GENE FLOW IN LIMBER PINE: EVIDENCE FROM POLLINATION PHENOLOGY AND GENETIC DIFFERENTIATION ALONG AN ELEVATIONAL TRANSECT

Limber pine (Pinus flexilis James) in Colorado occurs on sites from the upper treeline (3,350 m) to the lower treeline (1,650 m). In this study we examined pollination phenology at eight sites along this transect and allozyme frequencies in two populations at either end of the transect to evaluate the level of interpopulation gene flow. Pollen release and megastrobilus receptivity are synchronous within populations, lasting about 15 days. Pollination phenology is strongly affected by site elevation. Most sites that differ in elevation by more than 400 m do not have overlapping pollination periods, and thus interpopulation gene flow is restricted despite strong westerly winds. Gene frequencies are significantly different between upper and lower treeline populations at eight of the ten loci surveyed. Nevertheless, we estimate the level of gene flow to be 11.1 migrants per generation between these two populations. Stepping-stone pollen transfer between intermediate populations and a high level of gene flow via seeds may be responsible. Thus, despite the elevational restriction on long-distance gene flow via pollen, limber pine appears to maintain a high level of interpopulation gene flow.