Conserving mire ecosystems: linking flammability and moisture content of peaty substrates

The interaction of fire with climate change or land use change has strong implications for regional planning and conservation management. Mire ecosystems of tropical, temperate, boreal and montane environments are critical to the world's carbon stocks and their above-ground biomass may be naturally fire-prone. However, below-ground peat-accumulating biomass and carbon stores are largely protected from ignition by sustained high soil moisture. Anthropogenic disturbances, such as underground mining, ditching and climate change-related drying, dewater these ecosystems and change the likelihood of fire propagation. We sought to quantify the change in combustion risk across moisture gradients. We used samples within four undisturbed mire locations in southeastern Australia. Prior to laboratory simulations, we measured moisture, bulk density and organic matter for each sample. We then applied smouldering or flaming (either 35 kWm^-2 or 75 kWm^-2 intensity) ignition and observed moisture thresholds for substrate combustion. We assessed the combustion risk associated with underground mining using field data from moisture probes deployed in mined and unmined mires. We found that the probability of substrate combustion was negatively related to moisture levels for both smouldering and flaming ignition types. In the case of flaming ignition, high intensity fire had increased risk of burning at higher substrate moisture than moderate-intensity fire. When we compared moisture thresholds for substrate combustion with field moisture conditions, we found that underground mining more than doubled the risk of substrate burning for all ignition types. Anthropogenic disturbances that disrupt natural hydrological regimes and reduce substrate moisture, concurrently increase fire risk in the landscape.