by Nikolai SHCHIPANOV, Dr. Sc. (Biology), leading researcher at the A. N. Severtsov Institute of the Problems of Ecology and Evolution, Russian Academy of Sciences

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What is a forest?

Where does the dividing line run between a forest and, say, a grove? How can the importance of forest as a factor contributing to the diversity of Nature be measured?

Forest stands for many things, depending on what we want it to be. If we look at it as a source of wood, wood quality is what counts most. We don't care then what kind of animals it is home to, or what underbrush or grass grows in it. An ideal forest for a lumbering company is one that is easy to fell (in such a dream forest, trees mature simultaneously on as large an area as the loggers can possibly lay their hands on), with growing costs kept at a minimum and products fetching the highest prices.

Wood does not matter that much to an environmentalist. Its effect on the environment is much more important. For a nature worshipper, a perfect environment is one that supports the various animal species which have become endemic to the locality through evolution. In this respect, it will not be an exaggeration to say that forests are vital natural factors for much of this country's territory.

In fact, forests shape up the local climate to a large extent. This is not to mean that your locality gets warmer or colder next to a forest. It only means that it is just a bit warmer in the forest at low temperature than in the open, and cooler in hot weather. Snow thaws in a forest when the mean daily temperature becomes high enough. The thawing rate does not depend on whether the days are warm or cool, and the length of the snow thawing period in a forest does not vary as widely as it does in open country from year to year. Its greenery helps mitigate humidity fluctuations in the air and soil all around. People know from first-hand experience that while the fields are peeling from a prolonged drought the forest stays moist enough. A mature forest influences air current velocities at altitudes of up to 200 m, dust content (transparency) of the air, rainfall condensation (the breathing plants always keep the air above the forest more humid), and much else. To give the forest its due, we must say that it reduces the sweep of climate fluctuations in the environment.

The health of forests is largely affected by the quality of water

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bodies since they level up the run-off. Spring floods in woodlands differ little year by year and are generally calmer than in the open. Run-off after showers is equally tame. And more, rivers are kept full by forests that store up surplus water to smoothly feed the rivers as they dry up.

Not least, forests play a major role in keeping water bodies stocked with fish. In particular, moss-carpeted forests prevent the washout of soil particles, so the streams emerging from them have crystal-clear water. Should the plant cover be disturbed, large quantities of soil are washed away by heavy downpours. The tiny soil particles stick to the fish gills, making breathing difficult. Besides, many fish species spawn on flooded areas, which must remain under water long enough to allow fish fry to find their way to their larger permanent habitat. Finally, forests influence the chemical quality of the environment. They are the Planet's lungs not only because trees release oxygen as they breathe, but also because they absorb harmful industrial emissions, converting them into harmless deposits.

Back to that environmentalist, a good forest, in his view, is one that can keep the processes developing in it in equilibrium. To give the forest a viable chance, it must be assured conditions to which its plant and animal species have adapted in the course of evolution. By embarking on research in this area, this author has drawn on some traditional concepts of the Russian ecology school.

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Honestly, biological species interacting through stochastic processes cannot be judged at the level of individual organisms. To remain stable, a community must have a certain minimal size allowing it to survive variations in the quality of the environment within an acceptable range. It is called "population", if it shares a common territory. Given these basic facts, Stanislav Schwartz (1919-1976), an Academy member, held that population-related approach was at the core of environmental studies. Vladimir Beklemishev (1890-1962), a member of the Academy of Medicine, who studied the interaction of organisms coexisting within the system called biocenosis, believed that "Studying relationships among the populations of different species, so diverse and so different, is the cornerstone of biocenological research." Finally, Igor Shirshov, another Academy member, advanced a hypothesis about population homeostasis, under which a population tends towards a certain numerical size that can sustain it steadily in "habitual" conditions.

We started out from the view that, since a forest comprises different plant associations arising in areas overgrown in the wake of local devastations (fires, windfalls, etc.), a certain proportion of such areas is normal for it. For this reason, life in a forest is never stable. The range of its variations is significantly smaller than can be observed on a territory devoid of vegetation. In fact, the closer a forest is to the ideal, the shorter is the spread between the extremes to which the environment is flung by its natural fluctuations. A certain quality of life can only be maintained if the system stays intact over a definite

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area sufficient for a succession of generations in a community of some, including animal, species. Obviously, maintaining environmental conditions within margins typical of a "good" forest would not cause major changes in the territorial ethological structures of the species inhabiting it. As conditions move towards the margins of ecosystem variations, the population structures of these species can be expected to change and even, with forest quality deteriorating below the "normal" range of conditions, disappear from the forest territory.

We assumed that animals surviving in these conditions can be taken for a standard in identifying the tolerable limits of changes in forest ecosystems when they still hold on to their properties of a "good" forest and a minimum area needed to stay "good".

As we were looking for a suitable study area, our choice stopped on the Tver Region. Even though the region has long been settled by man, it still has forests that have been little disturbed or grown on recent farm fields. The area contained three experimental plots only a few kilometers from one another. One of them was a relatively well preserved moss spruce forest dotted with small islands of burned-out forest overgrown with pine and birch trees. Another was a heavily logged spruce forest with pine and birch patches standing on the sites of burned-out forests and large areas of commercial stands of various ages. Finally, the third plot was taken over by trees aged between 60 and 80 years where the land had been farmed in the 19th century. All the plots that we intended to study had, in a varying degree, experienced human influence, in particular, one plot was in an almost pristine condition, another could be called virgin in part only, and the third had in the past been felled almost completely. The question now is: which of these human behavior patterns can be tolerated and which will cause an irreversible change in the system? Will the forest in the second plot be capable of reviving to norm? To answer these questions, let us try to appraise the differences in the biological diversity of the plots. It is virtually impossible to do this in full, for this would require "winnowing" all life here, including bacteria and microbes. We settled on describing the diversity of the forest by cataloging the grass vegetation on 600 sample patches one meter square distributed over all the territory studied. We found nearly all plant species growing on these patches, the only difference being in proportions. How could we know, then, which of these proportions was normal and which suggested irreversible disarray in the system? To find out, we had to introduce additional criteria such as analysis of the condition of the population of a species associated with the community studied.

Species of this kind could be spotted among various groups of animals. This author, however, gives preference to the smaller mammals. Our undertaking being but an opening step to test the new approach, we confined ourselves to their analysis. To make the test more stringent, the mammals were to be associated with a specific ecosystem and immune to direct human influence (such as protection or management).

Our choice was the common shrew. This is a singular group of small insectivorous mammals, the most numerous and widespread species across Eurasia. Although they have been studied relatively little, their conspicuous presence puts them in the class of most typical targets.

First, shrews feed on various species of invertebrates (insects, spiders, and the like) inhabiting the forest floor. Which means that they depend for livelihood on the abundance and diversity of their prey. They need huge quantities

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of food, so the tiny creatures weighing between 2.5 and 10 grams and requiring a relatively low calorie input could find enough feed at any time of the year, even though different species of feed insects grow up to wholesome size in different periods. Second, the shrews have the highest metabolism rate of all mammals, so they die, if left without food for two to three hours.

Third, insect numbers are closely related to certain physical conditions in their habitats (like curled leaves, mosses, etc.). What's more, these conditions have, in any period of the year, to match the needs of the feed insects at a particular development stage. To sum up, the survival of shrew populations is closely tied up with the operation of the local ecosystem as a whole-a suitable terrain, decaying tree trunks, decomposing debris, moss, and grass. Besides, the forest has to be large enough to smooth out variations in daily and annual temperatures and humidity within ranges assuring the existence of species providing the bulk of shrew diet. In short, it is to be expected that shrew numbers are contingent upon the quality of the forest community in general.

This sealed our choice of the shrew as a tentative indicator of ecosystem conditions. Forests in this country's European part provide a habitat for three shrew varieties- Eurasian common shrew {Sorex araneus), masked shrew (S. caecutiens), and pigmy shrew (S. minutus). To select suitable specimens, we tagged some 30,000 animals, which we monitored simultaneously on the three forest plots of our choice for two years. At the end of our experiment, we found that the masked shrew only is sensitive to changes in the ecosystem and can, therefore, serve as the desired indicator, the two other species being well adapted to life under any conditions.

On the first forest plot that has escaped human influences, the common shrew was observed in all types of habitat. The animals were attached to their permanent territories, some of them having lived on the same spot all their lives 13 months long. In the good forest environment the shrew population reached the same size even in the alder grove, an extremely inconvenient habitat for the common shrew, as in the moss spruce stand which is the first preference for the animals. This is an important observation, showing that conditions on the forest plot as a whole are more important for this species than in their customary habitat. Even if some sections of the forest little suited for the animals occupy a sufficiently small area, they do not have an adverse effect on the lives of the shrews.

There are very few animals of this species in a forest tract diverging most from norm, even in spruce stands. The shrews made only occasional inroads into these territories. Since we monitored individually tagged

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creatures we can definitely say that those were roaming shrews only.

Finally, the plot we placed between the two extremes. We followed the shrews into the moss spruce stand and pine-beech forest, that is, habitats where the masked shrew was to propagate with little restraints on population numbers. Contrary to expectations, their numbers were nearly a third of those on the first forest plot.

The results of our experiment appear to accord fully with theoretical findings. And yet our studies are far from completed. First, we have only explored one, taiga type of forest community. We have no idea about the behavior patterns of other species in other forest types. Second, our observations lasted for only a very short period, so the chances of mere coincidence are not to be written off. Finally, and most importantly, the motivations of the animals we picked up are not clear at all. We have to learn about the kind of social relations that are accountable for the attainable population density and whether they change or not. How do individual animals of this species use their territory in different situations? If the population changes observed are due to natural variations in the territorial ethological structure, the masked shrew will behave similarly in all identical situations. Only when we have answered these questions will we be able to make more conclusive judgements.

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