A number of North American oaks are failing to regenerate, with explanatory hypotheses including a range of consumer and resource based limitations. These limitations are associated with a number of significant environmental changes including trophic collapse, climate change, and grass invasion. Using two factorial experiments, we demonstrated how direct and indirect consumer effects prevent recruitment by Quercus garryana, a keystone savanna species of northwestern North America.  The first experiment isolated the effects of large herbivores (black-tailed deer), small herbivores, summer rainfall, invasive grass cover, and ectomychorrhizal soil transfers on the survival of planted seedlings in an old-field over three years. The second experiment tested the generality of the herbivore and exotic grass impacts using naturally recruiting seedlings in an oak savanna remnant.

Results/Conclusions

In experiment 1 (planted seedlings), herbivory by small mammals severely damaged or killed ~100% of the unprotected seedlings during winter. Many damaged seedlings survived and recovered during the summer but were significantly smaller in size and produced fewer leaves.  Small mammal impacts indirectly affected the outcome of soil moisture limitation and competition with exotic grasses, by converting both from suppressive to lethal. Herbivore damage also increased the likelihood of severe insect attack, possibly due to reduced investment in foliar defence by recovering seedlings. There were no detectable impacts from deer or the soil transfers. In experiment 2 (naturally recruiting seedlings), herbivory by deer, instead of small mammals, damaged or killed 80% of seedlings. The switch in herbivore identity was determined by habitat structure (e.g., tree cover), likely due to its effects on perceived predator risk. Also, damage occurred in the absence of exotic grass cover, indicating that grasses facilitate seedling survival presumably by hiding the seedlings. This result was supported by a site survey, with densities of seedlings occurring throughout the 10 ha savanna remnant but none growing above the ~1.5 m grass canopy. These results show that herbivory plays a significant role in oak recruitment failure which may represent an almost insurmountable barrier for recruitment.