Pinus contorta populations from latitudes 47o to 63'N were cultivated in climate chambers and freeze tested at varying photoperiods. The results were compared with data from field trials comprising other populations from the same latitudinal range. A good agreement between our data on frost hardiness from the climate chambers and the ones front field trials was obtained. Breeding for frost hardiness in Pinus contorta ought to be done by early testing using the design developed in the present investigation. The longer the night length the greater the hardiness. Low temperature during the night promotes the development of hardiness. The more northern the origin the shorter the night length required to induce hardiness. Dry matter content of the upper 3 cm of the shoots and the lengthening of the secondary needles are strongly correlated with hardiness
