Contemporary fire regime and 33 years of black oak savanna dynamics at the Indiana Dunes

Background/Question/Methods

Over the last hundred years our understanding of the role of disturbances and especially fire in oak savanna ecosystems has increased.  Nevertheless, long-term studies of oak savanna dynamics are few and far between.  We report on 33 years of oak savanna change from two black oak (Quercus velutina) savannas sites studied by Norm Henderson in 1981: Miller Woods (MW) and Inland Marsh (IM) at the Indiana Dunes National Lakeshore in northwest Indiana.  Twenty-five permanent plots were resampled at each site respectively in 1998, 2000, and 2014, and 1986, 1994, 1998, and 2014. All woody stems greater than 2.5 cm dbh were tallied and identified to species.  We used ordination, multivariate analyses and regression to investigate the influences of landscape physiognomy and fire regime (time since last fire, mean fire return interval, fire frequency), on woody structure change in these oak savannas by understory, subcanopy and canopy size classes.

 Results/Conclusions

The mean fire return interval at IM (6.5 years) was almost twice that at MW (3.8 years) similar to historic regimes for the two sites.  MW experienced 12 wildland and 1 prescribed fire over the 33 years, whereas IM had 6 prescribed fires and one wildland fire.  No woody species at MW showed in change in density over the 33 years.  IM showed an increase in fire intolerant species including black gum (Nyssa sylvatica), quaking aspen (Populus tremuloides), and buttonbush (Cephalanthus occidentalis) especially at low elevation plots.  Over 33 years, MW showed little change in tree and stem density, and basal area.  In contrast, IM experienced dramatic increases in stem and tree density after an intense wildfire in 1986.  High stem densities persisted with seven years of fire suppression and until some sites were mechanically thinned in the mid-2000’s and all experienced an intense prescribed burn in 2013.  Change in stem density was positively correlated with increasing time since last burn.  The results illustrate the perils of intense wildfires followed by fire suppression in altering savanna/woodland structure and the role of landscape structure in modifying fire regime. The results will be discussed in the context of oak savanna management and restoration in regards to stability and alternative stable states.