Spatial concordance between seed rain and seedling establishment in bird-dispersed trees:: does scale matter?

We explored whether seedling recruitment was spatially predicted by seed rain (spatial concordance) at different scales (microsite, microhabitat and site) in the bird-dispersed trees Crataegus monogyna, Ilex aquifolium and Taxus baccata, in temperate secondary forests in north-west Spain. We propose that both spatial concordance within each scale and consistency of concordance patterns across scales are dependent on differences between seed rain and post-dispersal processes in the partitioning of spatial variance at each scale. We measured the density of dispersed seeds, the percentage of post-dispersal seed predation by rodents and the density of emerged first-year seedlings at sampling stations distributed throughout five microhabitats (under canopies of parental trees and in open gaps) and four localities over two seasons. Seed rain density of all tree species varied most at the microhabitat scale, but microsite and site differences accounted for most of the spatial variance in post-dispersal seed predation and, especially, in seedling establishment. All three species showed concordance between seed rain and seedling establishment at the microhabitat scale, because strong patchiness in avian-generated seed rain overrode the slight uncoupling effects exerted by the more homogeneous post-dispersal processes. Seed rain was also a good predictor of recruitment of Ilex and Crataegus at the microsite scale, but, for Taxus, the rather homogeneous dispersal across microsites contrasted with the heterogeneous post-dispersal losses. At the site scale, only Taxus showed a positive trend of concordance. Concordance patterns were maintained from microsite to microhabitat in Crataegus and Ilex, and from microhabitat to site in Taxus. Low-variance allocation to the site scale at the seed rain stage precluded complete consistency in Crataegus and Ilex. Positive responses of recruitment to seed dispersal depended on both species and scale, resulting in a complex template for dispersal-limitation effects on metapopulations and communities.