Because simple seed- or breeding-zone guidelines are inadequate for controlling the risk of maladaptation to environmental stresses, we are developing operational procedures to assess the risk of frost kill to genetically improved families of Douglas-fir (Pseudotsuga menziesii Mirb. Franco). We have (1) determined the time course of cold hardening and dehardening of nursery-grown Douglas-fir seedlings over four winters, by means of controlled freezing tests, (2) fitted curves to relationships between temperature sum and both fall cold hardening and spring dehardening, (3) applied the temperature sum models to daily temperature records of 80 weather stations to estimate frequency of years with significant frost kill at those stations, (4) interpolated frost kill probabilities for tree farms, using a thin plate spline procedure with elevation, latitude and longitude as variables, and (5) prepared a coarse-scale frost risk map from the resulting grid point estimates. With the exception of a few high-elevation stations, the most damaging frost at any station in any year occurred in either the fall (October and November) or late spring (mid-April to mid-May). In general, damaging spring frosts were two to three times more frequent than fall frosts, and areas in Oregon were at greater risk than areas at similar elevations and longitudes further north. The spline surface was less precise for predicting spring frost risk than fall frost risk.

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