We thank S. R. Carpenter, P. L. Firth, and Kevin H. Rogers
for comments that substantially improved the manuscript.
Formulation of ideas contained in this article benefited from
discussions with many colleagues and students; we truly appreciate
their willingness to share perspectives and information.
Financial support was provided by the U.S. Man and
the Biosphere Program, the A.W. Mellon Foundation, the Pacific
Northwest Research Station of the U.S. Forest Service,
and the National Science Foundation.