Phenology and growth of shoots, needles, and buds of Douglas-fir seedlingswith elevated CO2 and (or) temperature

Increased atmospheric CO2 and global warming may affect overall tree growth , but impacts of these combined stresses are largely unknown in terms of mu ltiple growing season impacts on specific flushes. Thus, the effects of amb ient or elevated CO2 (approximately 200 mu mol.mol(-1) above ambient) and a mbient or elevated temperature (approximately 4 degrees C above ambient) we re evaluated for both main and second (lammas) flushes of Douglas-fir (Pseu dotsuga menziesii (Mirb.) France). Established seedlings were grown for thr ee full growing seasons in outdoor, sunlit chambers, which maintained diel and seasonal variation in climate. A reconstructed forest soil was used wit h a seasonal wet and dry cycle and without added fertilizer. Compared with ambient CO2 elevated CO2 had no impact on overall phenology and growth of t erminal shoots, needles, or buds. In contrast, compared with ambient temper ature, elevated temperature resulted in higher shoot and needle growth rate s early in the season; reduced final terminal shoot length; and either redu ced, increased, or unchanged final needle length, depending on season. Init iation of the lammas flush was delayed and (or) decreased at elevated tempe rature. Leading terminal bud break and growth occurred earlier; however, re sting bud length was reduced, and bud width tended to increase with elevate d temperature. Thus, at least during early seedling growth, elevated temper atures may reduce both main- and lammas-flush growth, thereby altering tree productivity, whereas elevated CO2 may have little effect on main or lamma s growth at either the current or elevated temperature.