Aspen Bibliography

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

Document Type

Article

Journal/Book Title/Conference

Ecology

Volume

70

Issue

4

First Page

962

Last Page

970

Publication Date

1989

Abstract

We hypothesized that pocket gophers, Thomomys bottae, represent keystone herbivores in slowing down or preventing aspen, Populus tremuloides, invasion of mountain meadows in northern Arizona. Observations at the edge of three 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%. To separate cause and effect, aspen performance was compared in three replicate pocket gopher removal and control plots. 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. At the end of 16 mo all three replicates showed similar increases. Because of these findings we reasoned that aspens 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, we found that distributions of aspens and pocket gophers were nonoverlapping 93% of the time. Pocket gophers were almost always associated with the deep soils of mountain meadows while the centers of aspen clones were almost always associated with rock outcrops. Although tree ring analyses indicate that aspens grow best in the deep soils of meadows in the absence of pocket gophers, when pocket gophers encounter aspens, tree mortality can be high.

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