Pollen limitation of reproduction in a subarctic-alpine population of Diapensia lapponica (Diapensiaceae)

Determinants of reproductive success (measured as capsule mass) in Diapensia lapponica, an early-flowering pollen-risker dependent on insects for pollination, were evaluated in an exposed subarctic-alpine population in northern Sweden. The species was protogynous and self-compatible, but self-deposition of pollen was inefficient. Hand-pollination increased capsule mass significantly, indicating that reproductive success was severely pollen limited. An experiment designed to test the relative importance of insect pollination in comparison to water availability (precipitation) and shelter (temperature and wind), revealed that exclosure of insects decreased capsule mass significantly, as did watering, whereas shelter tended to increase capsule mass. Pollination was the most important determinant of reproductive success, and the study suggests that pollen limitation may be particularly severe for early-flowering species, i.e. pollen-riskers, in exposed locations.