Open Cut » Environment
This project assessed four controlled and five uncontrolled fires that occurred in rehabilitated and unmined areas in central and southern Queensland between 2015 and 2020. The aim was to improve the understanding of fire behaviour, fire risk and determine the trends in rehabilitation recovery and resilience using a combination of ground surveys and remote sensing.
Total fuel loads (grass biomass and litter) across the study sites were moderate to high, with pre-fire buffel grass biomass ranging from 2 - 12 t/ha and pre-fire detached litter (dead plant material) ranging from 1.3 - 4.5 t/ha. As a proportion of total pre-fire fuel loads, detached litter recorded between 22 - 42%, and these high levels contributed to fast moving and intense fires that were observed in buffel grass pastures. Litter levels remained at very low levels for up to two years following the fires, suggesting that fuel reduction burning in rehabilitated pastures has the potential to reduce overall fuel loads and fire risk for extended periods.
The average frontal fireline intensity (in the direction of spread) for the four fires was 6,679 (±1,025) kW/m and three of the four fires resulted in fire behaviour conditions that exceeded the threshold (6,500kW/m) for safe frontal fire suppression of a grassfire (QFES, 2014). That is, a fast-moving fire in buffel grass rehabilitation would be unsafe to fight from the front, and suppression efforts would only be possible from the flanks and the back of the fire.
Following the fires, re-accumulation of buffel grass biomass in central Queensland was rapid, with two pasture sites reaching pre-fire levels within 18 months to 2 years. Rehabilitated pastures (Dichanthium / Bothriochloa spp) in southeast Queensland also showed rapid recovery reaching values higher than pre-fire control levels within 6 months of the fire. Generally, biomass trends were closely associated with daily rainfall and wet season conditions, with fluctuations also noted in unburnt controls.
Ecological metrics such as species richness and vegetation cover recorded short-term recovery trends linked closely to daily and wet season rainfall. Species richness recovery was driven by native herbs and forbs in buffel grass pastures, but exotic species were the key drivers of species richness in Dichanthium / Bothriochloa spp pastures in southeast Queensland.
Significant changes in tree and shrub density occurred, with vegetative resprouting in Acacia stenophylla and A. salicina resulting in a 625% increase in stem density after 5 years at Mine Site 1. This is contrasted by Atriplex nummularia (Old Man Saltbush) on Mine Site 2 that saw an 88% decline in density two years after the burn. Both results are likely to impact on final land use at the respective sites, by changing the structural component of the new ecosystems with the potential to cause an ecological shift.
Satellite remote sensing analysis using the Normalised Difference Vegetation Index (NDVI) time-series quantified the severity of the fires and the temporal recovery of rehabilitated areas and comparison with unburnt rehabilitation and unburnt analogue (undisturbed reference) areas. NDVI was standardised to pre-fire conditions using the resilience index (RI) and then to unburnt controls by using the standardised resilience index (SRI). When sites demonstrated SRI values of 1 or above, they were considered to have reached equilibrium for pre-fire and unburnt NDVI conditions. Average recovery trends suggest that burnt analogue sites experienced a rapid recovery, reaching SRI equilibrium within a 12-18 month period. Burnt rehabilitation on average reached the SRI equilibrium value of 1 in the 30-36 month post-fire period and, although demonstrated extended fire severity compared with analogues, rehabilitation maintained this level for the duration of the project (5 years). The results suggest that burnt rehabilitation may experience extended severity and overall lower resilience compared to analogue areas, but by 30 - 36 months post-fire, both community types recover to record resilience on par with pre-fire and unburnt analogue conditions.
The project was successful in assessing multiple fires in rehabilitation and unmined areas for recovery and resilience. In general, the assessed pastures of central Queensland and southeast Queensland showed short to medium term resilience to fire impacts. However, the impacts on tree and shrub density have the potential to result in shifts in ecological trajectories, and future rehabilitation planning and fuel reduction burning should account for species recovery traits, seasonality and climate extremes as mines move towards closure.