Management and restoration of black oak dominated sand savannas rely on historic vegetative descriptions (settler accounts, surveyor notes, aerial photographs). It is commonly assumed that fire alone maintains savanna structure and composition, however little is known about the specific fire frequency needed to maintain these systems. The objective of this study was to quantify and correlate characteristics of stand structure with fire history of the Kankakee Sands savannas in northeastern Illinois. Fire history chronologies were determined through dendrochronological methods from 289 dated fire scars identified on 60 black oak (Quercus velutina) trees located throughout four study sites. Stand structure was characterized in 30 circular plots (0.04 hectares) per study site that were placed at 25-meter intervals along randomly established permanent line transects in the summer of 2007. Stand structure analysis consisted of the development of tree age-size relationships among presently dominant and suppressed trees in relation to fire history.

Results/Conclusions

Variations in stand structure were strongly related to fire dynamics among the four study sites. Specifically, components of stand structure such as tree density (n = 118; r = 0.440; P < 0.0001), basal area (n = 118; r = 0.347; P < 0.0001), and total woody stem density (n = 118; r = 0.417; P < 0.0001) all increased as a function of fire free interval. The population of black oak, the dominant canopy tree (I.V. ₃₀₀ = 244.66) appeared to be structurally unstable based on the historic fire regime of the four sites. Fire intervals less than two years maintained conditions of openness, as was referenced to 1939 historic aerial photographs, but eliminated potential future canopy trees. Under these conditions, we predict a dramatic shift in community structure toward prairie vegetation, as no smaller trees were present to assume canopy dominance. Fire intervals greater than two years were associated with transition to closed canopy forests. Other management considerations, including grazing and or selective cutting, are proposed to balance historic canopy openness and promote regeneration of characteristic savanna species.