ELEVATIONAL GRADIENTS IN ADULT SEX RATIOS AND SEXUAL DIFFERENTIATION IN VEGETATIVE GROWTH RATES OF POPULUS TREMULOIDES MICHX.

Since Fisher's (1930) early work on the effects of selection on the primary sex ratio which showed that mixtures should tend toward equal numbers of males and females, there have been several theoretical studies which have pointed out conditions in which deviations from 1:1 could be expected (e.g., Bodmer and Edwards, 1960; Emlen, 1970; Eshel, 1975). Empirical studies, usually of adult ratios, have shown strong imbalances in several cases (e.g., Putwain and Harper, 1972; Freeman et al., 1976; Bawa and Opler, 1977). The extent to which sex dependent mortality among adults influences changes in the primary ratio is still under theoretical investigation (Trivers and Willard, 1973; Werren and Charnov, 1978). The number of empirical studies which may be used to test these theories is still small, especially in plants (Godley, 1964; Putwain and Harper, 1972; Lloyd, 1973; Freeman et al., 1976; Bawa and Opler, 1977). We here report the results of field studies on adult sex ratios and concomitant patterns of vegetative growth rates for quaking aspen (Populus tremuloides Michx.) in the Colorado Front Range. Quaking aspen is the most widespread tree in North America, extending from Alaska to Mexico and from Maine to the state of Washington. Members of the genus are of substantial commercial, recreational and ecological significance (Little, 1971), as well as the subject of a large literature of physiological, silvicultural and general ecological attributes (Barnes, 1966, 1969, 1975; Reed, 1971; Anon., 1976). The extensive, beautiful stands or groves of quaking aspen familiar to travelers in the northern or western parts of Revised February 2, 1979