Stage and Size Structure of Three Species of Oaks In Central Coastal California

Oaks are foundational species in much of California, and many oak populations in the state may be in jeopardy due to a lack of recruitment of young trees. Despite considerable interest in this problem, there have been few comprehensive surveys of all stages of oak development. We surveyed all stages of three oaks: Quercus lobata, Q. douglasii, and Q. agrifolia in a forest plot with mixed land-use in central coastal California. We found abundant seedlings of all oak species, but an apparent paucity of Q. lobata and Q. douglasii saplings. First year seedlings of all species were less abundant in parts of the study site with cattle grazing, but later-stage seedlings of Q. lobata and Q. douglasii were equally abundant across land-use types. Quercus agrifolia seedlings were associated with non-grazed areas; Quercus agrifolia late-stage seedlings in the grazed area were smaller and less abundant than in nongrazed areas. Quercus agrifolia seedlings of all stages tended to be clustered around conspecific mature trees. Quercus lobata late-stage seedlings, and to a lesser degree those of Q. douglasii, were often distant from any potential parent tree. These data indicate that young stages of the three species of oak have different spatial distributions and occur in different abundances at two sites with different grazing regimes. They are also consistent with a relative lack of regeneration in Q. lobata and Q. douglasii, although it remains to be determined that the small number of saplings of these species observed is insufficient to replace mortality of mature trees. This survey provides a baseline from which future resampling can assess the long-term demographic success of three Californian oak species.