Introduction

Dry soil oak savanna in the Great Lakes region occurs in Minnesota; Wisconsin; Michigan; northeastern Iowa; the northern portions of Illinois, Indiana, and Ohio; and the Great Lakes plains of southeastern Ontario (Chapman et al. 1995). Dry soil oak savanna, where low moisture availability limits biomass production, includes vegetation with quite varied physiognomy; terms for variants include woodland, barrens, sand savanna, scrub oak savanna, and brush prairie. The term barrens has no standard usage (Curtis 1959; Heikens and Robertson 1994; Hutchinson 1994; J. White 1994); we use this term and other terms only as defined in this chapter.

Variation in savanna vegetation physiognomy and species composition in the region has been related to interaction of disturbance regime, especially fire, with broad edaphic gradients (Grimm 1984; Bowles and McBride 1994). Fire frequency and intensity are in turn influenced by landscape structure (Leitner et al. 1991; Will-Wolf and Montague (1994) interacting with climate (Grimm 1985). Vegetation classification, used to define units for mapping and for management, frequently incorporates the assumption that site environment determines vegetation composition and structure. However, in recent savanna classifications for the upper Midwest, dry soil savanna vegetation units distinguished by structure and species composition are described as a function of different disturbance regimes as much as of different site environments (Homoya 1994; Faber-Langendoen 1995; Haney and Apfelbaum 1995). These relationships are critical for understanding savanna dynamics and for successful site management.

Savanna vegetation associated with dry, low-productivity sites in the Great Lakes region falls into two groups (Figure 8.1).