Prescribed fire and natural canopy gap disturbances: Impacts on upland oak regeneration

Abstract

Across the central and eastern U.S., decades of fire exclusion have coincided with upland oak (Quercus spp.) regeneration problems and a compositional shift toward shade-tolerant, fire-sensitive species like red maple (Acer rubrum L.) and American beech (Fagus grandifolia Ehrh.). Because oaks are fire-adapted and moderately shade-intolerant, prescribed fire is commonly used as a management tool to decrease competition, increase light, and promote oak regeneration. However, prescribed fire alone often fails to sufficiently open the canopy and improve oak competitive status, suggesting the combination of fire and canopy gaps may be necessary for oak success. To better understand the effects of single and multiple prescribed fires alone and combined with naturally-formed canopy gaps on tree regeneration, we measured tree densities (both seed and resprout origin) in five height size classes (small seedlings: ≤0.5 m, large seedlings (>0.5–1.0 m), saplings (1.1–4.0 m), midstory (4.1–7.0 m), and poles (7.1–12 m)) within gaps and non-gap areas treated with no fire, single fire, or multiple fires (2–3) across six sites within the Knobs Region of Kentucky (U.S.A.) in 2017. Oaks were common as small and large seedlings without fire, especially within gaps, but they were largely absent from larger size classes. Instead, red maples and American beech dominated sapling and midstory size classes without fire, regardless of gap treatment. Single and multiple fires reduced both absolute and relative density of American beech saplings and red oaks (Q. velutina Lam., Q. coccinea Munchh. and Q. rubra L.) of all sizes, but single fires, both within and outside of gaps, increased red maple large seedling, sapling, and midstory dominance. Multiple fires, both within and outside of gaps, reduced red maple abundance, and this coincided with increased relative density of white (Q. alba L.) and chestnut oaks (Q. montana Willd.), but not of red oaks. Red oak small and large seedlings were the only oaks where absolute density increased in the small (~300 m2), relatively old (20–30 yr), naturally-formed canopy gaps in this study. Our findings suggest that management techniques that include multiple prescribed fires and large canopy gaps (>300 m2) created relatively soon after fire will likely be necessary to reduce competing species, increase oak density, and allow oaks to reach sapling and midstory size classes.