Elevated temperature but not elevated CO2 affects long-term patterns of stem diameter and height of Douglas-fir seedlings

Global climatic change may impact forest productivity, but data are lacking on potential effects of elevated CO 2 and temperature on tree growth. We determined changes in shoot growth for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings exposed to ambient or elevated CO 2 ( µmol·mol -1 ), and ambient or elevated temperature . Seedings were grown for 4 years (three complete growing seasons) in outdoor, sunlit chambers. In each season, height growth was initiated earlier and, in two seasons, ceased earlier for elevated compared with ambient temperature trees. Elevated temperature reduced intermediate and final plant heights. Stem diameter growth began earlier each season at the elevated compared with the ambient temperature, but temperature had no affect on final stem diameter. Elevated temperature tended to reduce leaf (p = 0.07) but not woody biomass. Elevated CO 2 had no significant effects on stem diameter, height, and leaf or woody biomass, and there were no significant CO 2 × temperature interactions. Thus, elevated temperatures (but not elevated CO 2 ) associated with climate change may decrease seedling canopy growth as indicated by reduced height and leaf biomass but have little or no effect on overall woody growth as indicated by stem diameter and woody biomass.