60 years of change in Wisconsin savanna plant communities
Across the Midwest, less than one percent of presettlement oak savannas exist today. Understanding how savanna communities respond to changing environmental conditions is critical to their continued presence on the landscape. Fire plays a critical role in savanna maintenance and altered fire regimes can result in large community changes. Winter temperatures have warmed within the Midwest, and may also influence savanna communities. To examine plant community change over the past 60 years, we are resurveying savannas that were originally surveyed in the 1950s. From 1951 to 1954, the Plant Ecology Laboratory at the University of Wisconsin – Madison surveyed 40 savanna plant communities across central and southern Wisconsin. In the summer of 2014, half of the savanna sites were resurveyed and the remainder of the sites will be visited in summer 2015. At each site, understory and canopy plant communities were surveyed following the same methods used for the 1950s surveys. Landowners provided information about land use and fire histories during the past 60 years. With these data, we examined how savannas changed over the past 60 years and how community change related to management and climate change.
Both savanna canopy and understory plant communities changed considerably over the past 60 years. Canopy density increased between the 1950s and today, with an average of 56 (±35) additional trees per acre. Tree composition also changed and many species present today were absent from savannas of the 1950s, including Acer saccharum, Acer negundo, and Morus rubra. The herbaceous understory changed dramatically as well. Most of the prairie species present in the 1950s are gone and replaced by species typical of forests and disturbed woodlands. Additionally, shrubs and lianas, including the problematic invasive species Rhamnus cathartica, Zanthoxylum americanum, and Celastrus orbiculatus, have moved into savannas. Across the sites fire frequency was very low or absent over the past 60 years. Current management varies between sites, and ranges from active cutting and burning, canopy thinning, grazing, or no management. In the absence, or near absence, of fire over the past 60 years, many savannas are showing signs of mesification, with a decrease in oak dominance and canopy replacement by more mesic tree species. Most of the sites surveyed in the 1950s still exist on the landscape today, but active management is required to restore and maintain savanna structure.