Underground » Environment - Subsidence and Mine Water
Temperate Highland Peat Swamps on Sandstone (THPSS) consist of natural ephemeral and permanent swamps. These swamps support distinctive vegetation that depends on soil water availability as well as organic rich sediments and can assist with carbon capture and storage. These swamps also contribute to regulating the water supply for major Australian eastern cities. A major concern often raised regarding THPSS is that mining induced drainage creates a drier soil moisture condition, reduces the swamps resilience to fire and leaves the swamps at a greater risk of permanent damage and loss of vegetation due to fire. This project aimed to assess the resilience of endangered THPSS, which have been mined under, in response to fire.
The fire severity maps of the study swamps were generated using satellite image differencing, and the studied sites were classified in terms of their fire severity of low, moderate, high and extreme burn. The Normalised difference vegetation index (NDVI) maps of swamps were generated and the NDVI and NBR time series for the studied sites were extracted. Then, Soil Moisture Index (SMI) maps of the study swamps were generated, and SMI time series of the studied sites were extracted. This study confirmed that NDVI values were indicative of the vegetation cover and SMI values reflected soil moisture content of the studied swamps. Therefore, the calculated NDVI maps and time series were representative of the vegetation cover recovery of the swamps and the SMI maps and time series indicated the soil moisture fluctuations of swamps' topsoil.
For Upper Nepean swamps, the NDVI and SMI values of the studied sites, which were not mined under, returned to pre-fire condition almost after one year from the fire events. The variability of SMI across the time series of the studied sites was low, regardless of the different fire severities. In contrast, for the Newnes Plateau swamps, SMI fluctuations were site specific and were dependent on precipitation and subsoil moisture. Greater SMI fluctuations were found for the site with high burn fire severity. Despite diverse soil moisture fluctuations in the Newnes Plateau sites, the NDVI time series showed a similar vegetation recovery pattern for sites with different fire severities. This study suggests that the vegetation recovery of swamps, which were mined under, was slower compared to the vegetation recovery of swamps which were not mined under. The post fire vegetation recovery depended on the post fire hydrology of swamps. This suggests that THPSS, which have been mined under, are not at risk of permanent loss of vegetation due to fire, although in this study the fire season was followed by a few years of high precipitation. No effect of fire severity was observed on the vegetation recovery rate of mined under swamps and non-mined under swamps. The post fire recovery of the swamp vegetation, which had not been mined under and affected by controlled fire, was comparable to the post fire recovery of swamps, which were not mined under but affected by the bushfire.
Fires altered soil chemical properties of the studied soil profiles in both Upper Nepean and Newnes Plateau. However, the changes in soil chemical properties of the swamps were possibly restricted by the land temperature, affecting organic matter loss, and by the soil moisture content. The fires affected the soil physical properties, in which greater soil hydraulic conductivity and total porosity values were measured post fire in Newnes Plateau soil profiles. Fires did not affect soil water repellency or soil texture.
It was concluded that the studied THPSS, which have not been mined under, showed a higher resilience to fires with a quicker recovery compared to the THPSS which had been mined under. Remote sensing imagery can be used as a tool to evaluate the post fire recovery of THPSS, both mined under and non-mined under swamps. However, it is important to note that the satellite imagery used in the project could not determine the specific vegetation species within swamp communities and thus recovery is expressed as a function of vegetation cover and greenness. Despite its limitations, the potential use of NDVI, NBR and SMI values derived from satellite imagery in THPSS can present broad recovery patterns of swamp vegetation and hydrology for both Upper Nepean swamps and Newnes Plateau swamps.