SOILS, SEED-ZONE MAPS, AND PHYSIOGRAPHY - GUIDELINES FOR SEED TRANSFER OF DOUGLAS-FIR IN SOUTHWESTERN OREGON

One procedure for guiding seed transfer is to partition the species habitat into zones within which there is little genetic variation from location to location. This report compares soil types and the existing regional seed zones as bases for classifying geographic genetic variation into zones. The data used were genotypic values of 135 Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) trees from 80 locations in a 15,000 km2 area in southwestern Oregon. Genotypic values were estimated by measuring traits expressing phenology and growth potential of 2-year seedlings in a common-garden experiment. Neither the soils model nor the seed-zones model satisfactorily classified geographic genetic variation (significant lack of fit). When physiographic variables (latitude, longitude, elevation, etc.) were added to the models, the added variables accounted for 16% to 23% of the total variation among locations. Gradients of genetic variation therefore exist within soil types and present seed zones. These gradients reflect the overall gradients of geographic variation with latitude, longitude, and elevation that occur in the region. The results suggest that zones constructed from a physiographic model may explain more of the genetic variation in southwestern Oregon than do either soils or seed-zones models.