No abstract

Many examples of biased sex ratios are known in natural populations of plants. Proximal causes of these biases are gender diphasy (sex changing), differential mortality between male and female genets, differential rates of clonal growth (numbers of ramets per genet), and differential flowering (differences in flowering frequency or age to maturity). In the western North American shrub Oemleria cerasiformis we determined sex ratios for 60 natural populations and found an excess of males in 56 populations. The male bias was greatest in populations with little recent recruitment. Sampling of young and old plants indicated that males flowered at an earlier age than females, which led to a transient flowering bias in very young plants, and that the genet sex ratio was 1:1 in young mature plants but male biased in old plants, as a result of differential mortality. Examination of dead genets confirmed that mature females have higher mortality rates. Females also have greater reproductive effort and slower growth rates than males. The major cause of biased sex ratios in O. cerasiformis is greater mortality of female plants during their reproductive years, which probably arises directly or indirectly from their greater allocation to reproduction.

We derived an index of reproductive effort (g reproductive tissue per g leaf) from data collected over two seasons on 28 males and 28 females of the dioecious shrub Oemleria cerasiformis. Males produced an average of three times as much flower and flower-stalk tissue as females, but because of their large fruits, females produced four times as much total reproductive biomass. Reproductive effort of both sexes was related to light. Fruit set in females (% carpels producing drupes) averaged 11.2% and was related to spring light levels. Male-biased sex ratios in this species may be related to the greater reproductive effort of females.

Summary 0We monitored~owering and fruiting of individual male and female plants of Oemleria cerasiformis over a 5!year period in a population in western Canada\ and calculated fruit set "percentage of pistils maturing# and reproductive e}ort "RE# "gram of reproductive tissue per gram of leaf#[ 1 Over 5 years\ male O[ cerasiformis had on average much lower total RE\ but much higher RE at~owering\ than females[ 2 In males\ strong correlations between RE and light suggested that investment in reproduction was largely determined by light levels[ There were strong positive correlations of RE between years\ with no evidence of periodic~uctuations[ 3 In females\ in contrast to males\ RE at~owering was not related to light[ However\ fruit set was strongly correlated with light[ Flowering RE and fruit set were uncor! related in females\ indicating that these are a}ected by di}erent factors[ 4 Correlations of RE between years in females\ although often signi_cant\ were lower than in males\ indicating that RE~uctuates more between years in females than in males and may respond to past levels of RE[ Flowering may re~ect adjustments in response to past reproduction\ or may be controlled by resources other than light[ Fruit set was not signi_cantly related to previous RE[ 5 The greater total RE of females and their limited ability to adjust fruit set are probably major factors contributing to the greater mortality rates of females and the male!biased sex ratios in O[ cerasiformis[ Keywords] control of~owering\ dioecious plants\ fruit set\ resource allocation Journal of Ecology "0888# 76\ 66Ð73

SummaryMany plant species comprising the present-day Arctic flora are thought to have originated in the high mountains of North America and Eurasia, migrated northwards as global temperatures fell during the late Tertiary period, and thereafter attained a circumarctic distribution. However, supporting evidence for this hypothesis that provides a temporal framework for the origin, spread and initial attainment of a circumarctic distribution by an arctic plant is currently lacking.Here we examined the origin and initial formation of a circumarctic distribution of the arctic mountain sorrel (Oxyria digyna) by conducting a phylogeographic analysis of plastid and nuclear gene DNA variation.We provide evidence for an origin of this species in the Qinghai-Tibet Plateau of southwestern China, followed by migration into Russia c. 11 million yr ago (Ma), eastwards into North America by c. 4 Ma, and westwards into Western Europe by c. 1.96 Ma. Thereafter, the species attained a circumarctic distribution by colonizing Greenland from both sides of the Atlantic Ocean.Following the arrival of the species in North America and Europe, population sizes appear to have increased and then stabilized there over the last 1 million yr. However, in Greenland a marked reduction followed by an expansion in population size is indicated to have occurred during the Pleistocene.