Population studies of alpine and subalpine races of conifers and Willows in the California high Sierra Nevada.

It is now generally admitted that growth forms are inherited among trees as well as among herbs. In the beginning of this century, Oppermann (1908) showed that twisted and dwarfish forms of beeches were inherited and not primarily the result of environmental influence. Based on field studies, transplantations, and seedlings, he also showed that oaks and pines had genetically distinct forms in exposed localities and that in smaller numbers such forms were scattered through Danish forests (Oppermann, 1925, 1929). Pinus mughus is recognized as a low elfinwood form of the Pinus silvestris-montana complex, and Larsen (1937) has shown that Larix gmelini and L. kaempferi are genetic low, twisted growth forms of the circumboreal Larix decidua complex. Stockwell and Righter (1946), Righter and Stockwell (1949), Righter and Duffield (1951), Larsen (1956), and Ehrenberg (1963) have discussed genetic variability and crossings in trees. Among herbaceous plants the gene bases for the prostrate coastal and erect inland growth habits of Viola tricolor and of Layia platyglossa and L. chrysanthemoides have been determined (Clausen, 1926, 1951; Clausen, Keck, and Hiesey, 1947; Clausen and Hiesey, 1958). The differences between races in growth form represent differences in adjustment to climate. Parallel situations exist among woody plants, for low bush forms have been selected at windswept coasts and also at high altitudes and latitudes where snow protects low growing plants against winter desiccation (Clausen, 1963). In herbaceous plants small systems of genes hold the non-hybrid coastal races prostrate even when they are moved to more protected inland habitats. Interracial hybrids are more modifiable. Genes promoting horizontal growth are balanced against genes promoting erect growth. Recombinations between genes of opposing effects unbalance the genetic controls, and genotypes arise that phenotypically are more modifiable. Such modifications do not indicate that the growth habit is controlled by environment only. Rather, it indicates a labile genetic control based in the action of opposing genes (Clausen and Hiesey, 1958). A tree species does not always disappear at its tree line. In many cases the subalpine tree is replaced by an alpine elfinwood, a low bush form (Clausen, 1963). In such cases the bush form represents the alpine ecotype of the tree species. The present investigation shows that the bush form is not limited to the alpine habitats but occurs in reduced numbers within the border zone between alpine and subalpine vegetations. Most trees are wind pollinated; the strong winds at high altitude carry the pollen between the subalpine tree forms and the alpine elfinwoods, and there is strong evidence of hybridization within the border zone. The severe climate in the neighborhood of the alpine zone exerts strong selection that is seen in numerous dead erect trunks developed during successions of more favorable years but killed during years of greater severity. Any upper subalpine zone bears evidence of dynamic interactions between heredity and environment, but the subalpine-alpine vegetations have not generally been studied from this point of view. Careful, detailed population studies in rough and relatively inaccessible terrains are necessary before the situation can be properly assessed. Area studied.-During the summers