Cold Hardiness of Twigs of Quercus Rubra L. As a Function of Geographic Origin

Abstract

Cold hardiness of twigs of young tress of Quercus rubra L. grown from seeds collected form 38 different geographic origins and growing on a single site varied widely from October to December, 1968, and was strongly related to estimated average annual minimum temperature of the origin. By March, 1969, differences among sources had disappeared. Hardening again was apparent as early as August 23, 1969, accompanied by a significant differentiation related to climate of the origin. Multiple regression analyses showed twig hardiness to be closely related to latitude of the origin. Longitude and elevation had significant but secondary effects in autumn. Average annual minimum temperature (AAMT), extreme minimum temperature (EMT), length of frost—free period (FFP), and biotemperature (BIOT) as defined by Holdridge, were highly correlated with each other and with latitude of the origin. All were strongly related to cold hardiness in autumn. Cold hardiness in all cases was greater than that required by climate of the origin, suggesting that twig hardiness in established trees is not an important factor currently in natural selection.