Competition, size and age affect tree growth response to fuel reduction treatments in mixed-oak forests of Ohio

Prescribed fire and thinning treatments are increasingly used to mitigate potential wildfire hazards, alter stand structure and restore forest functions. In this study, the effects of competition, tree size and age on tree growth following prescribed fire and thinning treatments were examined to better understand the consequences of these management tools on forest systems. Tree-ring data from 348 trees, comprising five species (white oak, black oak, chestnut oak, hickories, and yellow-poplar) were analyzed following standard dendrochronological protocols. Data were collected from 80 0.1-ha plots in two study sites, each with four treatment units (control, thin-only, burn-only, thin + burn) in mixed-oak forests of Ohio. A neighborhood analysis was used to assess the competitive status of each sampled tree. Basal area increment (BAI) and tree size were positively related, with the strongest correlation found in the burn-only treatment. Age was negatively related to BAI, though weakly. Competition was inversely correlated with BAI, with trees from the thin-only unit showing the strongest correlation. BAI was greater for larger trees when competition was low and declined at a steeper rate as competition increased. Smaller trees grew less in general but decreases in BAI were not as steep as competition increased. Overall, tree size, age, and competition explained ∼40% of total variance in BAI across all species. Values for individual tree species ranged from 30% to 57%, indicating considerable variation in the responses of species to these factors. Yellow-poplar exhibited greater sensitivity to competition than the other species analyzed. Altogether, competition was more important than size and age for tree growth in these managed stands. Variation in competitive status of trees within treatments supports the view that prescribed fire and thinning influence forest growth by creating heterogeneity among stands, and thus demonstrates the need for individual tree-based analysis to fully understand prescribed fire and thinning impacts on forest ecosystems.