Thinning, fire, and oak regeneration across a heterogeneous landscape in the eastern U.S.: 7-year results

We document an increase in oak and hickory advance regeneration, depending on landscape position, in the sixth year following mechanical thinning and repeated prescribed fires in southern Ohio, USA. Oak-dominated communities provide a multitude of human and natural resource values throughout the eastern United States, but their long-term sustainability is threatened throughout the region by poor regeneration. This study was established to assess regeneration following midstory thinning (late 2000) and prescribed fire application (2001 and 2005) at two sites in southern Ohio. Each of the four 20+ha treatment units (two thin and burn, two untreated controls) were modeled for long-term moisture regime using the integrated moisture index (IMI), and a 50m grid of sampling points was established throughout the units. Vegetation and canopy openness were sampled at each gridpoint before and after treatments, in 2000, 2001, 2004, and 2006. The thin and burn treatment generally resulted in more advance regeneration (>50cm height) of oak and hickory. The second fires in 2005 created additional landscape heterogeneity by causing variable tree mortality, and thus canopy openness, across the IMI gradient. The drier landscape positions generally had more intense fires, more canopy openness, and more oak and hickory advance regeneration; several other tree species also exhibited marked landscape variation in regeneration after treatments. Though advance regeneration of several competing species became abundant after the initial treatments, the second fires reduced the high densities of the two major competitors, Acer rubrum and Liriodendron tulipifera. Two simple models were developed: (1) a model of oak “competitiveness” based on the plot data related to advance regeneration of oaks and competitors and (2) a model estimating the probability of a plot becoming 'competitive for oak' based on canopy openness, IMI class, and number of oak and hickory seedlings present. For dry or intermediate sites with at least 5000 oak and hickory seedlings/ha, opening the canopy to 8.5-19% followed by at least two fires should promote oak and hickory to be 'competitive' over about 50% of the area. However, no appreciable oak and hickory regeneration developed on mesic sites. Overall, these results suggest promise for partial harvesting and repeated fires as a management strategy to reverse the accelerating loss of oak dominance in the central hardwoods region.