A MODEL OF FIRE SELECTION FOR SEROTINY IN LODGEPOLE PINE.

Abstract

The role of environmental heterogeneity in the maintenance of genetic polymorphism is a subject of much research and debate. Although a great deal of circumstantial evidence links polymorphism to spatial environmental variability, the importance of temporal variability is less well established, at least partly because of a lack of studies associating genetic diversity to temporal environmental changes (see the review by Hedrick et al., 1976). Herein we argue that a polymorphism in cone type of Rocky Mountain lodgepole pine (Pinus contorta var. latifolia) is maintained by temporal variation in the fitness of each type. Except for a few isolated cases in which it apparently forms climax stands, lodgepole pine in the Rocky Mountains is a seral species maintained by fire. In most areas, this fire adaptation is mediated by the serotinous cone, which is opened only by heat generally exceeding 45 C (Crossley, 1956; Lotan, 1970). Many years' seed production-millions of seeds per hamay be stored in serotinous cones that remain on trees. Fire triggers release of these seeds and rapid recolonization of burned sites. The importance of this mechanism in the northern Rockies is attested to by the expansive areas of dense, even-aged lodgepole pine forests. Serotiny in pines probably is genetically controlled (Crossley, 1956; Critchfield, 1957; Rudolph et al., 1959; Lotan, 1967), but the exact nature of this control is dif-