Fire and flammability characteristics in community dynamics: Inferring feedback mechanisms from the palaeoecological record

Background/Question/Methods: Plants and trees are known to have flammability characteristics that affect fire recurrence and intensity in ecosystems, which in turn plays a key role in determining community composition; what remains uncertain is the interaction between climate change and plant-fire feedback mechanisms on ecosystem structure and function. We studied long-term records of vegetation and fire dynamics over periods of critical climatic changes and applied a model-fitting and model-selection method to (1) infer the relationships between fire and population dynamics for key taxa in plant communities, (2) determine how these relationships affected the dynamics of other taxa in the community, and (3) ascertain how these complex relationships responded to significant climate changes. Data from three European sites covering the late-glacial/ early postglacial transition (16,000 to 8,000 years ago) demonstrate rapid climatic warming and shifts in community dominance from Pinus (pine) to Quercus (oak) forest; Calluna (heather) moorland to Betula (birch) woodland; and chalk grassland to oak woodland. Coincident with these vegetation changes are major shifts in the fire regime, a common phenomenon in northern, temperate latitudes during this period. The model-fitting and model-selection results demonstrate the most-likely mechanism underlying the relationships between each dominant plant population and the fire regime (i.e. whether a population fuelled fire levels or dampened them) and the susceptibility of each population to fire-induced mortality. By comparing these results across the dominant plant populations within a community, we were able to infer the impact of plant-fire feedback mechanisms on community dynamics. Furthermore, to determine whether there was evidence of a climate-threshold-induced change in the feedback mechanisms operating between dominant populations and fire level, we fitted separate plant-fire models for time periods before and after significant climatic changes and compared the results to the non-threshold model results (in terms of goodness-of-fit to the plant population data) for each site. Results/Conclusions: Preliminary results show that heather had a positive effect on fire levels when it was dominant and this was detrimental to birch; however an extended climatic deterioration reduced the cover of both populations. When both populations began to regenerate, the birch population quickly became dominant and this resulted in lower fire levels due to the dampening effect of birch litter on fire. There is clear evidence for a threshold-type climatic interference with plant-fire feedback mechanisms whereby climate change facilitates alternative plant-fire feedback strategies by changing community composition.