Grassland-Forest Transition in the Prairie Peninsula Region of Northwest Indiana as Recorded by the US Public Land Survey (1829-1835) as part of a Report to the Indiana Department of Natural Resources, Lake Michigan Coastal Program
AbstractTallgrass prairie extends eastward in a narrowing peninsula across northern Illinois into adjacent Indiana. This vegetation reflects a longitudinal gradient of decreasing drought severity, and local vegetation patterned by interactions between fire and landscape firebreaks. Understanding how these processes shaped vegetation provides an ecological context for restoration management as well as a baseline for projecting effects of ongoing climate change on vegetation. We analyzed vegetation across this region using maps, notes and witness tree data from the Government Land Office Public Land Survey (PLS) of Lake, Porter, and LaPorte counties, Indiana, which was conducted in 1829-1835. We examined how topography, landscape firebreaks and proximity to Lake Michigan interact to mediate the transition from grassland to forest across this region and compared it to vegetation pattern in adjacent Illinois. This region displayed an abrupt longitudinal (west to east) gradient of decreasing proportional abundance of prairie and increasing abundance of woody vegetation. Over a 5-10 km distance near the Illinois-Indiana border, prairie decreased from 80% to 20% cover, while barrens increased from 20% to 50% and timber from 25% to 50% cover. These changes appear to have been controlled by interactions between fire and local landscape features including water bodies, shifting alignment of the Valparaiso Moraine and the presence of Lake Michigan. Barrens were transitional between prairie and timber, and occupied habitats with intermediate topographic characteristics and had greater abundance of fire-adapted species. A tree density gradient across NW Indiana comprised increasing density of forest (>100 trees/ha) and decreasing abundance of savanna (<50 trees/ha). Oaks, which are fire-tolerant and shade-intolerant, remained dominant across this gradient. Fire-intolerant shade-tolerant species, including American beech and sugar maple increased in abundance across the gradient, reaching co-dominance in forest conditions in the eastern part of the study area. These effects varied among physiographic regions and proximity to Lake Michigan, which had greater abundance of coniferous species. Abundance of woody undergrowth data suggest that woody undergrowth dominated by American hazelnut was frequent throughout the region, especially in barrens. Managing and restoring the region's fire-dependent prairie and savanna vegetation should take into account evidence that this vegetation was strongly transitional and that woody plants (trees and shrubs) were a natural component. Management to restore this feature may be challenging in preserves that are smaller than the scale of original landscape processes.
Alternate TitleReport to the Indiana Department of Natural Resources, Lake Michigan Coastal Program