Introduction:
Persistence of the Prairie

Although fires were clearly important for the persistence of prairie in the Prairie Peninsula, they required a source of ignition. During the fall, when most of the fires occurred, thunderstorms were infrequent and typically accompanied by rain. However, Native Americans regularly burned the prairie for a variety of reasons. Many early observers believed and many modern ecologists also believe the Prairie Peninsula was primarily the result of Native American burning. However, throughout North America, Indians burned not only prairie, but also forest that did not convert to prairie. For example, Indian burning was common in the oak-hickory forests of the Ozarks and the pine forests of the Southeast. Thus, Indian burning could not have been the sole cause for prairie in the Prairie Peninsula.

In addition to requiring a source of ignition, fires require appropriate weather before they will burn. Seasonal dry periods in fall and spring encouraged fires. Obviously, rainy weather or snow cover would inhibit fires. Extended dry periods in late summer were probably particularly important for drying out the extensive areas of poorly drained wet prairies in the Prairie Peninsula and rendering the tremendous biomass growing in these prairies flammable. Thus, fire and climate are inextricably intertwined.

As early observers noted, trees rapidly invaded prairie following European settlement and the cessation of prairie fires. Thus, climate was favorable for tree growth once the fires abated. However, on a decadal to century scale, climate is highly variable in the Prairie Peninsula. In particular, drought is more intensive in the Prairie Peninsula during drought years than in surrounding regions. The drought years of the 1930’s resulted in the death of numerous trees in Iowa and Nebraska (McCoomb and Loomis, 1944; Albertson and Weaver, 1945). Transeau (1935) described the demise of thousands of trees along the prairie-forest border in Illinois following the drought of 1913-14, and Shimek (1911) reported that thousands of trees died during the droughty seasons of 1893, 1894, and 1895. Thus, while climate during most years may be favorable for tree growth in the Prairie Peninsula, occasional severe droughts set the forest back and greatly increase the damage caused by fires.