Interaction of multiple disturbances: importance of disturbance interval in the effects of fire on rehabilitating mined areas

Abstract

Abstract Multiple disturbance regimes are increasingly common as novel anthropogenic disturbances are added to existing natural disturbances. However, it is generally unknown whether simultaneous or sequential effects of different forms of disturbance are predictable from the independent effects of each disturbance. This study examines the short‐term effects of sequential disturbance by mineral sand‐mining followed by fire in a forest community in south‐eastern Australia. Four combinations of disturbance were sampled: unburned mined, burned mined, unburned forest (unmined) and burned forest (unmined, with between‐fire interval matching the disturbance interval between mining and fire of the burned mined treatment). All combinations were sampled approximately 12 months following fire on the burned sites. The impact of fire after mining depended on disturbance interval. Sites burned 0.5–2.4 years since mining had fewer native vascular plant species than unburned mined sites of the same mined age, whereas sites with 10–16 years or 20–26 years between mining and fire had greater native species richness than unburned mined sites of the same age. Burning 20–26 years after mining brought native species richness within the range of burned forest. For both unmined and mined sites native seedling densities increased with burning, and with longer disturbance intervals. Weed species richness and weed seedling densities were greater on mined sites than in forest, and burning mined sites elevated weed seedling densities further, particularly for short intervals. Both disturbance interval and fire intensity are likely to have contributed to these results, as intensity on mined areas increased with interval, and at 20–26 years post‐mining was equivalent to unmined forest. These results suggest that fire could be used to promote rehabilitation of these mined areas after at least 10 years, but should be excluded from earlier stages of post‐mining regeneration. However, other sources of spatial and temporal variability should be considered in addition to interval and intensity, as variation among mined areas was correlated with post‐fire weather conditions and available weed sources. Finally, the combined effects of mining and fire could not be predicted from knowledge of the disturbances operating separately, indicating that effects of multiple disturbance may be synergistic rather than additive.