Importance of belowground herbivory: pocket gophers may limit aspen to rock outcrop refugia

Thomomys bottae may represent keystone herbivores in slowing down or preventing Populus tremuloides invasion of mountain meadows in N Arizona. Observations at the edge of 3 aspen clones bordering meadows showed that where pocket gopher disturbance was relatively low (0.70 mounds/m²) aspen mortality was <20%, but where disturbance was high (5.6 mounds/m²), aspen mortality increased to 90%. Aspen release from pocket gopher herbivory on roots was rapid; for the single replicate monitored only 4 mo after the initial removal of pocket gophers, aspen survival increased to a value 3.5 times, vegetative reproduction 2.5 times, and average tree growth rates 3 times that of the control. Aspens thus might be limited to rock outcrops that are largely inaccessible to pocket gophers and act as refugia from belowground herbivory. Consistent with this hypothesis, in 32 aspen-meadow associations, distributions of aspens and pocket gophers were nonoverlapping 93% of the time. -from Authors