. The Canadian field-naturalist. 1999 Brunton ani:) Britton: Isohths maritima in Yukon 643. Approx. limit of Ice Sheet ^^ 10,000-11,500 years BP C^ ^ 12,000+ years BP Figure 5. Yukon Territory (dotted line) distribution of Isoetes in relation to Beringian land- base ca. 10 000 - 11 500 years BP. Solid square = Isoetes maritima, solid triangle = Isoetes echinospora. Location of inland /. maritima in Alaska also noted. (Quaternary information from Roberts et al. 1987; Prest et al. 1987; Schweger 1989). point where well-drained White Spruce {Picea glaii- ca (Moench) Voss)/ White Birch {Betiila pa
. The Canadian field-naturalist. 1999 Brunton ani:) Britton: Isohths maritima in Yukon 643. Approx. limit of Ice Sheet ^^ 10,000-11,500 years BP C^ ^ 12,000+ years BP Figure 5. Yukon Territory (dotted line) distribution of Isoetes in relation to Beringian land- base ca. 10 000 - 11 500 years BP. Solid square = Isoetes maritima, solid triangle = Isoetes echinospora. Location of inland /. maritima in Alaska also noted. (Quaternary information from Roberts et al. 1987; Prest et al. 1987; Schweger 1989). point where well-drained White Spruce {Picea glaii- ca (Moench) Voss)/ White Birch {Betiila papyrifera Marsh.)/ Black Spruce {Picea mariana (Mill.) BSP.) dominated forest gives way to more boggy Black Spruce dominated forest at the western approaches to the Macmillan Pass entrance into the Mackenzie Mountains (Porsild 1951). The Sheldon Lake - Sheldon Mountain area of the Ross River Valley has been identified as a centre of diversity for rare Yukon Territory flora (Douglas et al. 1981). This floristic richness is indicated by the presence of 19 territorially rare species of which Isoetes maritima, Potamogeton subsibiriciim Hagstr. and Potamogeton obtusifoliiis Mert. & Koch, are known elsewhere in the Yukon Territory from one other location at most (Douglas et al. 1981; Cody et al. 1998). Origin and dispersal At their closest, contemporary populations of Isoetes maritima in coastal southern Alaska are sepa- rated by ca. 500 km and across several mountain ranges from the Sheldon Lake population and are not connected to it by river systems. Disjunct interior British Columbia /. maritima, on the other hand, is less than 200 km from the species' primaiy coastal range and is connected directly to those populations by major river systems (Britton and Brunton 1995). The apparently isolated populations in the Fairbanks areas of eastern interior Alaska are over 900 km west of the Sheldon Lake population but occur within the same drainage system, the Yukon River. Much of the Y
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