Human-induced changes to climate and fire regimes are expected to have considerable consequences for future forest productivity and functioning. Additionally, there is increasing need for practical strategies to mitigate drought effects on forests. Manipulating tree population density and thus minimizing density-dependent competition for resources has emerged as one potential strategy to moderate drought-induced stress. Thus far, however, studies have focused on thinning experiments and have largely overlooked other commonly used management techniques, most notably prescribed fire. The long-term effect of increased fire frequency on the response of tree growth to climate remains poorly known. We measured the basal area increment (BAI) of adult oak (Quercus spp.) trees in the Missouri Ozarks, USA that experienced unburned, annual or periodic (every 4 years) prescribed fire treatments from 1950 – 2015. To assess whether increased fire frequency interacts with climate to determine long-term forest productivity, we modeled tree-level BAI as a function of potential evapotranspiration (PET), fire treatment and their interaction. We also quantified the effect of fire-driven reductions in tree population density on drought resistance (growth during drought) and resilience (recovery of growth after drought).
Results/Conclusions
Contrary to our predictions, frequently burned forests did not differ in drought resistance or resilience relative to controls. Rather, our results show that periodic, but not annual, fires reduced tree growth during wet periods favorable to growth (periodic fire x PET: P < 0.01), most likely due to observed, fire-driven reductions in inorganic N availability (fire frequency: P < 0.05). Our findings suggest that long-term manipulations of fire frequency can have negative effects on long-term forest productivity, but effects are contingent upon the return interval and therefore intensity of fires. These results highlight important differences in how fire and thinning disturbance influence density-dependent competition and the response of forests to climate. Although additional studies are needed from other forest ecosystems, this study suggests fire has limited effectiveness in mediating drought stress and instead, may constrain tree growth due to changes to nitrogen cycling.