ADAPTIVE SIGNIFICANCE OF INTERMITTENT SHOOT GROWTH IN DOUGLAS-FIR SEEDLINGS

We tested the hypothesis that intermittent (lammas) shoot growth in Do uglas-fir (Pseudotsuga menziesii var menziessi (Mirb.) Franco) seedlin gs from dry regions of southwest Oregon is adaptively significant. See dlings from open-pollinated families (160 total) from two inland (dry) and two coastal (wet) sources were grown under either well-watered or intermittent drought conditions (temporary drought followed by rewate ring) for two growing seasons. In the first growing season, the result s supported the hypothesis: the frequency of a second flush was geneti cally controlled (although weakly, h(f)2 less-than-or-equal-to 0.34); more seedlings, on average, from inland families than from coastal fam ilies displayed a second flush; and seedlings from inland families wer e more responsive to the intermittent drought regime in terms of incre ased frequency of a second flush (relative to the frequency in the wel l-watered regime). During the second growing season, the intermittent drought treatment did not promote intermittent shoot growth, although inland and coastal families had different patterns of shoot growth tha t reflected adaptations to soil water availability. We conclude that i nland families have adapted to dry summers and short growing seasons b y relying predominantly on predetermined growth for seedling height in crement after the first growing season. In response to wetter and gene rally longer growing seasons, however, coastal families have developed a less regulated pattern of shoot extension and rely more on free gro wth.