The impacts of pollination mode, plant characteristics and local density on the reproductive success of a scarce plant species, Salix arbuscula

Pollination failure represents one stage at which sexual reproduction of plants may be limited. It is therefore important to understand the pollination mechanism of the plant and how this may be affected by plant and population characteristics. This study examined the reproductive biology of a scarce montane willow species, Salix arbuscula, as part of a programme to determine ecological strategies for the conservation of montane willow scrub, an endangered habitat in the UK. The relative importance of insect and wind pollination, and the role of plant characteristics including plant size, catkin length, number of flowers and local density in determining reproductive success (number of fruit set) were investigated in three populations. Insect exclosures were used to determine the pollination mechanism. Our results suggest that S. arbuscula is predominantly insect-pollinated. In naturally pollinated plants, catkins with a higher proportion of ripe fruit were longer. Plants with more than two males in the local area and unbrowsed plants had a higher proportion of ripe fruit per catkin. The extent of wind pollination was very low, but differed between populations. The success of wind-only pollination was dependent on the number of males nearby, indicating that local density is more important in this type of pollination. Pollination mechanism and fruit set varied between the three populations observed, demonstrating the importance of multi-site comparisons. The number of males in the surrounding area had a positive effect on fruit set in both types of pollination, suggesting that Allee effects are likely to be operating in this species.