Consumer-based limitations drive oak recruitment failure

A number of North American oaks are experiencing recruitment failure, with explanatory hypotheses including a range of consumer- or resource-based limitations. Using a factorial experiment with transplanted seedlings, we demonstrate how direct and indirect consumer effects prevent recruitment by Quercus garryana, a keystone savanna species of northwestern North America. Foremost, intense herbivory by mostly exotic small mammals severely damaged or killed 100% of unprotected seedlings during winter. Many damaged seedlings survived but were 64% smaller in size and produced 75% fewer leaves. Herbivory by deer had no detectable impact despite being long hypothesized as a major contributor to oak decline. Indirectly, herbivory altered the outcome of moisture limitation and competition. Without small mammals, summer drought and a dense exotic grass cover associated with fire suppression significantly reduced growth but caused little mortality. With small mammals, both significantly increased mortality of herbivore-damaged seedlings. Herbivore damage also increased the likelihood of severe insect attack, possibly due to reduced investment in foliar defense by recovering seedlings. These results show that no single factor necessarily prevents seedling establishment by oaks, but that the combination of herbivory, undisturbed exotic grass swards, and summer drought creates an almost insurmountable barrier for recruitment.