Scale dependence shapes how plant traits differentially affect levels of pre- and post-dispersal seed predation in Scots pine

Abstract

To understand the demographic consequences of seed predation, it is essential to scale-up losses over space and time. We tested how individual tree characteristics, forest attributes, and the local environment affected rates of both pre- and post-dispersal seed predation in Scots pine Pinus sylvestris, in Scotland’s ancient native pinewoods. The sampling strategy was hierarchical, allowing assessment of how the influence of these factors differed with spatial scale. This approach revealed that the fate of a tree’s seeds depended on many factors, at multiple scales. Levels of seed predation were highly variable over space and time. Seed predation by local post-dispersal seed predators (invertebrates and small mammals) varied at the finest spatial scales, whereas mobile avian pre-dispersal seed predators discriminated among seeds at every spatial scale. Tree crown shape affected removal of seed by all seed predators; both crossbills Loxia spp. and generalist avian granivores selected trees with broad crowns. However, trees with narrow crowns were selected by red squirrels Sciurus vulgaris, and the associated tall understory vegetation supported the highest levels of post-dispersal seed predation. Seed chemistry was an effective seed defence that reduced seed loss to all seed predators. In contrast, increases in physical cone defence were ineffective at reducing seed loss and showed a potential trade-off with investment in seed viability. Large seeds carried a survival cost, being preferentially consumed by squirrels and post-dispersal seed predators. All post-dispersal seed predators consumed a higher proportion of seeds at the times and locations where seed fall was greatest.