Adaptive significance of intermittent shoot growth in Douglas-fir seedlings.

We tested the hypothesis that intermittent (lammas) shoot growth in Douglas-fir (Pseudotsuga menziesii var menziesii (Mirb.) Franco) seedlings from dry regions of southwest Oregon is adaptively significant. Seedlings 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 rewatering) for two growing seasons. In the first growing season, the results supported the hypothesis: the frequency of a second flush was genetically controlled (although weakly, h(f) (2) </= 0.34); more seedlings, on average, from inland families than from coastal families displayed a second flush; and seedlings from inland families were more responsive to the intermittent drought regime in terms of increased frequency of a second flush (relative to the frequency in the well-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 that reflected adaptations to soil water availability. We conclude that inland families have adapted to dry summers and short growing seasons by relying predominantly on predetermined growth for seedling height increment after the first growing season. In response to wetter and generally longer growing seasons, however, coastal families have developed a less regulated pattern of shoot extension and rely more on free growth.