Long-term growth response of trees to prescribed fire and thinning treatments in the mixed oak forests of southeastern Ohio

Background/Question/Methods

The application of prescribed fire and thinning as part of an integrated forest management solution has increased dramatically across the United States within the last decade, driven largely by the need to mitigate wildfire effects and to restore regional forests to historical structure and functions. However, it remains unclear how the treatments affect residual tree growth, particularly over a longer time scale, despite the increased literature on the subject. To understand a) how prescribed fire and thinning affect tree growth, b) variations in growth pattern due to the species type, size and age of trees, and c) to quantify the change in growth over time in southeastern Ohio, we analyzed tree ring widths of white oak (Quercus alba), chestnut oak (Q. prinus), black oak (Q. velutina), hickory (Carya spp.) and tuliptree (Liriodendron tulipifera), spanning a 20-year period (1991-2010) including 10 years pre-treatment and 10 years post-treatment. A total of 692 increment cores (346 trees ≥ 25 cm dbh) were collected from two forest stands in Vinton County, Ohio for the study. In each site, there were four treatments (control, thin, thin/burn, burn), each with ten 20 × 50 m permanent plots. Increment cores were prepared according to standard dendrochronological procedures and measured with the Measure J2X software.  Basal area increments (BAI) were computed from the ring widths as growth indicators. Results were evaluated using linear mixed effects models.

Results/Conclusions

Both standardized and raw BAI chronologies showed significant differences in the pattern of tree growth among the four treatments (P < 0.001) and over time (P < 0.001). Post-treatment mean BAI of the thin, thin and burn, and burn treatments all differed considerably relative to the control. However, after the initial surge (first five years post-treatment), growth generally slowed or decreased significantly among all treatments, except in the thin treatment where increases were still evident. Interspecific variations in tree growth response to the treatments were also evident, with tuliptree showing the highest growth rate while hickory recorded the least amount of growth. The oaks were intermediate in growth. Tree size and age also affected growth rates significantly. The present study clearly shows that the effect of prescribed fire and thinning manipulations on residual tree growth were significant but transient. Repeated silvicultural manipulations need to be considered to achieve management goals of restoring historical structure and functions.