Predicting fire regimes and their ecological effects in spatially complex landscapes
Creators: name: Cary, Geoffrey John
Publisher: name: The Australian National University
Year: 1998
Subjects: Fire ecology
Plant communities
Australian Capital Territory

description: Fire occurrence influences the distribution of plant species, and dynamics of plant populations, either independently from other factors or in interaction with them. Numerous studies have identified the effects of components of fire regimes (frequency, intensity and season of occurrence) on the population dynamics of individual plant species and the floristic composition of plant communities, both in Australia and in other fire-prone countries. Nevertheless, there has been considerably less research into understanding the causes of spatial variation in fire regimes and this will likely result in a major obstacle for the development of vegetation theory. Research into spatial and temporal patterns of fire regime and determining the extent to which this type of variation results in variation in the occurrence of plant species and hence the composition of plant communities, can help overcome this obstacle. Two hypotheses are constructed and addressed. They are:- (i) that because of the influence of a sites' neighbourhood, variation in fire regimes will exist for any particular set of sites that occupy a particular part of environmental space and are therefore otherwise similar, and (ii) that this variation in fire regimes will result in patterns of plant species occurrence and hence demonstrate that landscape induced pattems of fire regime are a fundamentally important component in determining the realised niche of plant species in spatially complex landscapes. These hypotheses are examined for a spatially complex landscape in the Australian Capital Territory region, Australia. Several distinct phases of research led to the conclusion that both hypotheses should be accepted for the study region. Firstly, a review of the available literature found that empirical approaches, and related statistical models, for determining long-term fire regimes provided data that was neither of sufficient length and accuracy nor of appropriate spatial resolution for examining landscape dependent patterns in fire regimes as outlined in hypothesis one. On the other hand, theoretical approaches which synthesise landscape patterns from the well­ understood processes that affect fire occurrence and behaviour proved to be a useful methodology, provided that the inadequacies in existing models, including their reliance on the North American approach to modelling fire spread, were addressed. These provisions were addressed by the construction of a new landscape-level process­ based dynamic simulation model known as FIRESCAPE. The development of the model involved parameterising and testing the Richardson weather generator for fire danger modelling and a re-assessment of the McRae lightning ignition model. The approach used in FIRESCAPE combines these models with existing models of terrain, solar radiation budgets, fuel moisture, soil moisture, fuel accumulation and fire spread to model spatial variation in fire regimes by the accumulation of information from spatially and temporally distinct fire events. Various analyses indicated that around 500 years was required to stabilise the variation associated with fire regime parameters for some sites, while sensitivity analysis found that the spatial variation in fire regime was not over-sensitive to any of the important fire spread parameters that summarise variation in the majority of input parameters. Further, the spatial pattern in fire regime was generally consistent with many general observations made in the real world, but v under-predicted the fire frequency of five stands of Eucalyptus pauciflora wh,ere detailed pyrodendrochronological measurements have been made. Model output demonstrated that considerable variation in fire frequency existed for all sets of environmentally similar sites that were analysed. This variation was la1rgely related to the position of the site with respect to its neighbourhood, and to a lesser extent the average annual solar radiation budget of the neighbourhood. This variation resulted in strong patterns of eucalypt species occurrence, as determined by generalised linear modelling of species occurrence in the lower Cotter River catchment. Many of the patterns were demonstrated to be ecologically sensible compared with general observations and independent models of the effects of variable fire frequency on the composition of plant communities. Therefore, it has been demonstrated that variation in fire regimes that arises from topographically complex landscapes can have important effects on the distribution of plant species within suites of environmentally similar sites. This is a major step forward in resolving the landscape-level effects of fire occurrence on plant species distribution.