Impacts of wildfire and salvage harvesting on water quality and nutrient exports from radiata pine and eucalypt forest catchments in south-eastern Australia

Abstract

Salvage harvesting may represent an important additional impact on sediment and nutrient transfer in forest environments affected by wildfire. However, few studies have examined the effect of this management practice on stream water quality and catchment nutrient exports. In this paper, we investigate nutrient losses following post-wildfire salvage harvesting of a radiata pine plantation catchment compared to an adjacent natural eucalypt forest catchment that was also burnt but not harvested. The study catchments form part of the long-term Cropper Creek Hydrology Project (established in 1975) that is situated in south-eastern Australia. Post-fire monitoring (2007–2009) involved collection of both weekly and flow proportional water samples that were compared with previously reported data from samples collected prior to the fire (1997–2003). It was found that median values of total suspended solids (TSS) and turbidity returned to pre-fire levels within 3 years in both catchments, whereas maximum levels during storm events in the harvested pine catchment continued to exceed the eucalypt catchment. This reflected a previously reported large increase in post-fire sediment exports from the harvested pine catchment that was a minimum 180 times the eucalypt catchment over the study period. In contrast, the impact of harvesting on solute concentrations (nitrate-N, P, S, Cl, Na, K, Ca, Mg) was minor and most solutes returned to pre-fire levels within 2–3 years in both catchments. Nutrient exports from the pine catchment exceeded the eucalypt catchment by 102 times for particulate P associated with suspended material compared to 1.9–4 times for solutes. The post-fire changes in solute concentrations were generally similar for both catchments and the increase in solute exports was largely a result of greater discharge after the fire and harvesting compared to the burnt eucalypt catchment. The post-fire loss of particulate P in suspended sediment and bedload from the pine catchment for the study period was estimated at a minimum of 11 kg ha −1 and together with the estimated loss of P from burning and the removal of the pines represented approximately 6% of the total P in surface soil and fertilizer applied to the plantation.