Effect of test environment on expression of clines and on delimitation of seed zones in Douglas-fir.

Clinal models of population structure in an indigenous tree species can be used to delineate seedcollection zones and breeding zones, and to devise transfer rules. Models may be developed by growing populations in test environments; however, a clinal description may be a function of test environments as well as of population genotypes. This possibility was studied by growing seedlings from 40 populations of northwestern U.S. Douglas-fir (Pseudotsuga menziesii var. 'menziesii' [Mirb.] Franco) in eight nursery-bed treatments which contrasted air and soil temperatures and nutrition. Growth traits measured were stem diameter, top height, and dry weight; phenological traits were bud-burst and bud-set dates, extension period, and extension period midpoint. Population samples interacted significantly with soil temperature for growth traits, and with soil and air temperatures combined for phenological traits. Interactions were at least partly explained by complex clinal associations of seedling performance with elevation, with latitude, and with distance from the ocean of the populations sampled. Both the complexity and the gradient of the clinal pattern depended on the trait and on the specific test environment.The clinal patterns of greatest complexity were expressed in warm air and soil treatments. Dry top-weights of population samples were associated with latitudes for samples grown in warm soils, but this relationship was not apparent in cool soils. A discrepancy in bud-burst dates between extreme coastal and more inland populations was greatest in warm soil-warm air treatments and was negligible in cool soil-cool air treatments. Populations X temperature interactions were attributed to the differential response of population samples to spring temperature and photoperiod. It is proposed that first attempts at devising a model can be based on nursery or growthchamber tests, and that test environments should stress contrasting photo- and temperature-regimes.The estimate of clinal structure in Douglas-fir suggests that there is more risk within northwestern U.S. in moving provenances east-west than north-south, that this risk increases with elevation of provenances, and that north-south transfers are more critical near the coast than inland.