Response of advance reproduction of oaks and associated species to repeated prescribed fires in upland oak-hickory forests, Missouri

Abstract

The Chilton Creek prescribed burn project was initiated in 1996 by The Nature Conservancy (TNC) to restore native oak woodlands and test the effect of frequent, low intensity surface fires conducted in the dormant season (March–April) on upland oak-hickory forests in the Ozarks of Missouri. Burning treatments on five sites totaling 1000 ha were initiated in 1998. The prescribed burn treatments included: annual burns (one site was burned annually) and periodic burns (four sites were burned with a mean fire return interval of approximately 4 years). Approximately 3000 stems of advance reproduction with basal diameters ⩽15.0 cm were measured in 1997 (pre-burn), and remeasured in 1998, 2001 and 2007 as the prescribed burn treatments proceeded. Mortality and changes in average and aggregate total height between 1997 and 2007 were analyzed for fourteen selected tree species. Over a ten-year period prescribed burning reduced stem density, mean height and aggregate height by 57%, 36% and 67%, respectively. However, individual tree species responded differently to the repeated burns. Oaks exhibited moderate mortality (36–54%) compared to hickories, sassafras and winged elm, all of which experienced relatively low mortality (18–35%). Flowering dogwood, shortleaf pine, blackgum and other minor species incurred higher mortality (60–76%). Oaks generally had slightly lower reductions in mean height and aggregate height than associated species with the exception of shortleaf pine, but the differences were mostly non-significant. Repeated burning tended to favor the white oak species (white oak, post oak, chinkapin oak) more than the red oak species (black oak, scarlet oak), although the differences were minor. Shortleaf pine was the only species to show increases in mean height following the ten-year period of prescribed burning. Effects of repeated burns on mortality of advance reproduction changed with stem size (e.g., basal diameter, total height). A logistic model quantified mortality changes as a function of the ratio between total stem height and the square of stem basal diameter. Based on this model, fourteen species were classified into four groups characterized by model intercept and slope: group 1: high intercept and high slope (shortleaf pine, flowering dogwood); group 2: high intercept and low slope (blackgum, other minor species); group 3: low intercept and low slope (sassafras, winged elm, black and pignut hickories); and group 4: low intercept and high slope (oaks, mockernut hickory). Species in groups 3 and 4 are more resistant to repeated fires than those in groups 1 and 2 as indicated by their lower mortality, particularly for stems with larger basal diameters. Among species in groups 3 and 4, oaks will likely be favored over the long term because of their conservative reproduction strategies as suggested by the significance of the regression coefficients for slope. If large advance reproduction is present, shortleaf pine will likely survive repeated low intensity fires and maintain its height and competitive position due to its ability to avoid shoot dieback.