Valley Oak (Quercus lobata Nee) is an endemic, keystone species in California oak woodlands and savannas, widely considered at risk from persistent recruitment failure over the past hundred years. Valley Oaks lack sapling recruitment in over 80% of the range and are 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 Valley Oak planting experiments from published and gray literature. We used the log response ratio and considered studies that manipulated herbivory, competition, grazing, moisture, and mycorrhizae. We also explore the relationship between results in the meta-analysis and patterns of recruitment in natural populations through range-wide stand surveys and narrative histories of local populations from ranchers and land managers.
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.
