Metla Project 640043

Changing fire regimes in northern coniferous forests

Duration: 2008-2011 Keywords: anthropogenic influence, dendrochronology, fire history, fire suppression, forest dynamics, forest fires, landscape ecology
Research project group:

Objectives

One of the global challenges is to understand the natural dynamics of forest ecosystems and past and present human influence on them. This is vitally important in order to alleviate the adverse effects of human influence on forest biota and to predict the future development of forests under a changing climate and alternative management options.

Our aim is to: 1) gain knowledge about the past fire regimes of poorly studied forest landscapes and regions in North Europe, Central Siberia and northern Canada, 2) to be able to conclude what have been the main driving factors of the observed changes in fire regimes since the 18th century. Based on that we will also have a better understanding of what the natural fire regimes would be without human influence.

Results

Northern Europe

In eastern Finnish Lapland we discovered exceptionally long fire cycle (350 years) for a Pinus dominated landscape. It demonstrated that fire regimes of poorly studied remote regions cannot be extrapolated from fire regimes of more carefully examined sites.

Estimating from the current lightning ignition densities and the past fire size distribution, the natural fire cycle can be as long as 1200 years. This suggests that the detected long historical fire cycles in the region were not necessarily natural. Human influence and changes in human living habits have probably considerably contributed to the number of fires as well as burned areas in different centuries.

More information: Wallenius, T., Kauhanen, H., Herva, H. & Pennanen, J. 2010. Long fire cycle in northern boreal Pinus forests in Finnish Lapland. Canadian Journal of Forest Research 40: 2027-2035.

Central Siberia

Contrary to expectations, climate change in the 20th century has not resulted in increased forest fires in our study area in Central Siberia. Average fire interval gradually lengthened from 52 years in the 18th century to 164 years in the 20th century. During the same time, the number of recorded fires decreased even more steeply, i.e. by more than 85%. Fires were more numerous but smaller in the past.

More information: Wallenius, T., Larjavaara, M., Heikkinen, J. & Shibistova, O. 2011. Declining fires in Larix dominated forests in northern Irkutsk District. International Journal of Wildland Fire 20: 248-254.

Northwestern Canada

A significant negative trend in the occurrence of forest fires was also observed in our large study area in northwestern Canada: average fire interval lengthened from 50 years in the first half of the 19th century to 300 years in the later half of the 20th century. Annually burned areas correlated significantly with many climatic indicators but none of them could explain the long-term negative trend in fires. Also fire suppression could not explain the decrease in annually burned areas that started already in the 19th century. Earlier interpretations that humans dominated the causes of forest fires in the past, even in sparsely populated regions, deserve further attention as a possible explanation for the decreasing trend in fires.

More information: Wallenius, T. H., J. Pennanen, and P. J. Burton. 2011. Long-term decreasing trend in forest fires in northwestern Canada. Ecosphere 2(5):art53.

Overview of the decrease in forest fires

The annually burned proportions declined over 90% in all studied regions. In three out of the four regions fires decreased decades before fire suppression began. Available drought data are annually well correlated with fires but could not explain the decrease of the level in annually burned areas. A rapid increase in the number of livestock occurred at the same time with the decrease in fires in the Western US but not in Fennoscandia. Hence, fire suppression in Central Fennoscandia and over-grazing in the Western US may have locally contributed to the reduction of burned areas. More general explanation is offered by human influence hypothesis: the majority of the past forest fires were probably caused by humans and the decrease in the annually burned areas was because of a decrease in human caused fires. This is in accordance with the old written records and forest fire statistics. The decrease in annually burned areas, both in Fennoscandia and the United States coincides with an economic and cultural transition from traditional livelihoods that are associated with high fire use to modern agriculture and forestry.

More information: Wallenius, T. 2011. Major decline in fires in coniferous forests – reconstructing the phenomenon and seeking for the cause. Silva Fennica 45: 139–155 (pdf).

Project leader: Wallenius, Tuomo

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Updated 02.02.2013
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