Genecology of Douglas-fir in a watershed in the Oregon Cascades

To gain insight into genetic microstructure of subregional populations of coastal Doug-las-fir (Pseudotsuga menziesii [Mirb.) Franco var. menziesii), genetic variability in a population foundon a 6100-ha, central Oregon watershed is described. Genotypic values of 193 parent trees locatedthroughout the watershed were estimated from progeny grown in a common garden. Then, geneticvariation was partitioned into components attributable to parent-tree location and to differencesamong trees within locations. Within-location variation appeared to be homogeneous in the watershed;between-location variation was related to topography, but the patterns of trait variation differeddepending on the trait measured. Growth traits exhibited strong gradients with elevation, but therelationship varied depending on position within the watershed. Based on estimates of proportionsof nonadapted seedlings in hypothetical transfers of seed from one part of the watershed to another.genetic differentiation within the watershed was surprisingly large. In one transfer of 3.5 km betweennorth- and south-facing slopes at the same elevation, ---80% of seedlings were estimated to be poorlyadapted.
Although other possibilities exist, the topoclinal variation in traits probably results from selectionas does the large within-location variation. Both kinds of variation are due to high selection intensitiesin the seedling stage, the former to selection by average environmental differences along gradients,the latter to microenvironmental heterogeneity. The combination of high within- and between-habitatvariation is suited to a species which episodically colonizes an environment that is extremely heter-ogeneous in time and space.
Key words: adaptation; Douglas-fir; genetic differentiation; genetic variance; natural selection;provenance; seed source; seed transfer; topocline.