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Population Studies of Alpine and Subalpine Races of Conifers and Willows in the California High Sierra Nevada

Jens Clausen
Evolution
Vol. 19, No. 1 (Mar., 1965), pp. 56-68
DOI: 10.2307/2406295
Stable URL: http://www.jstor.org/stable/2406295
Page Count: 13
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Population Studies of Alpine and Subalpine Races of Conifers and Willows in the California High Sierra Nevada
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Abstract

Three conifers, Pinus albicaulis, P. murrayana, and Tsuga mertensiana, and two willow species, Salix monica and S. orestera, evolved subalpine erect and alpine horizontal races between 10,000 and 11,000 feet in altitude in Slate Creek Valley, Harvey Monroe Hall Natural Area, on the east side of the Sierra Nevada. These races interbreed in the border zone, but strong natural selection tends to eliminate the intermediates at distances from the border zones. Interspecific crossing appears to augment the interracial crossings among the willows. The five species differ in the altitudes of their tree lines, in their frequencies according to slope exposure, and in other ecological factors. Only alpine elfinwood forms exist within the Slate Creek Valley of Juniperus communis, Sorbus sitchensis, Salix petrophila, S. nivalis, and S. reticulata, although the former two have tree forms in other parts of the world. Salix eastwoodiae is not known to have developed an elfinwood form but continues its erect form until high altitude. Elfinwood forms are also not known within Pinus cerebra of the Swiss Alps and the Altai Mountains, although within the Slate Creek Valley its close relative, P. albicaulis, has the best developed alpine elfinwood vegetations, except Juniperus communis. Ability to develop elfinwood depends upon the germ plasm of the particular species, as shown by the species to, species variation in the occurrence of elfinwoods. Erect subalpine trees may continue to the tree line of the species, as in Pinus murrayana. If the species has been able to develop an alpine race, such a race is a low bush or mat and extensive intercrossing and selection occurs within the upper range of the tree zone. Judging from the nature of natural hybrid swarms the segregation must be a compplex multigenic one.

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