Historical forest variability across a large Sierra Nevada mixed-conifer landscape
US Forest Service National Forests throughout the western US are beginning to revise their Land Management Plans, and ecological restoration will be the central theme across individual plans. While numerous studies have been conducted that describe historical forest conditions and fire regimes the concept of ecological restoration remains partially undefined. It is understood that fire in drier, mid- to low-elevation forests occurred fairly frequently and this generally maintained open and patchy forest structure. But, how open, how patchy, and how did this vary across large landscapes? This information generally cannot be extracted from the traditional tree-ring based studies. We have a historical dataset that can address some of these key questions that still remain unanswered. This dataset consists of early timber inventories conducted by the US Forest Service in 1911. These inventories were spatially explicit and included tree sizes and species, as well as understory vegetation cover by species.
Using K-means clustering we identified nine distinct forest structure/understory community groups across a 16,000 ha area. These groups ranged from nearly pure shrub-dominated to relatively dense forests, dominated by large trees. Regression tree analysis indicated that elevation had a strong influence on the spatial distribution of identified groups. Moisture deficit, and to a lesser extent slope gradient, also explained spatial distribution of groups, indicating topographic/productivity controls on forest structure/understory vegetation. This work has the potential to assist in developing restoration goals for land management planning by explicitly characterizing ranges of historical forest conditions across a large mixed-conifer forest rather than simply reporting average conditions.