For several reasons, less is known about the presettlement condition and composition of southern mesic forests in Wisconsin than other dominant community types (Kotar and Matthiae 1995). Mesic forests are dominated by shade‐tolerant, fire‐intolerant hardwood trees such as black walnut (Juglans nigra), red and sugar maple (Acer rubrum and A. saccharum), basswood (Tilia americana), and hackberry (Celtis occidentalis), with bitternut hickory (Carya cordiformis) and butternut (J. cinerea) common but not abundant in most stands and white ash (Fraxinus americana) and elm (Ulmus spp.) as associates. Historically, southwestern Wisconsin was alternatively vegetated by mesic forest and oak savanna/open oak woodland/prairie, with the dominant phase during any given time period determined by climate and its affect on the frequency of wildfires (Anderson 1998). During drier periods, wildfire was more frequent, and fire‐dependent communities dominated the landscape. Refugia of mesic forest probably persisted on north‐facing slopes that were less susceptible to frequent wildfire due to site conditions (north‐facing slopes receive less sunlight, making their interior conditions more humid than south‐facing slopes, and are generally lacking in flammable understory fuels). During wetter periods, fire frequency decreased and mesic forests expanded from their north slope refugia. As shade‐ tolerant mesic forest species migrated into oak savanna and open oak woodland, they inhibited oak regeneration by decreasing the amount of light penetrating to the understory. Evidence from fossilized pollen supports the hypothesis that mesic forests were increasing in area prior to Euro‐American settlement (Anderson 1998), although the rate of replacement was probably slowed due to use of fire by indigenous populations to drive game. Land surveys conducted in 1832 do not give any indication of the presence of southern mesic forest at the Preserve, and it is possible (perhaps even likely) that the mesic hardwood stand of
unit 23 is a post‐settlement artifact resulting from a combination of land‐use history factors, such as high‐grade logging, wildfire suppression, and grazing by cattle. In any case, southern mesic forest was never extensive in southwestern Wisconsin prior to settlement (Kotar and Matthiae 1995). Wildfire suppression and high‐grade logging of oaks during the first decades of settlement encouraged a temporary expansion of southern mesic forests, but much of this acreage was converted to farm land and all that remains are small, isolated forest remnants. The largest block of southern mesic forest occurs in the Baraboo Hills Region, which was spared conversion to agriculture by the presence of extensive quartzite talus. Southern mesic forest is a relatively stable vegetation association in the absence of wildfire, and the dominance of shade‐tolerant species often increases over time.