Fire Season, Overstory Density and Groundcover Composition Affect Understory Hardwood Sprout Demography in Longleaf Pine Woodlands

Andrew Whelan,Seth Bigelow,Steven Jack,Mary Nieminen

doi:10.3390/f9070423

Abstract

Seasonal timing of prescribed fire and alterations to the structure and composition of fuels in savannas and woodlands can release understory hardwoods, potentially resulting in a global increase of closed-canopy forest and a loss of biodiversity. We hypothesized that growing-season fire, high overstory density, and wiregrass presence in longleaf pine woodlands would reduce the number and stature of understory hardwoods, and that because evergreen hardwoods retain live leaves, dormant-season fire would reduce performance and survival of evergreen more than deciduous hardwoods. Understory hardwood survival and height were monitored over seven years in longleaf pine woodlands in southwest Georgia with a range of overstory density, groundcover composition, and season of application of prescribed fire. Hardwood stem survival decreased with increasing overstory density, and deciduous hardwoods were more abundant in the absence of wiregrass. Contrary to expectations, evergreen hardwood growth increased following dormant-season fire. Differences in hardwood stem survival and height suggest that low fire intensity in areas with low overstory density increase the risk that hardwoods will grow out of the understory. These results indicate a need for focused research into the effects of groundcover composition on hardwood stem dynamics and emphasize that adequate overstory density is important in longleaf ecosystem management.

Highlights

Loss of savannas and grassy woodlands to woody species encroachment is occurring globally [1], and could have profound effects ecologically and economically [2,3]
Ground cover ground cover dominated by wiregrass and growing-season fires give longleaf pine managers dominated by wiregrass and growing-season fires give longleaf pine managers flexibility in choosing flexibilitypatterns in choosing ignitionconditions patterns and conditions will to burn intensely enough to ignition and weather thatweather will burn intenselythat enough control both deciduous control both deciduous and evergreen understory hardwoods; variability inherent in and evergreen understory hardwoods; variability inherent in longleaf pine ecosystems can longleaf pine ecosystems can lead to unintended results
We found overall low stem survival which agrees with high rates of topkill in previous research in a frequently burned hardwood forest [56], but we expected that genet and stem survival would be lower under conditions predisposing to high fire intensity such as wiregrass ground cover, high overstory density, and growing-season fire

Summary

Introduction

Loss of savannas and grassy woodlands to woody species encroachment is occurring globally [1], and could have profound effects ecologically and economically [2,3]. Forest or livestock management can affect savanna and woodland vegetation configuration and structure, which can reduce fire’s efficacy in controlling woody species and could shift these ecosystems towards closed-canopy forest. Prior to European settlement, lightning- and human-ignited fires that occurred every 1–10 years created an open, pine-dominated canopy by minimizing recruitment of understory hardwoods into the midstory and overstory [7,8,9]. Naturally ignited fires today are typically limited in extent [10], and prescribed fire is presently the most common and effective means for maintaining longleaf pine woodland structure and ecosystem function [11,12]

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