Effects of repeated prescribed fire and thinning from below on understory components of southern Illinois oak-hickory forests

Fire has influenced species composition within the Central Hardwood Forest for millennia. Since the last glacial retreat, Native Americans followed by European settlers used fire as a tool to manipulate their environment. This fire use by humans helped maintain the dominance of well-adapted oak-hickory species across eastern forests. By the 1940's, land fragmentation from increased settlement and actively enforced suppression policies effectively eliminated fire from the landscape. Without the disturbance of recurrent fire that alters succession, the fertile loess-capped hills of southern Illinois have undergone several decades of compositional and structural change manifested by encroachment of mixed mesophytic competitors that are maladapted to fire. Today, land managers seek practical methods to restore declining oak-hickory forests. Southern Illinois forests in particular are lacking information on how cutting and prescribed fire techniques can be applied to encourage regeneration of oak-hickory species. In 2002, five sites were chosen across the Greater Shawnee Hills geographic region for similar ecological characteristics. A factorial combination of thinning and a fire treatment consisting of two burns was used to test the response of understory components including: seedling density, seedling height, seedling diameter, non-tree cover and available sunlight. Results showed a distinct improvement in oak-hickory seedling competitive position as compared to non oak-hickory species. Seedlings of sassafras out-competed all other groups in this study and were the only species to increase in both density and height following repeated fire. The non-tree vegetation layer increased as a result of thin from below treatments, while burning had no effect on the amount of available sunlight. Generally, woody seedlings benefited from thinning based on their physiological adaptations and fire essentially acts as a filter selecting for traits of disturbance-prone vegetation.