Legacies of past wildfires continue to influence the development of northern boreal forests long after the fires have disappeared

Forest fires were historically the most impor­tant disturbance agent in the northern European boreal forest, but have virtually disappeared in the 20th century. However, past fires have left persistent legacies in forest structure and, due to their strong influence on forest age structures, continue influencing forest dynamics long since their disappearance. It is hence clear that we need knowledge on past fire occurrence, to be able to attribute the changes we observe in the forest to their drivers.

We reconstructed past fires in three northern boreal landscapes (each 2 km x 2 km) in Värriö and Maltio strict nature reserves in northeastern Finland. For the reconstructions, we used dendrochronological methods (i.e., methods based on tree ring widths), and described the occurrence of fires in the past 300 years. For these three landscapes, the average fire cycles (a measure of fire activity) were 72 and 156 years in Scots pine-dominated landscapes, and 579 years in a Norway spruce-dominated land­scape. The numbers of fires during the past three centuries were clearly related with soil properties: forests on soils that retain water better had experienced fewer fires than forests on coarser soils with lower water holding capacity.

Tree age structures that we determined from 1800 live and dead trees showed strong age cohorts that were associated with large fires in two of the landscapes. The y-axis shows the cumulative number of trees – each horizontal line represents the life span of an individual tree. Years indicated fires that influenced most parts of the study landscape.

It is noteworthy that although tree growth and regeneration in these northern regions are considered highly climate-sensitive, fires have been a major driver of forest dynamics in these areas. It is also clear that the continued absence of fires will lead to considerable changes in the forest structure and species composition in the future, even if the reserves are strictly protected from direct human influence.

The study is published in Boreal Environment Research, and can be found here.

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