INTRODUCTION 



Cold acclimation in woody plants from temperate zones typically develops in two 

 phases (Weiser 1970) . The first phase is associated with the cessation of growth 

 and development. In most tree species of the northern temperate zone, this phase 

 coincides with the onset of winter dormancy or rest period that cannot be broken 

 until chilling requirements have been satisfied (Glerum 1973). The second phase of 

 cold acclimation results in the development of tolerance to extreme cold (Weiser 

 1970). This phase is triggered by the first frost, and hardiness develops in general 

 response to the minimum temperature (Levitt 1972) . Autumn frost injuries occur 

 during the first phase of cold acclimation when phenological events are not synchronized 

 with the local climate; injuries occur during the second phase when dormant tissues 

 have failed to harden sufficiently to withstand the minima of autumn and winter. 



The actual levels of hardiness that develop in response to given environmental 

 stimuli are under genetic control. For example, geographic races of Comus stotonifera 

 (Smithberg and Weiser 1968), Liquidambar styraeiflua (Williams and McMillan 1971) and 

 Quercus rubra (Flint 1972) acclimate differently. Throughout cold acclimation, 

 progenies of the coastal variety of Douglas- fir {Pseudotsuga menzies-li- var. menziesii) 

 are of lesser hardiness than those of the Rocky Mountain variety (P. m. var. glauoa) 

 (Rehfeldt 1977). And for the Rocky Mountain variety, genetic variation in hardiness 

 exists among populations and among trees within populations (Rehfeldt 1979) . 



Cold acclimation in populations of Douglas-fir from northern Idaho and eastern 

 Washington is the primary concern of the present research. In this area, tolerance 

 to cold temperatures is required for survival. Freezing temperatures can occur 

 during any month, and temperatures as low as -40°C are not rare. Previous research 

 has shown that tolerance to freezing is generally greater in Douglas-fir seedlings 

 from western Montana than in those from northern Idaho and eastern Washington (Rehfeldt 

 1979) . Differentiation of populations in cold hardiness may reflect ecological 

 adaptations that should be considered in limiting seed transfer for reforestation. 

 The present study emphasizes the effects of variation in hardiness on the delineation 

 of seed zones for northern Idaho and eastern Washington. 



MATERIALS AND METHODS 



Cold acclimation was studied in 2-year-old seedlings from 51 populations of 

 Douglas-fir (fig. 1). Each population was represented by seeds from at least 10 

 trees. Most of these populations were from northern Idaho and eastern Washington, 

 but populations from northeastern Oregon and north central Washington were also 

 included. Populations were selected to fully represent the geographic distribution 

 and ecological amplitude of the species in the region of study; longitude, latitude, 

 elevation, and habitat type were variables used to characterize a seed source. 

 Habitat type was classified according to the system of Daubenmire and Daubenmire 

 (1968) . 



About 100 seedlings from each population were grown for 2 years in plastic 

 containers (150 cm^) in a shadehouse at Moscow, Idaho. In order to promote branching, 

 seedlings were decapitated during the first growing season. 



1 



