Effects of Increasing Fire Frequency on Conservation Values in Eucalyptus Grassy Woodland in the Process of Invasion by Allocasuarina verticillata

Jamie B Kirkpatrick,Ian Jenkinson

doi:10.3390/fire5020031

Abstract

Woody thickening is a widespread phenomenon in the grassy woodlands of the world, often with deleterious effects on nature conservation values. We aimed to determine whether increasing the frequency of planned fire prevented woody thickening and improved conservation values in a Eucalyptus viminalis grassy woodland in the process of invasion by Allocasuarina verticillata (henceforth Allocasuarina) in Hobart, Tasmania, Australia. We used a before–after control intervention design. Ten plots from which detailed vegetation data were collected in 2018 (before the burns), 2019 (between burns), 2020 (between burns) and 2021 (after the burns) were randomly located in each of four blocks. Two of the blocks were burned in both 2018 and 2021. One block was burned only in 2021, and another was not burned at all. Mechanical thinning of Allocasuarina took place in 2021 in six plots in one unburned block and in three twice-burned plots. The fires were low intensity and patchy, reflecting the reality of planned burns in this environment. Thus, there were unburned plots mixed with burned plots in each of the three burned blocks. We compared changes in vegetation and cover attributes between a preburn survey in 2018 and a postburn survey in 2021, between five fire history/thinning classes (unburned, no thinning; unburned, thinning; twice burned; burned in 2018 only; burned in 2021 only). Fires in both 2018 and 2021 resulted in lower litter cover and higher exotic species richness than one fire in 2021. Exotic species richness increase between 2018 and 2021 was greater after fires in 2018 and 2021 than after a fire in 2021 alone. Exotic species richness was lowest six years after fire and highest one to three years after fire. The basal area of Allocasuarina was, counter-intuitively, less reduced by two fires in four years than by one. Mechanical thinning reduced shrub layer cover, which largely consisted of small trees, but did not affect basal area. Our data suggested that grass cover increased until five years after a fire, declining back to a low level by eight years. The implications of the results for conservation management are that the mechanical removal of young Allocasuarina may be successful in preventing its thickening and that burning at a five-year interval is likely to best maintain understorey conservation values. The counter-intuitive results related to Allocasuarina basal area emphasise the importance of understanding cumulative effects of fire regimes on fuel cycles and the consequent effects on tree mortality.

Highlights

With the exception of sedge cover, which was significantly higher in the plots that were burned only in 2018 than those burned only in 2021, the richness, basal area and cover variables did not vary between the 2021 burn/thinning classes in the prefire sampling
Eucalyptus viminalis was an occasional emergent from the Allocasuarina canopy (basal area = 1.1 (0.3) m2 /ha)
Our data suggest that fire more frequent than once every five years is likely to be counterproductive in preventing woody thickening in this ecosystem

Summary

Introduction

Temperate grassy ecosystems are one of the vegetation types of the world that have been most transformed by agricultural development [1,2,3,4,5]. In their preagricultural condition, the interaction of fire and grazing/browsing regimes [6] and the role of ground disturbance in reducing root competition with tree seedlings in the grass sward [7] appear to have been critical in determining their woody content. Stock can remove competition for threatened native plants and suppress tree and shrub regeneration, sometimes producing a desirable conservation outcome for flora [8]

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