Forest Club model projects and sites

Fire Dynamics

Fig. 16. Slash-and-burn agriculture had significant influence on the dynamics of taiga forest. This influence is not limited to areas of current cultivation, but extends over large areas affected by previous cultivation and occasional escaped fires. At any particular time, the affected area could exceed the current cultivated area tens or even hundreds of times. Significant parts of the southern and middle taiga were affected by slash and burn cultivation. With a low population density, influence was concentrated mainly around the most suitable areas, while remote forests were affected to a lesser degree or escaped cultivation altogether.

Fire dynamics is the main mechanism for rejuvenation of the taiga forests in the examined areas. Despite the existence and even firm documentation of individual cases in which forest fires have occurred due to lightning, this forest rejuvenation mechanism must not be regarded as entirely natural. The overwhelming majority of fires in taiga forests of European Russia are related to human economic activity, as evidenced by many historic and contemporary studies. In fact, not even in 1999, a year of extremely dry conditions and many fires, did any fires occur within the remaining intact (unpopulated, and very rarely visited by people) forest areas that was detectable in high-resolution satellite images. At the same time, the number of forest fires exceeded practically all previously recorded levels in areas with dense population and a dense transportation network.

Fire or pyrogenic dynamics are associated with the periodic partial or total burning of a forest as a result either of natural causes or human activities and the establishment of a new tree generation on the cleared area.

Forest fires have always accompanied the economic activities of people, and people have lived in the taiga almost from the very time it was formed. The main cause of fire was shifting (slush-and-burn) cultivation, existing in Northern European Russia up until the end of the 1930's, in which the farmer would move to a new spot every few years and clear it by means of fire (see Figure 16). Given the fact that people have been setting fires for the entire period during which the taiga forests were formed, it is reasonable to regard fires associated with low intensity human management as an ancient semi-natural mechanism for shifting tree generations in taiga forests (this excludes such frequent or catastrophic fires that are associated with the modern development of the taiga and upset the fire structure that has shaped these landscapes for centuries).

There is a big difference between forests that have been subject to frequent fires over a long period of time, and forests that have only experienced occasional fires separated by centuries of "fire-free" natural dynamics. Exposure to periodic fires for centuries or millennia favors so called "light" (fire-dependent) coniferous taiga forests in which pine (everywhere) and larch (in the north-east) species dominate. The light coniferous taiga is most typical for the Baltic Shield encompassing Karelia Republic, Murmansk and western Leningrad Regions, and also on sandy sediments of the lowlands and along large rivers. For pine and larch, fresh fire scars and forests affected by ground fires provide optimal conditions for establishment and development of new tree generations. Mature pines and larches are capable of surviving even severe ground fires (see Figure 17 and 19).

Fig. 17. Example of canopy structures in landscapes formed under the influence of forest fires A. Fire refuge (area having escaped fires during a period longer than one tree generation). Such areas are usually found in moist locations along rivers and lakes but may also be found on well-drained sites. B. Areas having escaped fire long enough for a layer of spruce undergrowth to appear under the pine canopy. Spruce trees are beginning to replace old pines in the canopy. C. Uneven-aged pine forest formed by repeated ground fires. Each fire allows a new tree generation to appear, while at the same increasing mortality among the old trees. D. Area affected by crown fire which has destroyed almost the entire original stand. Crown fires promote the development of dense undergrowth of pine which eventually develops into an even-aged stand.

Seeds of pine and larch are rather heavy, however, and don't spread very far with the wind - usually a hundred meters at the most. When fires occur only very occasionally and new fire scars are several kilometers or more apart from older ones, other pioneer species such as birch and aspen, which have seeds capable of travelling further or which naturally are part of the forest even without any fire, will dominate the post-fire succession. Abundant growth of birch and aspen on fire scars is common on the wide water dividers, where fires used to occur more seldom and most sources of pine and larch seeds (if they were present at all) have been cut down during the last century and a half (see Figure 18).

Fig. 18. Re-growth on a water divide which seldom burns. In the absence of seed trees of pine and larch, pure or almost pure stands of birch, sometimes with aspen, will develop.

Taiga forests that are more or less periodically exposed to fires have the following characteristics:

1. A mosaic of parts that have been subject to different frequencies, intensities, and types of fire (ground fires, which burn only the organic layer of the ground and fallen branches, and crown fires, which burn the entire stand either superficially or totally). The effect of the fire determines the structure and dynamics of the forest, including age structure, the intensity of tree death, the amount of fallen trees and organic debris on the ground, and the composition and structure of the lower layers of the forest. The type and frequency of fire determines whether even-aged forests with homogeneous structure or multi-aged forests consisting of several different tree generations will be formed (see Figures 17 and 19). These forests are complex mosaics of parts that have burned with different frequency, which also contain fire refuges (i.e. pockets in the forest which, for some reason or another, have escaped fires for centuries and therefore display a gap structure of the stand).

2. A broken age structure for the dominating tree species. At the most fire prone sites, stands of pine or larch develop containing several clearly distinguishable tree generations, each of which was established after a ground fire. On sites that have been without fire for a long time, a secondary layer of spruce will develop. This will typically consist of groups of trees established over a rather brief period of time, usually 60-80 years. Even at sites that have escaped fires for a very long time (200-250 years and more), where spruce is often the dominating species in the stand, the age structure of the spruce is not continuous in the way that a sustainable population would be. Such spruce stands are only partially mixed ages.

Fig. 19. Frequent fires promote uneven-aged pine forests. Karelia Republic, Kostomuksha Nature Reserve. Photo: V. Kantor.

The deciduous species in these forests (birch, aspen, goat willow) in most cases grow in stands with an uneven age distribution, typically with a few clearly distinguishable tree generations.

3. A presence of downed trees of different diameters, unevenly distributed over the area as well as through stages of decomposition. Pine or larch typically account for most of the large-diameter fallen trees. Only on sites that have escaped fire long enough to establish gap dynamics is it possible to find downed spruce.

4. An absence of a thick organic layer, except for on wet sites, and a rather small amount of organic debris on the forest floor, except for those sites that are part of a fire refuge. The soil profile is more flat and less developed than in the typical forests with gap dynamics. This is connected with the fact that many trees die and gradually decompose "on the stump" instead of falling along with their root systems. Overall, these forests have a much smaller capacity for retaining moisture than forests with clearly developed gap dynamics.

5. The process of old trees dying is very different within various parts of the forest, and is regulated by several factors in concert: with the effect of fires (including superficial ground fires, which cause trees that are weak or more harmed by fire to dry out), the death of old and weak trees from pests and diseases, and the effect of extreme weather conditions (drought, which is especially significant on sites with a thin layer of organic matter).

Modern forest management leads to significant changes in forests with strong fire influences. Fires that develop around clearcuts and roads fundamentally alter structures that have been established over centuries in the taiga landscape; fire frequency is increased, fire refuges disappear, and the forest mosaic is simplified. An equilibrium that has evolved over centuries is disturbed in a catastrophic way and it is not always possible to predict how biological and landscape diversity will be affected. The large fire scars that have occurred during a brief period in areas of intensive development in the Northern European Russia are not fully analogous with wildfires, which have occurred over millennia as a result of dispersed human activity and lightning. It is also clear that any attempt at dividing fires into "natural" and "unnatural" is artificial, because it is rarely possible to tell the origin of a fire from the character of a fire scar. A discussion and description of formal criteria for separating "natural" fires, those not regarded as affecting the natural dynamics of a taiga forest ecosystem, from "unnatural" fires that are seen as a human disturbance of natural dynamics, is provided in the section Identification and mapping of intact forest landscapes.