al. found white spruce in the northern edge of continuous forest in central Canada at Ennadai Lake, about 60°45′ N, 101°’W, just north of the northwest corner of Manitoba. Bryson et al. noted that the forest retained “the same general characteristics as when it was first described [by Tyrrell] in 1896”. Collins and Sumner reported finding white spruce within 13 km of the Arctic coast in the Firth valley, Yukon, at about 69°30′ N, 139°30′ W, and Sargent noted that white spruce in Alaska “reached Behring Strait in 66°44′ N”. Climate, especially temperature, is obviously a factor in determining distributions of northern flora. Halliday and Brown suggested that white spruce's northern limit corresponds “very closely” with the July mean monthly isotherm of 10 °C in Ungava, but that the northern limit west of Hudson Bay was south of that isotherm. Other climatic factors that have been suggested as affecting the northern limit of white spruce include: cumulative summer degree days, position of the Arctic front in July, mean net radiation especially during the growing season, and low light intensities. Topography, soil conditions, and glaciation may also be important in controlling northern limits of spruce. The southern limit of distribution of white spruce is more complex. From east of the main range of coastal mountains in British Columbia, the southern continuous limit of white spruce is the forest/prairie interface through Alberta, Saskatchewan, Manitoba, the northern parts of Minnesota and Wisconsin, central Michigan, northeastern New York, and Maine. Sargent and Harlow and Harrar also included Vermont and New Hampshire; and, while Dame and Brooks excluded New York and states further west, they included Massachusetts as far south as Amherst and Northampton, “probably the southern limit of the species” in that area. Nisbet gave the range of white spruce as extending to “Carolina”, but he did not recognize red spruce as a species and presumably included it with white spruce. Towards the southern parts of its range, white spruce encounters increasingly effective ecological competition from hardwoods, some of which may reinforce their growth-rate or sprouting competitiveness with allelopathic depredation of coniferous regeneration. Further southward extension of the distribution is inhibited by white spruce's cold requirement. White spruce as an exotic species As an exotic, white spruce is widespread but uncommon. It was introduced into England and parts of continental Europe in or soon after the year 1700, into Denmark about 1790, and into