Recognizing loss of open forest ecosystems by tree densification and land use intensification in the Midwestern USA

Abstract

Forests and grasslands have changed during the past 200 years in the eastern USA, and it is now possible to quantify loss and conversion of vegetation cover at regional scales. We quantified historical (ca. 1786–1908) and current land cover and determined long-term ecosystem change to either land use or closed forests in eight states of the Great Lakes and Midwest. Historically, the region was 35% grasslands (31 million hectares), 38% open forests of savannas and woodlands (33 million hectares), and 25% closed forests (22 million hectares). Currently, the region is about 85% land use (76 million hectares), primarily agriculture, and 15% closed forests (12 million hectares). Land use intensification removed 75% of open forests, while 25% of open forests have densified to closed forests without low severity disturbance to remove understory trees. Historical forest ecosystems included a gradient of oak savannas and woodlands with open midstories (50 to 250 trees/ha), along with closed old growth forests. Open forests have become dense (200 to 375 trees/ha) and are cut frequently, resulting in the extremes of closed canopy forests and clearcut openings across forested landscapes. We demonstrated that forests have transitioned from a historically wide gradient in canopy closure to either dense young closed forests with clearcut openings or to various land uses (agriculture, grazing, residential and commercial land development). The historical abundance of open forest ecosystems, composed of both forest and grassland layers, often is not recognized, and thus, these forests are undervalued for conservation and management.