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THE NATURAL DYNAMICS OF TAIGA FORESTS

"Dynamics of random disturbances" is a phrase that denotes a whole set of natural mechanisms that allow forests to maintain themselves over long periods of time. The essence of the concept of random disturbances is as follows: a natural forest, having developed without any catastrophic human disturbances during many tree generations, is a complex mosaic of small (in relation to the size of the area as a whole) patches, which develop without synchronization. The tree stand on any of these small patches will sooner or later die, for a variety of reasons of a more or less random nature, thus yielding space to a new generation of trees. The reasons for the death of a forest patch under natural conditions may vary considerably and include fires, pest outbreaks, disease, extreme weather conditions, or ageing. Different causes are connected with different sizes of the affected part of the stand, from the size of an individual tree up to hundreds or thousands of hectares. Each large block of natural taiga consists of hundreds or thousands of such parts, each of which is developing according to its own progression depending on the type of disturbance and the time when it took place. This cyclic flow allows a taiga block as a whole to be in a condition close to equilibrium and maintain itself for an unlimited amount of time. Russia's intact taiga has two main types of random distur-bances dynamics which are of particular importance: dynamics associated with the development of a tree population without any catastrophic disturbances (gap or tree-fall dynamics) and dynamics associated with the effect of fires (pyrogenic or fire dynamics).

Gap Dynamics

Gap dynamics is a basic natural mechanism for self-maintenance of taiga forests, developed over the course of many tree generations in the absence of catastrophic external influences, and most characteristic of dark coniferous forests (spruce and spruce-fir forests, sometimes with Siberian Stone Pine mixed in and usually with a component of birch). Gap dynamics is associated with the death of individual old trees, causing openings to appear in the canopy, letting in light and giving smaller trees the possibility to grow and assume a place in the stand (see Figure 11). At equilibrium, old trees die more or less evenly spread over the forest and over time. A stand will develop in which trees of all ages are present and the age distribution of woody species will be that of a sustainable population. The younger trees will be the most numerous, while older trees will be gradually less numerous with increasing age (see Figure 12). Forests with equilibrium gap dynamics are very robust over time. In the taiga of European Russia such forests are typically rather open with an abundance of gaps in the crown layer (see Figure 13). Forests with equilibrium gap dynamics are very rare and it is much more common to find forests in which gap dynamics have begun to appear but not yet reached equilibrium. This happens as stands with a more uniform structure due to some past disturbance, usually fire, start to collapse. Such stands are dominated by one or another tree generation and even in density so that gaps that appear are rather large as clusters of trees of similar age die and fall at the same time.

Gap dynamics is associated with the death of individual old trees or tree groups, causing openings (gaps) to appear in the canopy, letting in light and giving smaller trees the possibility to grow and assume a place in the stand.

Fig. 11. Canopy gap in dark (spruce and fir dominated) coniferous forest. Perm Region, Basegi Nature Reserve. Photo: A. Morozov.

Forests with clearly developed gap dynamics make up only a small part of the total forest area of the European North of Russia. This is also true for the intact forest areas examined in this work. Gap dynamics are most common in forests on the moist western slopes of Urals, where they locally dominate the structure of the taiga landscape. Here, one can find the greatest areas of forests with gap dynamics at an absolute equilibrium, having developed without catastrophic disturbances for at least a few hundred years. Such forests are less typical of the plains of European Russian taiga where they usually appear as pockets in a much larger forest with evident signs of some relatively recent catastrophic disturbance such as fire. These pockets are generally small in size (from tens of hectares up to a few hundred or thousands of hectares). In most cases on the flatlands, gap dynamics are associated with the moistest areas, such as the floodplains of rivers, along creeks and where ground water comes close to the surface. However, pockets also exist on well-drained slopes and elevations where for centuries they have avoided the effects of fire.

Fig. 12. Distribution of spruce trees (not including seedlings) by number (green line) and volume (blue line) over age class in an intact Myrtillus-type spruce forest. Komi Republic (S.A. Dyrenkov, 1966).

Forests with gap dynamics have a large accumulation of dead wood on the ground, and of dead organic matter in the soil. They also have a special soil profile, formed as a result of the continuous falling of old trees along with their root systems. This gives them a higher soil water retention capacity and a lower amount of surface run-off during the spring thaw and heavy rains and, as a consequence, a less variable microclimate in terms of moisture and temperature under the canopy during the whole vegetation period (Zubareva, 1967). The special ecological conditions under the canopy - high degree of light, moisture, variety of substrates - provide forest ecosystems with a diverse plant community as well as dominance in ground vegetation of some tall ferns and grasses (Dryopteris austriaca Schinz et Thell., Diplazium sibiricum (Turcz. Ex Kunze) Kurata, Aconitum septentrionale Koelle, Delphinium elatum L., and others). The layer of tall grasses and ferns, sometimes reaching a height of 1.5 - 2 meters, either hinders or makes it impossible for young plant specimens to develop on the forest floor or on small-diameter fallen trees. Large-diameter fallen trees therefore play a key role for tree regeneration (see Figures 14 and 15), and their disappearance as a result of selective cutting of the largest trees may considerably disturb the natural regenerative dynamics of these forests.

Fig. 13. Canopy density distribution, based on systematic circular 5-meter radius sample plots, and corresponding typical forest structure in two different forests on the same site type: A - an intact spruce-fir forest with gap dynamics in balance B - a secondary birch-spruce forest on a 64-year old clearcut. Perm Region, Basegi Nature Reserve, western slope of the Ural Mountains. The intact spruce-fir forest is dominated by sections with low and medium density canopy. The rather even-aged secondary forest, on the other hand, has a higher and more uniform density.

The particular characteristics of gap dynamics are regulated by various processes in the forest ecosystem and also by some external factors. Wood decomposing fungi, for example, by lowering the mechanical strength of trees and eventually causing old trees to fall down or be broken by wind or snow, sometimes while still fully alive, are usually the main regulators of the tree life span in such forests. By determining tree longevity, wood decomposing fungi have a strong indirect influence on the pace of tree generation change and the size of individual canopy gaps, and also contribute to the occurrence of large areas of wind-fall. Extreme weather conditions may encourage mass infestations of tree-eating insects, decimating sizable areas and thus producing particularly large gaps.

Fig. 14. Regeneration of spruce on a dead fallen tree. Arkhangelsk Region. Photo: V. Potansky.

Fig. 15. Relationship between diameter of dead wood and amount of spruce regeneration (higher than 10 centimeters) growing on it under a canopy of an intact spruce-fir forest. Perm Region, Basegi Nature Reserve.