Historical range of variability for restoration and management in Wisconsin

In Wisconsin, as in other states, management goals sometimes include restoration of historical forest conditions, which may prepare forests to be more compatible with future climates, disturbances such as drought and fire, and forest health threats. We quantified historical (1830–1866) composition and structure to develop historical reference conditions for restoration and documented changes based on current (2005–2009) forest surveys in Wisconsin. We provided structural metrics, functional group composition, and forest types for 186 ecological land types, and we also summarized trends in composition and structure. Wisconsin forests historically were comprised of 46% oak or pine savanna or woodland, 6% pine forest, and 48% forests primarily consisting of late-successional eastern broadleaf forest species and early-successional northern mixed forest species; densities of these forest types ranged from 60 to 460 trees/ha. In the Eastern Broadleaf Forest ecological division, increased composition of the early-successional and mid-successional eastern broadleaf forest groups (from 10 to 40%) and (planted) pine group (8–23%) occurred along with decreased fire-tolerant oak composition (from 65 to 23%). Density increased in current forests compared to historical forests by a factor of 2.2; despite increased density, basal area increased only slightly due to the presence of larger diameter trees in historical tree surveys. In the Northern Mixed Forest ecological division, increased composition of the mid-successional eastern broadleaf forest group (from 12 to 24%) and late-successional northern mixed forest group (from 10 to 17%) occurred due to decreased composition of the fire-tolerant pine group (from 17 to 9%) and late-successional eastern broadleaf forest group (from 30 to 20%). Density remained similar in current forests compared to historical forests but current basal area was 50% of historical basal area. The transition from open fire-tolerant oak and pine forests, with rarity of early-successional tree species, to closed forests composed of a variety of early- and mid-successional tree species parallels results from other research. Replacement of open oak or pine forest ecosystems by dense forests has moved Wisconsin outside of the historical range of variability, likely reducing plant and wildlife species associated with open oak and pine ecosystems.