A project undertaken at University of Canberra, and supervised by Ben Kefford
Fire is an important disturbance in many ecosystems, especially in Australia. Fires can be a natural disturbance, but human management and climate change alter fire and thus disturbance regimes. In stream ecosystems, a wide range of responses to fires have been documented from large and long-lasting (>10 years) effects to relatively small and short-lasting (< 1 year) effects. The mechanisms for this variation in effects is not clear. Additionally, studies of effects of fires on stream ecosystems have mostly been opportunistic, and consequently had non-optimum study design e.g. limited before-fire control sampling or lacked control unburnt sites and with irregular or short-term assessment post fire. Some of the variation in the effects of fires on stream ecosystems may be a consequence of non-optimal study design.
In this project we sought to understand the effect of fire via two processes: upstream catchment disturbance and local (site) scale disturbance. We used a dataset which regularly sampled a set of sites in NSW and the ACT from 1994 or 1996 until 2020 with a fire in 2003. This fire either burnt sites sampled (site burnt sites) or the site itself was unburnt but the upstream catchment was burnt (catchment burnt sites). Additional, control sites that had no burning either locally or in their upstream catchment were examined. The analysis of this dataset allowed for a near-optimum study design, in that we had a long baseline time-series (at least 1996 to 2002), before the fire, a long baseline after the fire to assess the duration of effects, replicate unburnt control, site burnt, and catchment burnt sites. In general, the fire had a longer lasting effect on water physiochemistry and habitat at sub-alpine sites than sites at a montane elevation, likely because of the slower recovery of vegetation in the sub-alpine than the montane vegetation. Several differences in the effect of fire on water physiochemistry and habitat between site burnt and catchment burnt sites were noted. For example, total nitrogen increased at burnt sites yet was unchanged at catchment burnt sites.
Effects of the fire on stream macroinvertebrate communities were greater and longer lasting at catchment burnt than site burnt sites. For example, effect of the fire on macroinvertebrate community indices were detectable for 12 months post fire at the site burnt sites compared to 4 years post-fire at catchment burnt sites. We were able to identify that the water physiochemistry and habitat variables (substrate composition metrics, muck and/or detritus) were most associated with changes in macroinvertebrate communities. The recovery of riparian grasses at site burnt sites appeared to be more important for macroinvertebrate community recovery than recovery of riparian shrubby vegetation, probably because grasses effectively filter water flowing over the soil surface. We are awaiting genomic analysis of our specimens and plan to write up and publish the findings of these analyses when available. These analyses will allow us to determine if there were population bottlenecks and subsequent recovery mechanisms, e.g. did individuals repopulate sites from a few individuals that locally survived the fire or via migration from outside the site. This project has shown, with a near-optimum study design, that the effects and duration of fires on stream ecosystems is associated with elevation zone (sub-alpine vs. montane), whether the site itself is burnt or whether the upstream catchment is burnt.