Growth and resource use of birch seedlings under elevated carbon dioxide and temperature

The effects of elevated CO2 and temperature on the growth, resource acquisition and resource allocation of small birch seedlings (Betula pendula Roth.) were examined under conditions of non-limiting soil, water and nutrient supply. Seedlings were planted in pots and placed in controlled environment chambers either under normal ambient conditions (CON), or in the presence of elevated CO2 (approx. 700 mu mol mol(-1); Elev. C), elevated temperature (approx. 3 degreesC above the outside ambient temperature; Elev. T) or a combination of elevated CO2 and elevated temperature (Elev. C + T). Both Elev. C and Elev. T significantly increased biomass accumulation, but the extent of the increase depended greatly on the stage of development of the seedlings. Furthermore, the theoretically expected positive effect of the warmer temperature on the CO2-induced stimulation of growth was not observed. By analysing resource acquisition (i.e. CO2, nitrogen and water uptake), seedling development, leaf area production and the allocation pattern, it was deduced that the CO2-stimulated increase in biomass resulted mainly from the initial 'fertilization' effect of CO2 while the temperature-induced increase in biomass stemmed from higher net carbon intake during the middle and latter parts of the growing season achieved by Virtue of the increased leaf area and larger photosynthetic capacity. The lack of positive stimulation by temperature under Elev. C + T may be related in part to (1) CO2-induced acceleration of seedling development, which led to a small or no response to CO2 enrichment and lower leaf area production during the latter part of the growth season, and (2) a cumulative delay in the response of growth to the warmer temperature, which did not increase net carbon intake when the seedlings were at a juvenile stage. Neither Elev. C nor Elev. T altered the root:shoot ratio during early growth, but Elev. C increased it during the latter part of the growth season while Elev. T decreased it, possibly on account of a change in leaf area retention. Finally, the nitrogen and water use efficiencies of seedlings at different stages of development are discussed. (C) 2001 Annals of Botany Company.