People working to restore ecosystems around the world have, broadly speaking, used combinations of two different approaches (SER 2002). A structural approach seeks to quickly return the structure of a given ecosystem to what it was before it was disturbed. A process approach seeks to return core ecological processes to the role they had before disturbance took place. In southwestern ponderosa pine forest ecosystems, a structural approach seeks to quickly return forest structure—that is, the placement and growth patterns of both trees and understory plants—to what it was before the disturbances of Euro-American settlement. A process approach, on the other hand, seeks to allow the keystone processes that shaped the forests, especially fire, to shape the forests again without making an explicit decision about precisely what the forest’s structure should look like.

There is a great deal of overlap between these two approaches, and in fact every workable restoration solution for heavily altered forests needs to incorporate both. The “presettlement model” approach developed at Northern Arizona University seeks to quickly return tree densities to what they were in pre-Euro-American settlement times through thinning, but it does so in order to allow low-intensity fire to safely shape forest structure in the future without the threat of unnaturally intense crown fires (Covington et al. 1997). In other words, this approach alters structure so that the process can be changed. Approaches that emphasize the use of fire while calling for less structural manipulation—such as the Natural Processes Restoration Model prescription—also do require some thinning before fire can be reintroduced, since even a carefully planned prescribed fire in an unthinned forest can have disastrous consequences.

The Treatment Options section details how land managers in various areas have combined structural and process changes to craft restoration treatments that make sense. It also looks at the tradeoffs inherent in the various approaches, for every restoration treatment is effective in meeting some objectives and less so in others. Any meaningful restoration treatment has to address both forest structure and ecological processes. Any successful treatment has to permit the reintroduction of the frequent low-level fires that will shape the forest in the future.