Recruitment of Quercus agrifolia in central California: the importance of shrub-dominated patches

Many perennial plants strongly enhance the survival of seedlings of other s pecies. We studied patterns of long-term recruitment of Quercus agrifolia ( Coastal live oak) associated with shrub-dominated communities by counting Q . agrifolia recruits on a time sequence of historical aerial photographs an d comparing recruitment among mapped patches of coastal sage scrub, chaparr al, and grassland in an 1120-ha landscape. Because we could not identify ne w recruits in existing woodlands with aerial photographs, we studied the re cruitment of Q. agrifolia in this vegetation type indirectly by comparing p opulation size structures and the spatial relationships between shrubs and recruits among woodlands that varied in understory community type. At the l andscape scale, recruitment was higher in coastal sage scrub vegetation tha n predicted by the extent of its coverage, commensurate with the spatial co verage of chaparral, and very low in grassland. Recruitment within woodland communities also varied considerably. In woodland communities on sheltered , north-oriented topography with understories dominated by shrubs, there we re large numbers of small Q. agrifolia, and recruits were not significantly spatially associated with shrubs within plots. In woodlands with herbaceou s understories there were few individuals in the small size classes, and re cruits were strongly spatially associated with shrubs within plots. Woodlan ds with shrub-dominated understories have population structures that appear to be stable, but woodlands with herbaceous understories exhibit size stru ctures associated with declining populations. Quercus recruitment into shru b-dominated patches corresponds with previous documentation of facilitative relationships between shrubs and oak seedlings, and suggests the occurrenc e of an unusual form of patch dynamics in these landscapes.