K. Kuusela
Dr Kullervo Kuusela is professor emeritus of the Finnish Forest Research Institute.
The changing impact of fire on the forests of Finland.
Under natural conditions, fire is the most important factor regulating tree species composition and development in the boreal forests of Finland. After fire has cleared an area of spruce-dominated climax forest, pioneer trees establish themselves in the burnt-over areas and, in a gradual succession, the area progresses back to climax conditions and further fires. The species composition of the pioneer stands as well as the number and distance (from the burnt-over area) of the spruces providing a source of seeds will determine the amount of time needed to reach climax conditions after fire. If there are seeding trees in the vicinity, spruce reaches dominance over alder in 30 to 40 years, and over birch and aspen in 80 to 100 years.
Although, historically. wildfires were the dominant factor causing change, most of the individual fires occurring in the boreal forests of Finland covered relatively small areas, destroying sensitive plants and creating characteristic mosaics of areas at various stages of regeneration. However, under particularly dry or hot and windy conditions, large areas were affected, creating homogeneous pioneer tree communities.
Under natural conditions, wildfires are the most important factor assisting the decomposition of the dead understorey material and humus. This is particularly important in terms of maintaining the availability of usable nutrients in the forest soils. The longer the period between fires, the greater the proportion of spruce in the stand and the thicker the layer of humus, which contains large amounts of nutrients in a form that is inaccessible to trees and other plants. For example, under a mature spruce stand there can be as much as 1500 kg/ha of nitrogen, but often only 15 to 22 kg/ha is available to the trees; the rest is trapped in the humus layer. Fire releases the mineral nutrients in a usable form and renders them alkaline, thus decreasing the soil acidity which generally increases under coniferous forests. The decrease in acidity improves nitrification and favours decomposing microbes. On the whole, therefore, fire increases the amount of nitrogen available to plants and improves site fertility.
The occurrence of fire in nature is controlled by climate during the growing season and by soil parent material. The warmer the temperature, the smaller the ratio of precipitation to evaporation and the drier the soil, the more often fire will occur. According to a study done in northern Sweden (Zackrisson, 1977), the average cycle between successive forest fires before the twentieth century was shortest (46 years) on sandy soils and gravelly riverside terraces, while it was somewhat longer on moraine of medium fertility and longest (122 years) on fertile morain soils and northern slopes. The cycle was shorter on southern slopes than on northern slopes and shorter on the upper elevations of hilly landscapes than it was in depressions.
Although, initially, fire was exclusively a natural part of the boreal forest environment, by the eighteenth and nineteenth centuries much of the fire ecology was human-made. It has been estimated that shifting cultivation - where forest was burnt and then cultivated for two to four seasons before being allowed to regenerate for 20 to 40 years - covered more than four million ha of forest land in Finland (Heikinheimo, 1915). Between 50 and 75 percent of the country's forest land was under a shifting cultivation regime.
During the era of shifting cultivation and forest pasturing, human activities disturbed the natural successions and maintained a kind of false climax or perpetual pioneer tree forest. Shifting cultivation especially favoured grey alder and white birch. As a heritage of this time, Finland and parts of Russia have the largest good-quality white birch resources in Europe. The period of shifting cultivation left large areas of pioneer broad-leaved and mixed broad-leaved pine stands of variable density as well as residual stands on sites that, under natural conditions, would have been dominated by spruce.
On the other hand, these practices led to good soil fertility and a high diversity of landscape and plant and animal communities. This was also the case in inland waters which were enriched by leached nutrients. The grazing of wild and domestic animals also contributed to the circulation of nutrients.
Shifting cultivation decreased rapidly at the end of the nineteenth century and in the beginning of the twentieth century. Agricultural practices became more efficient. The wood from Finland's forests was recognized as a valuable commodity, especially for export, and legislation was enacted to ensure a sustained supply. Spruce began to return to its former sites. However, in an area of 1.3 million ha in southeastern Finland, spruce accounted for only 12 percent of the growing stock volume when Finland's First National Forest Inventory was carried out at the beginning of the 1920s. Broad-leaved species accounted for 48 percent and pine for 40 percent. According to the results of the Eighth National Forest Inventory, in 1988, spruce accounted for 38 percent, broad-leaved species for 21 percent and pine for 41 percent of the total growing stock, which had increased by 67 percent over the preceding 60 years while the volume of spruce had quadrupled.
Over the past few decades, the incidence of wildfires has decreased dramatically. In fact, for all practical purposes, fire has been eliminated. The average annual area burnt by wildfires decreased from 7803 ha in the period 1957 to 1961 to less than 500 ha at present. At the same time, Finnish forestry has drifted into a state of permanent underutilization of resources. Without repeated fires, and in the absence of productive silviculture, spruce would gradually become the dominant tree on all mineral soils, with the possible exception of the most barren sands and gravels. Other species would accompany the spruce to various extents, depending on their ability to occupy gaps in overmature, degenerating stands.
Norway spruce (Picea abies)
A spruce stand that has been protected from fire in northern Finland; note the heavy humus layer
The long-term dominance of spruce will lead to negative changes in the micro-environment. The amount of raw humus will increase, the nutrient regime of the soil will be degraded, the recycling of nutrients between the soil and the trees will decrease and the stands will not be able to regenerate. Paludification will be promoted and, finally, the site may develop into a peat-forming mire. The degeneration of old-growth forests may also lead to a decrease in the number and diversity of animals that traditionally inhabit these stands.
Therefore, in the absence of fire, active management measures are the only means with which to maintain the nutrient and genetic quality of boreal forest ecosystems. From a financial standpoint, the only feasible way to pay for this management is through the production of wood on a large proportion of the forest area, involving sufficiently intensive final cuttings, small-scale successions in the regeneration stages and silvicultural measures such a human-caused change in the species composition, site preparation and fertilization.
Admittedly, this is a double-edged sword. Although the nutrient status, which was degraded in mature and overmature stands, would become available thanks to regeneration measures and site treatments and would allow small-scale plant community successions, the profit motive of commercial forestry requires stands to be dominated by species that are valuable as a raw material and uniform in diameter structure. Such economic requirements tend to limit ecological diversity. From the point of view of stumpage income, pine, spruce and birch continue to be the most valuable timber species in Finland while the production shares of alder, aspen and low-quality birch are being deliberately reduced.
This having been said, however, it is important to note that mixed stands are currently common in managed forests. There are many individual broad-leaved trees and pine on sites where the succession toward the dominance of spruce continues. Mixed forests have benefits and environmental values that have been recognized in the development of silvicultural methods. For example, the addition of birch in a spruce stand has been observed to help prevent fungal damages to the spruce and improve the regenerative quality of the site for the spruce seedlings.
Regeneration of pine and birch 40 years after fire
Given the current state of Finnish forestry, in which fire has been virtually eliminated, the functions and expected services of forests should be reformulated and stated objectives should be tested against the perceived or actual needs of citizens. Based on this action, appropriate management measures should be implemented to achieve the desired targets in terms of both environmental and production values.
The desirable intensity of wood production, diversity of landscapes and species composition should be analysed and described. Existing tree and plant populations have genetic qualities adapted to a fire climax ecology. When fires are eliminated, the productivity and diversity of forest ecosystems can only be maintained by active measures designed to create open regeneration gaps, pioneer plant communities and various successional steps toward climax stands. Management regimes designed to attain this goal should form a part of economically profitable silviculture and should be complemented by the maintenance of marginal landscape mosaics and habitats for plants and animals that have adapted to living conditions different from those prevailing in production forests.
Heikinheimo, O. 1915. Kaskiviljelyn vaikutus Suomen metsiin. Acta Forestalia Fennica, 4.
Kuusela, K. 1990. The dynamics of boreal coniferous forests. Helsinki, Finnish National Fund for Research and Development (SITRA).
Zackrisson, O. 1977. Influence of forest fires on the northern Swedish boreal forest. Oikos, 29: 22-32.
