Canada's plant hardiness zones revisited using modern climate interpolation techniques

Canada's plant hardiness zones are well known to Canadian gardeners. The or iginal hardiness indices and zones were developed in the early 1960's throu gh regression models of several climatic parameters and plant survival data from numerous locations across the country. Since that time Canada's clima te has changed and climate interpolation techniques have improved. We have remapped Canada's plant hardiness zones using data from the period of the o riginal analysis (roughly 1930-1960) and for the 1961-1990 period using thi n plate spline interpolation methods. Trials of bivariate and tri-variate s plines were undertaken and evaluated using withheld data. A trivariate func tion of position (longitude and latitude) and elevation performed best. Sta ndard errors of the surfaces were about 0.5 degreesC or less for temperatur e variables and 5 to 28% for rainfall depending on the month (winter months bring the worst). The creation of a new digital elevation model (a regular grid of position and elevation) of Canada enabled the mapping of each vari able required for the plant hardiness formula at spatial resolutions of 1 k m to 10 km. These models better capture the spatial variation in climate th an previously possible and hence should provide a stronger basis for applic ations such as the determination of plant hardiness zones. Comparisons of t he zones between the two time periods are consistent with what is known abo ut climate in Canada. The hardiness index has declined or has stayed stable in eastern Canada and has increased in western areas. The results also sug gest that more station data are required in western Canada to better captur e the inherent spatial variability of climate, particularly precipitation, in mountainous terrain.