Prior to European settlement, the dominant trees in these forests were fire-tolerant species of oak. They were relatively open-canopied forests (large trees spaced widely apart). They had either a multi-layered structure including a shrubby understory containing hazel, or containing prairie plants such as big and little bluestem.

The fires favored the open nature of the forests, allowing adequate light to keep the prairie species growing and allow the regeneration of oaks, which require high light conditions for growth. 

The effect of lack of fires on forest
European settlers affected the structure and composition of Illinois forests through fire suppression. European settlers sought to protect their farms and livestock with firebreaks. 

The reduced burning resulted in the growth of less fire-tolerant species such as sugar maple. Sugar maple saplings can grow well in low light conditions in the understory. When a gap in the canopy appears, they grow to maturity and cast dense shade. The shade prevents the regeneration of oaks because oak seedling need more light to grow to the sapling stage. The low light conditions created by the maple trees also prohibits the growth of prairie species in the understory of former savannas and barrens (see panoramas of barrens for its lack of shrub layer).

Overall, as a result of fire suppression in Illinois, oaks are not regenerating and sugar maple has increased dramatically.