Increased growth in elevated [CO2]: an early,short-term response?

Saplings of four clones of Sitka spruce and cherry were grown for three and two growing seasons, respectively, in open top chambers at two CO₂ concentrations (≈ 350 and ≈ 700 μmol mol^–1) to determine whether the increase in total biomass brought about by enhanced CO₂] is a result of a transient or persistent effect in nonlimiting conditions. Classical growth analysis was applied to both species and mean current relative growth rate of total dry mass (R_T) and leaf dry mass (R_L), and period relative growth rate of total dry mass ( ) and leaf dry mass ( ) were calculated. Sitka spruce saplings and cherry seedlings showed a positive growth response to elevated CO₂], and at the end of the experiments both species were ≈ 40% larger in elevated CO₂] than in ambient CO₂]. As a result, the period mean and were significantly higher in elevated CO₂]. The differences in plant dry mass at the end of the experiments were a consequence of the more rapid growth in the early phase of exposure to elevated CO₂]. After this initial phase mean R_T and R_L were similar or even lower in elevated CO₂] than in ambient CO₂]. NAR of both species was much higher in elevated CO₂], whereas both LAR, SLA, and LMR showed the opposite trend. The higher LAR and SLA of plants in ambient CO₂] contributed to a compensation by which they maintained R_T similar to that of elevated CO₂] saplings despite lower NAR and photosynthetic rate. However, when the same size the trees were similar amongst the CO₂] treatments, indicating that one of the main effect of elevated CO₂] on tree growth is to speed-up early development in all aspects.