Genecology of Douglas‐Fir in a Watershed in the Oregon Cascades

To gain insight into genetic microstructure of subregional populations of coastal Douglas—fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii), genetic variability in a population found on a 6100—ha, central Oregon watershed is described. Genotypic values of 193 parent trees located throughout the watershed were estimated from progeny grown in a common garden. Then, genetic variation was partitioned into components attributable to parent—tree location and to differences among 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 differed depending on the trait measured. Growth traits exhibited strong gradients with elevation, but the relationship varied depending on position within the watershed. Based on estimates of proportions of 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 between north— and south—facing slopes at the same elevation, °80% of seedlings were estimated to be poorly adapted. Although other possibilities exist, the topoclinal variation in traits probably results from selection as does the large within—location variation. Both kinds of variation are due to high selection intensities in 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—habitat variation is suited to a species which episodically colonizes an environment that is extremely heterogeneous in time and space.