. The Canadian field-naturalist. Natural history. 172 The Canadian Field-Naturalist Vol. 84. Figure 2. Hybrid index values for Popiilus bahamifera and Populus trichocarpa. 0 — 2 Populus bahamifera 8 — 9 Populus trichocarpa 3 — 7 Intennediates It is obvious from the map that additional in- formation is needed from areas where the ranges overlap or are separated by low moun- tain passes such as the Stikine River, the Lynn Canal and at Yakutat in southeastern Alaska, the Susitna valley, and Kodiak Island. In addi- tion, population collections are needed from several localities to determine the de
. The Canadian field-naturalist. Natural history. 172 The Canadian Field-Naturalist Vol. 84. Figure 2. Hybrid index values for Popiilus bahamifera and Populus trichocarpa. 0 — 2 Populus bahamifera 8 — 9 Populus trichocarpa 3 — 7 Intennediates It is obvious from the map that additional in- formation is needed from areas where the ranges overlap or are separated by low moun- tain passes such as the Stikine River, the Lynn Canal and at Yakutat in southeastern Alaska, the Susitna valley, and Kodiak Island. In addi- tion, population collections are needed from several localities to determine the degree of variation within stands and within local areas, especially in regions of hybridization. Discussion A possible explanation of this distribution and hybridization is discussed by both Hulten (1937) and Brayshaw (1965a). During one or more of the major ice advances of the Pleisto- cene epoch, perhaps as late as the Wisconsin, the common ancestral stock of both species was separated by the ice sheets. What is now balsam poplar was isolated in the Tanana-Yukon up- land refugia and south of the ice border but east of the cordilleran ice sheet of the Rocky Mountains. The black cottonwood ancestor was isolated west of the Rockies. As the ice sheets diminished, both species would have expanded rapidly as pioneer species on glacial moraine and outwash. Where the two populations have met in post-Wisconsin time, hybridization and introgression have occurred freely. In Alaska, this has been limited by the mountain barrier along the coast and by the lateness of the ice retreat at the end of the Wisconsin Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Ottawa Field-Naturalists' Club. Ottawa, Ottawa Field-Naturalists' Club
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