What does fire do to boreal trees?
- nicolakokkonen
- Sep 12
- 3 min read
The answer to this question is not as straightforward as it seems - it depends on both the trees
and the fire.
Not all trees die when the forest around them burns. Many trees survive, albeit with some level of damage. Some tree species are adapted to survive low-severity and low-intensity fires with thick bark that insulates delicate growth tissues from heat. High branches prevent the fire from climbing up into the tree's upper leaves and consuming them. These trees are said to resist fire damage.
Damage
Fire impacts trees from their roots in the soil, up through their stems and branches, and into the upper canopy, or crown. Fire can burn away upper soil layers, which heats tree roots and kills sensitive tissues there and can even consume the roots themselves meaning that trees are unable to uptake water and nutrients from the soil. Ultimately, this means that trees are not well anchored after deep burning (i.e. high severity) fires and are prone to falling down causing hazards for humans. Without thick bark, or in high severity and intensity fires, the heat of the fire can penetrate the delicate tissues that are just below the bark that are essential for tree growth and sugar transport. If flames or intense heat are able to reach the leaves (usually by climbing lower branches), these thin structures are easily killed and even burned away in extreme fire behaviour. Trees with leaves lower to the ground are more susceptible to this, and certain species are more flammable because of compounds in their leaves (such as true firs Abies spp).
After the most intense fires, it is often evident where fire has reached the tops of the trees and killed them immediately. In lower intensity fires, however, most trees do not immediately appear dead. It takes time to see the true impact of fire damage to the roots and stems of the trees over the months and weeks following fire. Beyond the direct impact of the fire, insects such as bark beetles and wood borers can cause more trees to die over years following fire. Those trees that die go on to provide valuable habitat for many species (such as fungi, insects, birds) that depend on dead trees.
Consistency
Fortunately, fires do not burn uniformly across the landscape. Often, small changes in fuels, such as moisture, quality or quantity, combined with subtle fluctuations in weather conditions means that fire can have a very diverse impact within a single burned area. For example, in windy conditions, the upwind side of the tree may be nearly unscathed, while the downwind side can be burned quite deeply. This means that the tree can usually survive, but with a scar on the downwind side of the tree. With all of these variables across the landscape, the impact of fires on trees can be incredibly diverse.
Regrowth
Some survive and are able to produce seeds that become the next generation of trees on the burned lands. Certain species of pine release their seeds after exposure to intense heat, meaning that its seeds are likely to land in freshly burned areas. Here, most competing vegetation is burned away and the soil surface is suitable for these seeds to sprout and grow. This allows a new forest to establish and grow in open areas and in between surviving trees.
Some trees are damaged by fire , but are able to grow, or even spread from damaged stems and roots (for example aspen, Populus tremula and P. tremuloides). These can form dense patches and grow more quickly than seedling.
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