Consequences of Flowering-Time Variation in a Desert Annual: Adaptation and History

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Can observed life histories be successfully explained in terms of their current adaptive significance, or do we need historical data as well? This issue is examined for the case of the desert annual Eriogonum abertianum Torr. (Polygonaceae). Individuals of this species flower in the spring, or following the onset of summer rains, or both. In order to estimate survivorship probabilities and fecundities, large samples of marked plants were followed in each of two years at two sites. In both years the Sonoran Desert (lower summer rainfall, hotter) population experienced significantly more mortality during the foresummer droughts, and had a significantly greater proportion of spring—flowering plants, than the Chihuahuan Desert (greater summer rainfall, cooler) population. This result was reproduced in a greenhouse study (Fox 1989). The model of selection comparing the spring—plus—summer flowering habit with spring—only flowering suggests that, in the years studies, expected summer fecundity may not have offset the risk of foresummer mortality in the Sonoran Desert population. Distributional data indicate that, rather than switching to a spring—only phenology as predicted by the model, the species' range ends where summer rainfall declines abruptly. Fossil and biogeographic evidence lend support to an adaptive interpretation of earlier flowering in the Sonoran Desert: They suggest a mainly summer—flowering origin for the species and an invasion of the Sonoran Desert during a period with greater summer rainfall than is the case at the present. However, the persistence of the spring—plus—summer habit is not explained by either the demographic or the historical data.

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Ecology, v. 70, issue 5, p. 1294-1306