Growth response of branches of Picea sitchensis to four years exposure to elevated atmospheric carbon dioxide concentration

Branch bags were used to expose branches on mature Sitka spruce trees to ei ther ambient [CO2] (A) or elevated [CO2] (E) for 4 yr. This paper reports t he effects of this treatment on the growth, development and phenology of th e branches, including shoot expansion, shoot numbers, needle dimensions, ne edle numbers and stomatal density. The effect of elevated [CO2] on the rela tionship between leaf area and sapwood area was investigated. Exposure to e levated [CO2] doubled photosynthetic rates in current-fear shoots and, desp ite some downregulation, 1-yr-old E shoots also had higher rates of photosy nthesis than their A counterparts. Thus, the amount of assimilate fixed by E branches was substantially more than that fixed by A branches; hen-ever, this increase in the local production of assimilate did not lead to an incr ease in non-structural carbohydrate or stimulate growth or meristematic act ivity within the E branches. There was a very consistent relationship betwe en leaf area and stem cross-sectional area that was not influenced by [CO2] . However, unbagged branches had thicker stems than bagged branches, result ing in a slightly lower ratio of leaf area to cross-sectional area. The imp lications of the results for the modelling of growth and allocation and the potential utility of the branch bag technique are discussed.