Phenology and growth of shoots, needles, and buds of Douglas-fir seedlings with 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 multiple growing season impacts on specific flushes. Thus, the effects of ambient or elevated CO2 (approximately 200 mu mol.mol(-1) above ambient) and ambient or elevated temperature (approximately 4 degrees C above ambient) were evaluated for both main and second (lammas) flushes of Douglas-fir (Pseudotsuga menziesii (Mirb.) France). Established seedlings were grown for three full growing seasons in outdoor, sunlit chambers, which maintained diel and seasonal variation in climate. A reconstructed forest soil was used with 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 terminal shoots, needles, or buds. In contrast, compared with ambient temperature, elevated temperature resulted in higher shoot and needle growth rates early in the season; reduced final terminal shoot length; and either reduced, increased, or unchanged final needle length, depending on season. Initiation of the lammas flush was delayed and (or) decreased at elevated temperature. Leading terminal bud break and growth occurred earlier; however, resting bud length was reduced, and bud width tended to increase with elevated temperature. Thus, at least during early seedling growth, elevated temperatures may reduce both main- and lammas-flush growth, thereby altering tree productivity, whereas elevated CO2 may have little effect on main or lammas growth at either the current or elevated temperature.