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Effects of ambient and elevated temperature and atmospheric carbon dioxide concentration ([CO₂]) on CO₂ assimilation rate and the structural and phenological development of shoots during their first growing season were studied in 45-year-old Norway spruce trees (Picea abies (L.) Karst.) enclosed in whole-tree chambers. Continuous measurements of net assimilation rate (NAR) in individual buds and shoots were made from early bud development to late August in two consecutive years. The largest effect of elevated temperature (T_E) was manifest early in the season as an earlier start and completion of shoot length development, and a 1–3-week earlier shift from negative to positive NAR compared with the ambient temperature (T_A) treatments. The largest effect of elevated [CO₂] (C_E) was found later in the season, with a 30% increase in maximum NAR compared with trees in the ambient [CO₂] treatments …