Delayed Tree Mortality After Prescribed Fires in Mixed Oak Forests in Northwestern Ohio

Abstract

Delayed tree mortality can contribute to variability in fire effects in forests, but its prevalence is not well understood in eastern North American oak forests where a management goal is using prescribed fire to shape forest density and composition. To assess potential delayed mortality after prescribed fires, we tracked the fates of 690 trees of four species in burned and 542 trees in unburned oak forests in northwestern Ohio, USA, and modeled survival using tree diameter and bole char. Delayed mortality, occurring 3–4 growing seasons after fire and in addition to initial mortality (1–2 growing seasons after fire), varied with species and tree diameter. Compared to initial mortality, delayed mortality resulted in eleven times more small-diameter (1–13 cm) red maple (Acer rubrum) dying after fire. White oak (Quercus alba), 1–25 cm in diameter, also incurred delayed mortality (five-times increase in dead trees). Neither sassafras (Sassafras albidum) nor black oak (Quercus velutina) displayed delayed mortality. Background tree mortality in unburned sites was minimal (0.4% per year across species). Logistic regression to model canopy survival selected only stem diameter for burned red maple trees, whereas both diameter and bole char related to survival in other species. Results suggest that (1) monitoring postfire tree mortality in oak forests should extend for at least four growing seasons to detect delayed mortality in some species, and that (2) single surface fires may eventually reduce encroaching red maple in oak forests more than initial postfire years indicate. Study Implications Delayed tree mortality, a poorly understood phenomenon in eastern North American oak forests, varied with species and stem diameter after prescribed fires in northwestern Ohio oak forests. Accompanied by minimal mortality of large oaks (>25 cm in diameter), single surface fires appear capable of reducing density of encroaching red maple stems, but these reductions may not appear for at least 3–4 growing seasons after fires.

Rapid increase in sensitive indicator plants concurrent with deer management in an oak forest landscape

“…These results suggest 3 further management applications or questions. First, while results revealed that browse damage to oak seedlings declined after deer culling, the seedlings are not transitioning to larger size classes (Abella et al 2021), consistent with widespread findings in eastern North American oak forests (e.g., Thomas‐Van Gundy et al 2014). Advancing oak seedlings to saplings often requires a combination of canopy gaps, fires, and low levels of deer herbivory (Nuttle et al 2013, Walters et al 2020).…”

Changes in Groundlayer Communities with Variation in Trees, Sapling Layers, and Fires during 34 Years of Oak Savanna Restoration

“…The threshold for tree density was below 100–200 trees/ha. Trees generally remained above these thresholds for the first 23 y of the study, as prescribed fires had minimal influence on survival of large oaks, consistent with their resistance to low-severity fires (Abella et al 2021). It was after 2010, after wind damage halved overstory tree basal area and density, when some of the highest savanna plant cover and richness values appeared.…”

“…As tree canopy cover is also often used to characterize midwestern oak savannas, we developed regression equations with tree canopy cover data we collected some years to estimate canopy cover from tree density and basal area to enable converting among these measures (Supplemental Appendix S1). We selected <10 cm as a cutoff for saplings because these smaller stems are more readily top killed (although root systems can survive) by typical low-severity fires (Abella et al 2021) and to represent the dense sapling layer below overstory trees that can form on savanna sites during fire-free periods (Taft 2009; Figure 2). We based the number of fires in the last 3 y on calendar years preceding a sampling year, but excluding any fires in autumn the calendar year of sampling as autumn fires would be after summer sampling that calendar year.…”

“…The low-severity, prescribed fires commonly used in contemporary savanna restoration and management do not necessarily have much influence on large, thick-bark, fire-resistant oaks (Cole et al 1992). However, low-severity fires can affect the sapling layer, especially by top-killing less fire-resistant species encroaching into savannas (e.g., red maple [ Acer rubrum L.]) and small oaks not yet fire resistant (Abella et al 2021). Fire can influence soils through processes such as incinerating seeds and reducing thickness of surficial organic layers (O horizons), which can filter groundlayer species composition (Plenzler and Michaels 2015).…”