Blue Oak (Quercus douglasii Hook. and Arn.) and Valley Oak (Quercus lobata Nee) are endemic, keystone species in California oak woodlands and savannahs. These oaks are widely considered at risk from persistent recruitment failure over the past hundred years. Blue Oak shows very limited seedling or sapling recruitment, and Valley Oak lacks recruitment in more than half of its range and is showing long-term declines in certain populations. Factors that may contribute to the lack of regeneration include climate, herbivory, seed predation, disease, cattle grazing, stand thinning, land fragmentation, proximity to agriculture, competition from annual grasses, and fire suppression. We performed a meta-analysis on all Blue or Valley Oak planting experiments from published and grey literature. We used the log response ratio and considered studies that manipulated herbivory, competition, grazing, moisture, and mycorrhizae.
Results/Conclusions
Results indicate that winter/early spring cattle grazing and increased moisture have a positive effect on seedlings and that late spring/summer grazing, presence of exotic grasses, nitrogen fertilization, and presence of wild herbivores have a negative effect on seedlings. Timing of cattle grazing is significant in determining the direction of effect on Oak seedlings. While cattle grazing in winter may remove competition, cattle grazing in the spring and summer may browse seedlings in absence of other available forage.