The legacy of fire: long-term changes to the forest understory from periodic burns in a New England oak-hickory forest

Caroline G Borden,Mark S Ashton,Marlyse C Duguid

doi:10.1186/s42408-021-00115-2

Abstract

BackgroundOver the last century, fire exclusion has caused dramatic structural and compositional changes to southern New England forests, highlighting the need to reintroduce fires into the historically pyrogenic landscape to study the response. We investigated the effects of a single overstory thinning and midstory removal to create an open oak-hickory woodland structure, followed by repeated prescribed burns. We hypothesized that burning would create greater floristic diversity comprising fire-tolerant woody regeneration and shade-intolerant herbaceous flora. We followed shifts in plant structure, composition, and diversity over a 23-year period, using a before-after-control-impact design with data collected once prior to burning and twice after burn treatments had begun and with soil samples collected after nearly 20 years of burning.ResultsWe observed a dense ingrowth of saplings on unburned plots that were largely absent from burned plots and a shift in midstory composition to favor mesic sweet birch (Betula lenta L.) in the unburned treatment, as opposed to the hickories (Carya Nutt. spp.) and oaks (Quercus L. spp.) that dominated the burned treatment. Burning resulted in a significantly greater density, richness, Shannon diversity, and evenness of understory vegetation (forbs, shrubs, tree seedlings). These four measures remained high on burned plots, despite a decrease in both floristic diversity and evenness on unburned plots and a reduction in unburned site-level richness. Understory composition varied significantly by year and burn treatment, with unburned plots largely characterized by shade-tolerant species while burned plots showed an enhanced abundance of heliophilic plants.ConclusionsOur results suggest that periodic burning increases nutrient microsite heterogeneity and periodically maintains greater understory light, both of which in turn increase understory plant density and diversity and cause a shift in understory composition. This study shows that repeated prescribed burns in an open New England woodland have lasting structural and compositional effects capable of restoring pre-settlement, pyrogenic vegetation patterns.

Highlights

Over the last century, fire exclusion has caused dramatic structural and compositional changes to southern New England forests, highlighting the need to reintroduce fires into the historically pyrogenic landscape to study the response
We investigate the effect of overstory cutting followed by repeated prescribed burns on a southern New England oak-hickory ecosystem over a 23-year period
We identified each understory plant to the species level, with the exception of the genera grape (Vitis L. spp.), hawkweed (Hieracium L. spp.), hawthorn (Crataegus L. spp.), serviceberry (Amelanchier Medik. spp.), and hickory, which we identified to the genus level

Summary

Introduction

Fire exclusion has caused dramatic structural and compositional changes to southern New England forests, highlighting the need to reintroduce fires into the historically pyrogenic landscape to study the response. Numerous studies have documented this structural change triggering a positive feedback cycle dubbed “mesophication,” whereby microenvironmental conditions under dense forest canopies favor shadetolerant, mesic species such as maple (Acer L. spp.), cherry (Prunus L. spp.), and birch (Betula L. spp.) over the shade-intolerant, fire-adapted species such as oak, hickory, and pine that used to dominate this landscape (Brose et al 2001; Hall et al 2002; Nowacki and Abrams 2008; Kreye et al 2013; Poulos 2015) The impact of this mesophication on overall floristic diversity and richness is more nuanced (Anderson and Schwegman 1991; Nowacki and Abrams 2008). It is possible that diversity could decline due to increased canopy shade and the progressive dominance of fire-sensitive, shade-tolerant species, as has already been observed by Anderson and Schwegman (1991) (Ryan et al 2013; Stambaugh et al 2015)

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