Fire Severity and Plant Community Dynamics in the Australian Alps, Southeastern New South Wales
The effects of fire on plant communities are a subset of a broader range of spatial and temporal influences on plant community dynamics. Widespread unplanned fires in the Australian Alps in 2003 provided a unique opportunity to investigate the response of vascular plant species to high severity fire and to test whether there was a differential response to variations in fire severity that may affect long term vegetation dynamics within and between subalpine, montane and tablelands plant communities. A series of pre-existing sites established in the late 1990s in the northern parts of the Australian Alps in Brindabella National Park, Bimberi Nature Reserve and Burrinjuck Nature Reserve were resurveyed and monitored from 2003-2012. Two research questions were posed: 1) what is the response to high severity fire of vascular plant species found in subalpine, montane and tablelands plant communities? 2) does high severity fire result in significant or permanent change to vascular plant species richness and plant community composition as compared to low severity fire in subalpine, montane and tablelands plant communities and does the recovery rate vary with severity? The fire responses of over 400 vascular plant species in 21 plant communities were documented. There was no loss of plant species from the study areas and all vascular plant species recovered rapidly post-fire, but some fire-killed species exhibited slow recovery rates. Although vegetation structure is affected by fire severity, it is fire severity interacting with fire frequency that has the potential to change vegetation structure in the long term, particularly via the loss of structurally dominant obligate seeder species. The overall trend in vascular plant species richness was an increase in species richness post-fire irrespective of fire severity, with a time since fire decline. Significantly for forest ecosystems, this peak and decline response of vascular plant species richness post-fire is modified and varies over time in relation to rainfall, decreasing in dryer years. While there was an initial difference in floristic compositional dissimilarity between low and high severity sites in the Brindabella sites and greater dissimilarity in the short to medium term, this dissimilarity diminished over time and there was no evidence of permanent compositional change in high severity sites. In high severity sites, there was an increase in the density of many dominant shrub species leading to differences in the short term post-fire but by 10 years post-fire any compositional difference due to severity was not detectable. The Burrinjuck study area did not exhibit these differences in relation to fire severity but did exhibit differences in richness and composition based on season of sampling. The results of this research show that site identity is strong and that the plant communities investigated are buffered against long term shifts in composition, even after a large infrequent disturbance event such as the 2003 fires. Post-fire effects are transient rather than permanent. Infrequent high severity fire events in the Australian Alps should be regarded as natural disturbances, and not as disruptive perturbations, as they do not lead to permanent changes in the plant communities investigated. However, if high severity fire events become more frequent under climate change then resultant changes in structure, composition and richness are likely to occur.This research has vastly improved our understanding of how subalpine, montane and tablelands plant communities respond to high severity fire and provides reserve managers with detailed species and community level responses to better inform fire management. The research also provides a baseline with which to compare the effects of future changes to fire regimes in these communities under climate change. The research has shown how long term monitoring plays a crucial role in understanding vegetation dynamics.
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