History of fire regimes in the biosphere: a climatological bias
There is an understandable bias in the thinking of many ecologists that large-scale fire regimes are driven primarily by climate. More specifically, it is often assumed that variations in historical versus current fire regimes have arisen predominantly by climate change. Perhaps the word ‘bias’ is not appropriate, for there are data from several parts of the world that present a compelling argument linking increased fire frequency with global warming. Nonetheless, studies that do not consider the potential for vegetation—and its associated dynamics—to alter fire regimes present an incomplete picture. Indeed, although some studies (e.g. Carcaillet et al. 2001) have considered both potential drivers and found climate to be overriding, others (e.g. Pausas 2004) have investigated only climate.
Invasion biology: a temporal bias
Another potential bias among ecologists is that plant invasions are not only somewhat recent phenomena, but are also primarily facilitated by activities of modern civilization. Although this is certainly true of many invasive species representing current threats to a variety of terrestrial and aquatic ecosystems, plants have been ‘invading’ new regions since the evolution of land species, a phenomenon also known as ‘migration.’ One such invasion involves Norway spruce (Picea abies) and boreal forests of Europe. Range expansion for this species reached northern Sweden ~4000 years BP, and has further extended to the south-west for reasons that are not entirely clear.
Vegetation-mediated changes in fire regimes
In their paper ‘Invasion of Norway spruce diversifies the fire regime in boreal European forests’, the Editor's Choice in this issue of Journal of Ecology, Mikael Ohlson and colleagues synthesized data from a wide-ranging region of northern Europe (Norway and Sweden), totalling 75 sites from 25 spruce stands. They combined an extensive network of peat, humus and tree-ring records from these forest landscapes to analyse stand-scale forest composition and fire disturbance of the late Holocene. Part of the novel aspect of this study was in combining these paleoecological investigations with knowledge of the rate and direction of spruce expansion. The authors focused primarily on forest peat and humus records because they contain stratified pollen and charred particle sequences exhibiting the chronology of change in local forest composition and stand-scale burning at a high spatial resolution over a millennial time-scale of the Holocene. In short, by comparing charcoal and spruce pollen records, the authors were able to assess simultaneously the influence of both climate and vegetation composition on fire regimes throughout the study area. More important, however, is that these results have implications for a broader region of European boreal forest which encompasses oceanic and continental climates.
Macroscopic charcoal data exhibits localized fire history and forest landscape patterns
Although several studies of this nature have demonstrated connections between macroscopic charcoal and local fire history, among the notable findings of this study is that, by comparing charcoal deposition in pre- versus post spruce-established sites, it became clear that spruce had a profound impact on fire frequency. Indeed, macroscopic charcoal deposition was five-fold lower in post-spruce invasion stands compared to pre-spruce stands. Results of this study also yielded an unanticipated finding: a challenge to the long-held paradigm of fire as a ubiquitous ecological factor in boreal forests. In fact, nearly 20% of their 75 cores contained no macroscopic charcoal at all. Furthermore, their patterns of charcoal suggest prevalence of fires of low intensity in boreal forests of Europe, in contrast to the high-intensity crown fires more common in North American boreal forests (Carcaillet et al. 2001).
Shifts in dominant vegetation trump regional climate change in explaining altered fire regimes
I found this paper to be interesting for several reasons. As already noted, the authors were able to challenge commonly held notions, including that fire has not been as wide-spread an ecological force as once thought, and that there are profound differences in the nature of fire between boreal forests of Europe versus North America. Also, I found the most intriguing result to be the novel finding that invasion of Norway spruce and the subsequent shift in dominance of tree species had a profound influence on the fire regime. More important than this conclusion alone, however, was that this effect of vegetation change on fire regime exceeded that of late-Holocene shifts in climate. A final conclusion goes a step beyond this climate-versus-vegetation debate by addressing broad-scale spatial heterogeneity. The authors conclude that invasion by Norway spruce diversified the fire regime of this region, increasing at the landscape scale the spatial variability of occurrence of fire.
Frank S. Gilliam
Associate Editor, Journal of Ecology
- Carcaillet, C., Bergeron, Y., Richard, P.J.H., Fréchette, B., Gauthier, S. & Prairie, Y.T. (2001) Changes of fire frequency in the eastern Canadian boreal forests during the Holocene: does vegetation of climate trigger the fire regime? Journal of Ecology, 89, 930-946.
- Ohlson, M., Brown, K.J. Birks, H.J.B., Grytnes, J.-A., Hörnberg, G., Niklasson, M., Seppä, H. & Bradshaw, R.H.W. (2011) Invasion of Norway spruce diversifies the fire regime in boreal European forests. Journal of Ecology, 99, 395-403.
- Pausas, J.G. (2004) Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean Basin). Climate Change, 63, 337-350.
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