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

Summary Saplings of four clones of Sitka spruce and cherry were grown for three and two growing seasons, respectively, in open top chambers at two CO 2 concentrations (≈ 350 and ≈ 700 μmol mol –1 ) to determine whether the increase in total biomass brought about by enhanced [CO 2 ] 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 [CO 2 ], and at the end of the experiments both species were ≈ 40% larger in elevated [CO 2 ] than in ambient [CO 2 ]. As a result, the period mean and were significantly higher in elevated [CO 2 ]. 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 [CO 2 ]. After this initial phase mean R T and R L were similar or even lower in elevated [CO 2 ] than in ambient [CO 2 ]. NAR of both species was much higher in elevated [CO 2 ], whereas both LAR, SLA, and LMR showed the opposite trend. The higher LAR and SLA of plants in ambient [CO 2 ] contributed to a compensation by which they maintained R T similar to that of elevated [CO 2 ] saplings despite lower NAR and photosynthetic rate. However, when the same size the trees were similar amongst the [CO 2 ] treatments, indicating that one of the main effect of elevated [CO 2 ] on tree growth is to speed‐up early development in all aspects.