Repeated burning alters the structure and composition of hardwood regeneration in oak-dominated forests of eastern Kentucky, USA

Abstract

The exclusion of anthropogenic fire is a primary factor responsible for the 'mesophication' of eastern oak (Quercus) forests and resultant oak regeneration problems. Consequently, the reintroduction of fire is increasingly used to promote the establishment and growth of oak and hickory (Carya) and control competition from shade-tolerant species (e.g., red maple (Acer rubrum) in the forest understory. In this study, we examined the effects of fire frequency on the abundance of prominent species in the woody regeneration layer in oak-dominated forests of eastern Kentucky. Treatments included: (1) fire-excluded (FE); (2) frequent fire (FF)five burns over nine years, and (3) less-frequent fire (LFF)two burns over seven years. Prior to burning (2002) and again five and seven growing seasons following the cessation of burning in the FF and LFF treatments (2015), respectively, we inventoried tree species in the woody regeneration layer into three size classes: (1) small seedlings (stems <0.6 m), (2) large seedlings (≥0.6 m and <1.2 m) and (3) small saplings (≥1.2 m and <3.8 cm diameter at breast height). Pre-and postburn, the regeneration layer was dominated by non-oak-hickory species, and although oak-hickory regeneration was abundant the majority of stems were <0.6 m. For oak-hickory, significant treatment effects were limited to the large seedling and small sapling size classes. For large oak-hickory seedlings, density was significantly greater in the LFF than FE treatment. For small oak-hickory saplings, density in the LFF treatment was ~17 and ~4 times greater than in the FE and FF treatments, respectively. This study provides support for the notion that fire-free periods may be more of a factor controlling the abundance and composition of the woody regeneration layer than simply the number of burns. However, despite greater stem density, the fate of the oak-hickory regeneration layer that developed in response to the LFF treatment is uncertain, as the density of non-oak competitors remains high. Additional treatments (e.g., targeted herbicide application, additional burning) may be necessary to reduce the abundance of non-oak species and increase the likelihood of continued recruitment oak-hickory should natural or silvicultural release events occur.

Reversing Mesophication Effects on Understory Woody Vegetation in Mid-Southern Oak Forests

“…We observed an obvious exception to this general trend; an aggressive, positive response of shade-tolerant red maple followed canopy disturbance. Extended time under closed canopies probably promoted the observed premanagement dominance of red maple (Arthur et al 2015, Keyser et al 2017, which sprouted vigorously from well-established rootstocks during our study. This illustrates the positive feedback nature of mesophication and how reversal effort can accumulate over time in the absence of disturbance.…”

“…To illustrate, stands burned for >60 years had dense midstories if fires occurred every 4 years, but virtually no midstory if fires occurred annually (Knapp et al 2015). Controlling understory densification is often a more immediate concern (Hanberry et al 2014), but maintaining overstories dominated by disturbance-adapted trees will eventually require an understanding of how pauses in fire recurrence influence recruitment (Keyser et al 2017) and subtle differences in how fire affects individual tree species (Keyser et al 2018). Our study lacked a substantial fire-free period, and should be interpreted accordingly.…”

“…Seedling and sapling density of red maple at CWMA, and of red maple and yellow poplar at GRGL, increased from premanagement to postfire. Red maple has been described as a supergeneralist, capable of adequately performing in a wide variety of ecological conditions (Abrams 1998), and often sprouts persistently after multiple fires (Albrecht and McCarthy 2006, Arthur et al 2015, Keyser et al 2017. Yellow poplar can be similarly described where light is not limiting (Iverson et al 2017).…”

“…] and sassafras [Sassafras albidum Nutt. Nees]) are well suited to sparse overstories maintained by fire (Burns and Honkala 1990b), and sprout readily or establish quickly after fire (Abrams 2007, Keyser et al 2017. Many of these woody species also promote the fires they are adapted to by contributing highly flammable and decay-resistant fuel (Varner et al 2016).…”

“…However, knowledge gaps related to correcting mesophication effects on woody vegetation remain. Many recent evaluations of repeated fire effects on woody vegetation in the Mid-South have not occurred in conjunction with canopy disturbance (Hutchinson et al 2012, Arthur et al 2015, Keyser et al 2017). The few studies that have combined partial harvests with repetitive burning have also promoted disturbancedependent trees Van Lear 1998, Iverson et al 2017).…”