Ecotones are transitional areas between contrasting ecological systems where species from both systems co-occur. Spatial variability in the position, shape, and composition of regional-scale ecotones can influence how ecological systems respond to environmental changes. For vegetation, patterns of species co-occurrence in ecotones can indicate ecotone properties—such as ecotone width and transition sharpness—and can reflect processes driving the transition between systems. The Tension Zone in Wisconsin is an ecotone between southern deciduous-dominated woodland and northern mixed conifer-hardwoods forests. To determine the historical spatial variability of ecotone properties in the Tension Zone, we used spatial records of trees that occurred across Wisconsin in the mid-1800s from the Public Land Survey database. Following indicator species analysis to determine characteristic species from each region, we mapped the Tension Zone based on the ratio of spatial density estimates. To determine whether northern and southern trees were related to each other similarly across the Tension Zone, we compared nearest neighbor distances between northern and southern trees. Last, we analyzed clustering patterns of northern and southern species at different scales and in different parts of the Tension Zone using standardized Ripley’s K for northern and southern trees.
The map and examination of species co-occurrence patterns in the Tension Zone revealed great variability along the length of the Tension Zone. The width of the Tension Zone ranged from narrow (27.2 km) to comparatively wide (162 km), suggesting variable rates of transition between northern and southern vegetation. Based on nearest neighbor distances between northern and southern species, southern species more frequently occurred farther away from northern species than vice versa. Northern and southern species occurred farther away from each other where Tension Zone width was narrow, as well as in the eastern (wetter) region. This may be due to environmental conditions leading to competitive species interactions. Patterns of clustering in northern and southern species also varied along the Tension Zone; for example in some regions, northern and southern species had similar clustering patterns, whereas in other regions northern species are more dispersed from each other than southern species at small scales (regions < 1.5 km) but are more clustered at larger scales (regions > 2 km). Because the spatial patterning of northern and southern species varies with scale and location along the ecotone, we conclude that a range of scale-dependent processes control vegetation composition along the Tension Zone.