PARENT SESSION
Oral Session 10: Physiology I: Temperature, Light, and Growth.
Presiding: C Knight
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room C 123.

How does temperature impact the phenology of bud break in young Douglas-fir plantations in western Washington?

Bailey, John^*,1, Harrington, Constance¹, ¹ USDA Forest Service Pacific Northwest Research Station, Olympia, WA, USA

ABSTRACT- Douglas-fir buds have a chilling requirement; that is, they need to experience cool temperatures (0-5^oC) during the winter in order to break bud normally in the spring. Based on growth chamber and greenhouse trials, all Douglas-fir genotypes are considered to have had their chilling requirement fulfilled at 1200 hours, and there is no additional reduction in time to bud break with additional chilling. Once fully chilled, warm temperatures (>5^oC) must be accumulated to drive bud break. We wanted to test this model to confirm that seedlings in cooler microsites indeed break bud after those in warmer microsites. For three years we have monitored terminal bud break of Douglas-fir seedlings as well as soil and air temperatures on large, replicated harvest plots that created different forest floor conditions: accumulated woody debris following bole-only harvest (cooler microsites) and bare soil left by whole tree removal (warmer microsites). In year one, seedlings on these cooler microsites broke bud 3-5 days before those on warmer microsites despite having accumulated lower heat sum hours during April and May. We believe that the increased number of chilling hours in colder plots (2140 vs. 1840 hours) and/or earlier date to the minimum 1200 hours set the stage for accelerated bud break once warm conditions existed. Thus, we propose that under natural spring conditions, which are substantially cooler than in a greenhouse, plants experiencing more than 1200 chilling hours have a reduced time to bud break. Colder temperatures can, however, delay bud break. Based on differences in bud break in successive years, temperature events less than 5^oC successively reset to zero any prior heat sum accumulation. Warm temperatures experienced early in the spring are thus ineffective in promoting bud break when followed by cooler temperatures. Our phenological model has implications for interpreting silvicultural treatments as well as potential climatic change scenarios.

Key words: budbreak, phenology, Pseudotsuga, temperature