Branch bags were used to expose branches on mature Sitka spruce trees to either ambient [CO2] (A) or elevated
[CO2] (E) for 4 yr. This paper reports the effects of this treatment on the growth, development and phenology of
the branches, including shoot expansion, shoot numbers, needle dimensions, needle numbers and stomatal
density. The effect of elevated [CO2] on the relationship between leaf area and sapwood area was investigated.
Exposure to elevated [CO2] doubled photosynthetic rates in current-year shoots and, despite some down-regulation, 1-yr-old E shoots also had higher rates of photosynthesis than their A counterparts. Thus, the amount
of assimilate fixed by E branches was substantially more than that fixed by A branches; however, this increase in
the local production of assimilate did not lead to an increase in non-structural carbohydrate or stimulate growth
or meristematic activity within the E branches. There was a very consistent relationship between leaf area and stem
cross-sectional area that was not influenced by [CO2]. However, unbagged branches had thicker stems than bagged
branches, resulting in a slightly lower ratio of leaf area to cross-sectional area. The implications of the results for
the modelling of growth and allocation and the potential utility of the branch bag technique are discussed.