Mechanical Mastication Reduces Fuel Structure and Modelled Fire Behaviour in Australian Shrub Encroached Ecosystems

Abstract

Shrub encroachment of grassland and woodland ecosystems can alter wildfire behaviour and threaten ecological values. Australian fire managers are using mechanical mastication to reduce the fire risk in encroached ecosystems but are yet to evaluate its effectiveness or ecological impact. We asked: (1) How does fuel load and structure change following mastication?; (2) Is mastication likely to affect wildfire rates of spread and flame heights?; and (3) What is the impact of mastication on flora species richness and diversity? At thirteen paired sites (masticated versus control; n = 26), located in Victoria, Australia, we measured fuel properties (structure, load and hazard) and floristic diversity (richness and Shannon’s H) in 400 mP2 plots. To quantify the effects of mastication, data were analysed using parametric and non-parametric paired sample techniques. Masticated sites were grouped into two categories, 0–2 and 3–4 years post treatment. Fire behaviour was predicted using the Dry Eucalypt Forest Fire Model. Mastication with follow-up herbicide reduced the density of taller shrubs, greater than 50 cm in height, for at least 4 years. The most recently masticated sites (0–2 years) had an almost 3-fold increase in dead fine fuel loads and an 11-fold increase in dead coarse fuel loads on the forest floor compared with the controls. Higher dead coarse fuel loads were still evident after 3–4 years. Changes to fuel properties produced a reduction in predicted flame heights from 22 m to 5–6 m under severe fire weather conditions, but no change in the predicted fire rate of spread. Reductions in flame height would be beneficial for wildfire suppression and could reduce the damage to property from wildfires. Mastication did not have a meaningful effect on native species diversity, but promoted the abundance of some exotic species.

What Do the Australian Black Summer Fires Signify for the Global Fire Crisis?

“…In Australian forests long-term fire management strategies focus on reducing fuel loads and altering fuel structure, with the aim of reducing the rate of spread and severity of wildfires and to increase opportunities for safe fire suppression. Fuel reduction is most commonly achieved through the use of hazard reduction burns-i.e., planned fire-although fuel mastication is also increasingly undertaken in areas where it can be Fire 2021, 4, 97 challenging to implement burns, such as close to houses [31]. Many jurisdictions have annual targets for the area of land subject to hazard reduction burns.…”

Long-Term Response of Fuel to Mechanical Mastication in South-Eastern Australia

“…All measurements took place from September to December 2020. These data were supplemented with similar measurements made between November and December 2018 at nine masticated sites by Grant, Duff [12]. In total, 63 plots were used in the analysis and represent a space-for-time sampling approach.…”

“…The mastication process uses heavy machinery to grind, chip or shred standing fuel (commonly understorey and midstorey vegetation) and deposit it on the forest floor [8]. Mastication aims to disrupt the vertical continuity of fuel through the forest profile (i.e., from the surface to canopy), thereby reducing fire intensity, flame heights and reducing the potential for crown fire [9][10][11][12]. However, the relocation of standing fuel to the ground results in larger amounts of fine (<6 mm diameter) and coarse (≥6 and ≤25 mm diameter) dead fuel particles on the forest floor.…”

“…Similar differences exist in studies examining the recovery of standing fuel at masticated sites. Understorey shrub density and cover in pine forests return to pre-treatment levels after 4 years [23][24][25] whilst in shrublands, shrub density and cover can remain low for up to 15 years post-treatment [12,18,26,27]. Rates of vegetative growth can vary across small and large spatial scales (e.g., 100-1000 m vs. 10-100 km [28,29]) as a function of environmental factors such as precipitation, radiation and temperature.…”

“…This information will enable us to fully evaluate the effectiveness of mastication against other fuel management strategies [37,38] and its potential to alter wildfire behaviour and risk [14]. Yet, there is relatively little known about the long-term efficacy of mastication in south-east Australian ecosystems as existing studies focus on shorter time periods post-mastication (e.g., 4 years) [12,15]. The aim of our study was to quantify the longer-term (9 years) effect of mastication on fuel across an aridity gradient.…”