Before humans began leaving their mark on the forests in northern Sweden, before the mid-1800s, and started with firefighting they were heavily marked by fire. Fire was the most important ecological factor in the forest and all the plants and animals of the forest were adapted to recurring fires or lived in the smaller, moist areas that rarely burned.
In the past, forests often consisted of a number of old pines that had survived several fires, and then younger pines, spruce and deciduous trees largely divided based on how long it had been since the last fire. Approximately one-third of all standing volume was dead timber: trees that had died — perhaps from the last fire — but remained standing, dry and impregnated with resin. They were excellent fuel for the next forest fire, which — like many of this summer's fires — often started with a lightning strike.
Open forest is more inflammable
An old-growth forest of this kind was usually quite open. The standing volume was lower than the well-managed forests of today that approaches final felling. While there were often a number of very large old pines, there weren't very many of them, and as for the rest there were varying numbers of trees spread unevenly over the area of greatly varying sizes, most quite small.
As a result of the forest being open, it dried out quickly when the weather was warm, dry and windy. An old-growth forest in northern Sweden is both marked by fire and a great deal inflammable.
Pine trees are well adapted to fires
The pine tree is well adapted to recurrent forest fires. The bark on the lower part of the trunk is thick, and insulates against heat. The branches on an older pine sit high up, and if the fire only sweeps along in the ground vegetation — which is the most common occurrence — an old pine has good possibilities of surviving a forest fire. There are examples of old pines that bear the scars of five or six, or even more, forest fires.
After the fire, the pine could spread its seeds over the burnt grounds, and a new generation of pines could grow up without competition from other trees. If the ground was fertile — on a slope near a river, for example — a generation of deciduous trees, primarily birch and aspen, would follow the fire instead, a phenomenon known as broadleaved succession.
When the spruce is taking over
The spruce, with its low-hanging branches, is susceptible to fire. It can grow up in the shade of other trees, and as a rule comes into an area razed by fire a few decades after a new generation of pine has been established. If the next fire is some time in coming, the spruce trees could grab more and more space at the expense of the pines. If all the fires and all the forestry activity ceased, practically all forests would ultimately be spruce forest, and the animal and plant species that rely on fire and open ground would disappear.
After humans began large-scale use of the forests in the late 1800s, the number of fires — and their size — have dropped radically. Modern firefighting is one factor. Another factor is that the forests of today have another structure. A planted forest closes up more rapidly than a forest that regenerate after a fire. The planted forest shades the ground, which does not dry out as rapidly and is thus not as inflammable.
A well-managed, cultivated forest
- Grows faster and therefore binds more carbon dioxide.
- Has a greater standing volume and thus a larger carbon stock.
- Is more dense and shaded and does not dry up as rapidly as the more open old-growth forest.
Fires are an important ecological factor in SCA forest land, which is why we burn forested areas under controlled forms in order to ensure that there are living environments for the plants and animals that depend on burnt ground and burnt timber.