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Guak, S.H., David M. Olszyk, L.H. Fuchigami, and David T. Tingey. 1998. Effects of elevated CO2 and temperature on cold hardiness and spring bud burst and growth in Douglas-fir (Pseudotsuga menziesii). Tree Physiology 18:671-679.

We examined effects of elevated CO2 and temperature on cold hardiness and bud burst of Douglas-fir (Pseudatsuga menziesii (Mirab.) Franco) seedlings. Two-year old seedlings were grown for 2.5 years in semi-closed, sunlit chambers at either ambient or elevated (ambient approximately equal to 4 degreesC) air temperature in the presence of an ambient or elevated (ambient approximately  200 ppm) CO2 concentration. The elevated temperature treatment delayed needle cold hardening in the autumn and slowed dehardening in the spring. At maximum hardiness, trees in the elevated temperature treatment were less hardy by about 7 degrees C than trees in the ambient temperature treatment. In general, trees exposed to elevated CO2 were slightly less hardy during hardening and dehardening than trees exposed to ambient CO2. For trees in the elevated temperature treatments, date to 30% burst of branch terminal buds was advanced by about 6 and 15 days in the presence of elevated CO2 and ambient CO2 respectively. After bud burst started, however the rate of increase in % bud burst was slower in the elevated temperature treatments than in the ambient temperature treatments. Time of bud burst was more synchronous and bud burst was completed within a shorter period in trees at ambient temperature (with and without elevated CO2) than in trees at elevated temperature. Exposure to elevated temperature reduced final % bud burst of both leader and branch terminal buds and reduced growth of the leader shoot. We conclude that climatic warming will influence the physiological processes of dormancy and cold hardiness development in Douglas-fir growing in the relatively mild temperate region of western Oregon, reducing bud burst and shoot growth.

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