Effects of prescribed fire on oak regeneration, midstory retention, and mycorrhizal symbiosis in the Central Hardwood Region
Oak (Quercus) is being replaced by maple (Acer) and other mesic species across the Central Hardwood Region (CHR) and other eastern deciduous forests. This replacement is due to a self-reinforcing process called mesophication, in which the environment becomes increasingly mesic and hinders both oak retention and the disturbances which aid in the maturity of an oak-dominated forest. Within the last couple decades, forest managers have been prescribing surface fire in an attempt to mimic the historic disturbance regime that likely created these forests centuries ago. There is limited guidance on how many fires are needed and the temporal frequency with which they should be applied, leaving room for confusion and potentially detrimental burn practices. Fire is used for both seedling establishment and midstory tree retention, which may be conflicting goals with how surface fire is usually carried across the CHR landscape. Even less is known about the effects of prescribed surface fire on mycorrhizal communities and their dispersal agents, which are necessary parts of the oak ecosystem and enhance seedling establishment and maturity through their symbiosis. Fungal spore dispersal by small mammal is an important step in recovery after a disturbance such as fire, but the fires themselves may be endangering this relationship.
To better understand the relationship between immature trees, prescribed fire, topography, and stand structure, I surveyed 63 stands both with (n = 47) and without (n = 16) a known prescribed fire since 1990 in the Hoosier (IN) and Wayne (OH) National Forests. Here I quantified tree seedlings (height > 15 cm, DBH < 10 cm) and midstory saplings and poles (10 cm < DBH < 25 cm) in two concurrent studies. Across all sites, white oak (section Quercus) comprised 47.8% of all seedlings, red oak (section Lobatae) comprised 8.2%, and the common interfering species maple and American beech (Fagus grandifolia) comprised 5.4%. Mid- and overstory basal area, percent slope, years since the last burn, and total number of burns all significantly affected oak seedling density, but site characteristics like slope and midstory basal area were just as important of predictors as number of burns. However, all oak and hickory (Carya) midstory trees averaged 0.65 ± 0.003 m2 ha-1 basal area of stocking, while sugar maple (A. saccharum) midstory averaged 0.87 ± 0.001 m2 ha-1 basal area. Furthermore, overstory stand structure variables were often more important than burn variables when predicting midstory basal area and composition. Prescribed fire does favor oak and hickory over mesophytic species, but mesophication may be preventing fire alone from significantly reducing midstory maple. Likewise, prescribed fire may condition a site for regeneration of oak seedlings, but if an established midstory already exists or fire has not been used on the landscape for almost a decade, repeated applications of prescribed fire will likely have a negligible effect on oak regeneration unless the site is burned more than three times with less than four years between burns.
Finally, I extracted fungal spores from eastern chipmunk (Tamias striatus) scat collected across the Hardwood Ecosystem Experiment from plots in oak-dominated stands without a recent burn history (n = 33 trap stations) and with one burn since 2015 (n = 32 trap stations). I identified 17 fungal taxa in 64 samples; 70.6% of taxa were found in at least 10% of all samples. Arbuscular mycorrhizal taxa richness was not affected by any measured variable, but ectomycorrhizal taxa richness decreased after burning. However, environmental variables like leaf layer depth, EM tree and AM tree basal areas each helped explain some variation in spore composition and were correlated with fungal community composition in scat. Therefore, prescribed surface fire appears to differentially affect the truffles of mycorrhizal functional groups, but due to the nature of this pilot study, additional research is needed to more closely examine the relationship between mycorrhizal fungi, their dispersal agents, and prescribed fire.