Although a variety of factors contribute to global wildfire activity, climate is thought to be a major determinant. Ohlson et al. now find that tree species composition has had at least as strong an influence as climate on wildfire activity over time in late Holocene boreal forests. By analyzing humus and peat records in northern Europe, the authors show that the invasion of Norway spruce, Pkea abies, southwesterly across the region starting 4500 years ago was accompanied by a reduction in the frequency and severity of fire. The presence of Norway spruce would tend to make the forest more dense and shaded in the drier summer months, which would increase the local humidity of the understory and inhibit the spread of fire. Although the spruce invasion corresponded with a period of cooler and wetter climate, the tight spatial correlation of the arrival of spruce and the reduction in fire in specific locations argues for a key role of species composition in governing the fire regime. In turn, the reduction in fire would have led to other ecological consequences, notably increased sequestration of carbon by the vegetation and the establishment of other species requiring longer-term continuity. Thus, the results show how changes to the dominant forest species can exert a cascading effect on the regional ecology for centuries or millennia to come.
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