Community-Level Impacts of Management and Disturbance in Western Michigan Oak Savannas

Midwestern oak savanna systems are typically defined by their open canopy and the co-existence of scattered mature oak trees and a ground layer dominated by herbaceous vegetation. The structure of these systems is thought to be primarily maintained by disturbance such as fire. In this study we examined the plant community of 21 different oak savanna sites in western Lower Michigan, U.S.A., across a coarse disturbance gradient created by different management practices. Herbaceous community composition differed significantly across a variety of management approaches, while overall diversity remained similar. Indicator species analysis (ISA) identified several species commonly associated with mixed oak forest understories (e.g., Maianthemum canadense) as indicators for recently abandoned oak savanna sites, whereas the indicators identified for managed or heavily disturbed sites included common savanna-associates (e.g., Lupinus perennis). Variation in soil characteristics (C:N ratio and pH) and canopy cover may be driving these differences in plant community composition between management approaches. These results reinforce the importance of disturbance to Midwestern oak savanna ecosystems. Furthermore, if long-term management goals include encouraging the establishment and maintenance of herbaceous oak savanna-associated plant species, disturbance created through management activities, such as hand cutting, will likely yield better results over inaction, especially where using fire is not an option.