue. It is likely that increased fine root density assisted the trees in meeting their nutrient demands. Changes in the species composition of the ectomycorrhizal fungi associated with the trees grown in elevated CO2 in favour of late successional species supports the hypothesis of an acceleration of the ontogeny of the trees in elevated CO2. (C) 1997 Annals of Botany Company.