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 (approximate to 350 and approximate to 700 mu mol 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 (R-T(t)) and leaf dry mass (R-L(t)) 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 approximate to 40% larger in elevated [CO2] than in ambient [CO2]. As a result, the period mean R-T(t) and R-L(t) 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.