Effects of Canopy Disturbance and Fire on Vegetation and Fuel Dynamics in Oak Forests of the Mid-South USA

Removing fire’s influence from Southern Appalachian and Central Hardwood forests (Mid-South) has 1) virtually eliminated communities defined by shortleaf pine (Pinus echinata) and native warm-season grasses, 2) greatly altered fuel-bed properties, 3) limited the regeneration of shade-intolerant and fire-adapted woody species, and 4) decreased herbaceous groundcover and diversity. We evaluated the ability of canopy-disturbance (none, 7, and 14 m² ha^-1 residual basal area) and fire-season (none, October, and March) combinations to reverse such trends by monitoring vegetation and fuels from 2008 to 2016 at three sites located across the Mid-South. Shortleaf pine regeneration and native warm-season grasses occurred when canopy closure was reduced below 65 % and the dominance of understory woody vegetation was reduced. Regardless of degree, thinning doubled (+19.6 Mg ha^-1) coarse woody fuels (diameter >0.66 cm) and 3 biennial fires did not affect this difference. A net reduction of fine-fuels (reduced woody [litter and 1-hour], increased herbaceous) followed thinning and burning; however, maintenance of this reduced level required biennial burning, and the rate of herbaceous fuel increase under the biennial burning regime suggested future compensation for reductions in fine woody-fuels. Thinning and fire shifted understory woody communities towards shade-intolerant and fire-tolerant woody species. Management nearly doubled (+2,256 stems ha^-1) oak (Quercus spp.) seedling density across all sites, but mesophytic species (largely red maple [Acer rubrum]) persisted and perhaps precluded an even greater response of disturbance adapted woody species. Herbaceous diversity increased 3.4- to 5.2- fold across sites from pre- to post- treatment. Fire-season did not have strong effects on any monitored components of this system. Overall, our results question restoration associated thinning and burning as regionally effective fuel reduction treatments but demonstrate the ability of such disturbance to increase diversity, function, and sustainability of oak communities throughout the Mid-South region.