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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
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17_ Axiom 7-1doc
7(1) EVOLVING of STRUCTURE of ECOSYSTEM STABILITY to PLANT
PESTS and PREREQUISITES of its PERFECTION
In nature, where ecosystems arise without human assistance, evolving of SES might be
considered as appearing an Order from Chaos. Numberless factors take part in this process. The
factors operate as limitations, which determine some ways among unlimited possibilities. The
totality of these factors is defined as natural selection, and it is possible to be classified by a
number of types, as follows.
7.1. Darwinian or evolutionary selection
The theory of evolution teaches that a group of organisms having the same hereditary
properties (traits) and defined as “taxon” (species, subspecies, ecotype) is a result of natural
selection over foregoing generations. In the context of present report, it is important the traits
providing the self-protection of plants of a given taxon against PPs, which are adapted to
consume this taxon. Every of the traits is a FESPPs, which is effective on condition that a
presence of certain circumstances (prerequisites) that composes some complex defined as a
CESPPs. These CESPPs have been classified in the category 2.1. “Plant resistance to PPs.” Thus,
there are the grounds to suppose that all the subcategories of 2.1. are consequences of natural
selection of foregoing generations that corresponds the Ch. Darwin’s understanding of natural
selection. Genealogy of plants shows sequence in an evolutionary history of the traits, which
concern resistance to PPs. 2.1.1.1. “Nonpreference” and 2.1.1.2. “Antibiosis” are the most
ancient means of self-protection indispensable at the time, when diversity of pests’ natural
enemies else do not emerged. Afterwards, along evolving of the natural enemies, development of
2.1.1.3.“Tolerance” and 2.1.1.4.“Evasion” becomes possible. Prosperity of plant taxa with these
traits demonstrates that all these subcategories of 2.1. are effective depending on environmental
conditions.
This type of this of natural selection, which should be referred to as Darwinian or
evolutionary. Its role as a prerequisite of perfection of SES becomes clear if to compare the
situations with wild plant taxa and those bred by humans. Being bred, such cultivars usually
were not exposed to all the complex of their PPs. Indeed, this is difficult to do taking into
account that the number of biotypes within a species of phytopathogens reaches hundreds, and
their occurrence cannot be predicted. Therefore, such cultivars having a bouquet of valuable
traits can be defective as to resistance to some PPs.
7.2. Local selection (due to abiotic factors and interspecific competition)
The soil is abundant everywhere with seeds of diverse species of plants. The number of the
seeds is evaluated to reach eight million per square meter of the soil. Only a minor part of them
germinates every year, whereas the most part is forced to stay in an inactive state waiting
favorable conditions for germination. It takes place competition both among plant species and
within the species. Here, natural selection is exerted by diverse factors – competition on the part
of species most adapted to given abiotic conditions or most vigorous seeds of the same species.
As to biotic factors, it is used diverse means, for example an excretion of substances, which
inhibit germination of seeds or poison plantlets of other species and own one, shading of the soil
surface by sharp growth leaves in horizontal direction, etc. A change of situation in the soil
surface, for example by tillage, affection by ground fire or increased insolation due to cutting of
trees in an upper story gives advantage to seeds of other species. If existing cover is wiped out,
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
abiotic factors become the main driving force of the selection. Then other plant species gain
from the change, and character of vegetation occurs different.
The phenomenon of competition of the type 7.2. was shown in literature, though under other
name – the unequal vitality of diverse species in a given ecosystem (Braun-Blanquet and
Pavillard, 1928, cited in M.B.Markov, 1962, p. 16). The scholars noted four gradation of the
vitality of species in ecosystems, as follows:
i)
All the normal life cycle is completed.
ii)
The life cycle is incomplete, but vegetative reproduction is well-developed.
iii)
The vegetative reproduction is weak.
iv)
Plantlets are sometime produced, but later on the species disappear.
Also, the selection of the type 7.2. results in developing of groups of a number of taxa with
symbiotic relations. For example, some forms of the clover, Trifolium repens grow better in
cooperation with diverse cereal species (Turington and Harper, 1979, cited in T.A.Rabotnov,
1983, p. 108). This phenomenon should take into account at establishing of articenoses.
In biocenoses, the process of 7.2. proceeds without destruction of ESPPs. The elimination of
unfit plant species takes place mainly on early stages of their development (germination of seeds,
plantlets, or seedlings). In so doing, a composition of dominants does not change.
In articenoces, especially at establishing of woody stands of the protective concern, it is
common quite another situation. The matter of facts, such stands are established in severe
environmental conditions – steppe, plots affected by soil erosion, areas of reclamation after
mining, moving sands, etc. It is complicated problem to choose taxa, which are able to
withstands the abiotic conditions, and measures to assist the plants to survive over the time as
long as possible. High skill of silviculture is needed for achievement of this task. Mistakes in this
deal result in mass mortality of dominants long before their natural longevity. The decline is
accompanied by affection of the dominants with PPs, so that destruction of the general stability
of such ecosystems includes disturbance their ESPPs. The ways of solution of this vast problem
need a special consideration.
7.3. Hereditary selection
This type of natural selection begins to operate at shortage of the vital resources (nutrients
and moisture in the soil, light). The plants need to compete with each other to keep balance
between growing stock of them and available supply of the resources. At the competition, it is
revealed different possibilities to survive within a taxon. It implies survivorship the individuals
having the traits, which the most correspondent for given conditions. The presence of such
selection is obvious, because organisms in a species are nonequivalent as to their heredity. Thus,
the type 7.3. differs from 7.2. by the important role of the intraspecific competition comparing
with the interspecific one.
An operation of such a selection might be observed at growing of cereal crops. A
conventional sowing rate (the number of seeds per unit of area) in the crops is much more than
the number of plants surviving up the their maturity. Necessity of such a practice was proved by
numerous experiments. For example, D.A.Sabinin (1948, p.40) has reported that mortality of
spring wheat plants composes usually 30-40% even at absence of any stressors (drought, etc.).
An explanation of this phenomenon consists in an understanding of the beneficial role of
competition for vital resources in the soil among separate plants of the wheat. Such a
competition results in surviving most vigorous plants and stems, which produce maximal grain
yield. Vigor of wheat plants can be uneven due to diverse causes, in particular because a
significant percentage of seeds having inner infection. The role of heredity in the affection of
seeds is very probable. Again, let us turn to cereal crops. At growing a winter wheat variety
during a number of years showed a trend of an increase affection of it by the leaf rust (Remeslo,
1977, p.101). This fact might be considered as a progressing decrease of resistance to a
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
phytopathogen on the hereditary level at a lack of natural selection. In wild plants, internal
infection in seeds was recorded in the English oak (Shcherbin-Parfenenko, 1963). By means of
elimination of plants within a taxon with traits, whose expression is insufficient for local
conditions, natural selection of the type 7.3. provides ecosystems with plants having most
adapted genotypes.
7.4. Physiological selection
Within a plant taxon, there exist a wide variety of physiological state of organisms
independent on their genotypes. The causes of this diversity are very various, for example
unequal conditions of insulation, worst chances to win in competition with older plants, random
damage by herbivores, etc. Natural selection of this type operates over all the plant life. It is well
noticeable in forest ecosystems.
This type of the selection concerns interrelations within plants of one age group, rather than
the change of plant generations. This selection display itself in mortality over all the age groups
of plants beginning with 6.c. Seedlings of dominants. In the older groups (6.b. Undergrowth of
dominants, and 6.a. The main stock of dominants) this phenomenon is called the annual stem
fall.
At the selection of trees on the base of an inequality of their physiological state,
phytopathogens play the role of pioneers. Activity of these PPs over a number of years weakens
of affected trees up to the level, when they become liable for colonization by the bark beetles.
The beetles kill the trees during few weeks. Such succession of events was shown for the western
white pine in Idaho by D.L.Kulhavy et al. (1984).
It is logical to put an idea of the benefit role of phytopathogens and stem borers for existence
of ecosystems. True, as to the bark beetles, such an idea was declared by F.P. Keen (1936, cited
in R.C. Hawley and P.W. Stikel, 1948, p. 231) in the following excellent words: “Nature’s
silvicultural agents which relieve the pressure of severe tree competition or of critical growth
conditions and tend to preserve a natural balance between growing stock and available supplies
of food and soil moisture.” These words concern the ponderosa pine and Dendroctonus
brevicomis LeConte. Nevertheless, they are true for any forest stand and bark beetle species.
When Selection 7.4. operates in an ecosystem, the annual stem fall in it does not exceed the
normal level year after year. One may see the rate of the fall, which should consider as normal
one, in the Table 26, Section 5(1), and quoted from the book by A.I.Vorontsov et al. (1991, p.
271). The Table 26 demonstrates the values of probable annual mortality of trees at the process
of natural thinning in closed Scots pine stands. The rate of mortality is insignificant comparing
with all the stock of dominants. If their density is less than that in “closed” stands, the rate is
some less. These data were obtained in monocultured pine plantations. Nevertheless, they give
an idea of the processes in natural ecosystems.
Such a rate of the annual stem fall is a sign of well-being of ecosystems. In them, ESPPs in
the biomass of the category D-I does not decrease to the levels 3.2. “Lag control” and 3.3. “Late
control”, when mortality spreads on the main stock of dominants.
Although, in overstocked stands of coniferous tree species, damage of crowns with the
Intermediate density by needle-eating insects (the decrease of ESPPs in the category biomass DI) increases a rate of the annual stem fall, but this process is favorable for survivorship of an
ecosystem by means of a relief of competition among the trees. This is a case of LS 5.4.6.”Pine
plantations in the Steppe biome.”
In the category biomass D-II (foliage of deciduous tree species and the larch), a decrease
ESPPs to levels 3.2. or even 3.3. does not increase a rate of the annual stem fall or does it in
insignificant extent.
A benefit of the Selection 7.4. becomes obvious if to pay attention on the situation in
monocultured forest plantations with high stem density. Here, it is planted of trees of the same
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
age and of the close physiological state. In such a situation, possibilities for the Selection of type
7.4. occur shortened. These plants struggle for life with the equal success. Because as plant’s
growth, soil resources become insufficient to maintain all the stock trees in a plantation, the
competition among them is inevitable, and possibilities of the trees in the competition are nearly
equal, it comes a general weakening all the trees. Then, abiotic stress, usually drought, decreases
physiological state of all the stock to loss of the resistance, firstly 2.1.1.1.”Nonpreference” and
2.1.1.2. “Antibiosis.” The destructive consequences of the competition is especially expressed on
the poor and dry soils.
Exactly, heavy competition among the trees with approximately equal possibilities in the
conditions of extreme deficiency of vital resources is a cause of the decline of overstocked
monocultures rather than “…narrow genetic variability…” as W.E. Wallner (1987, p. 322) and
many scholars suppose.
7.5. Through-generations selection
This selection concerns a change of generations in dominants. It includes two subtypes:
7.5.1. Mild change of the generations,
7.5.2. Catastrophic change of the generations.
In turn, 7.5.1. includes:
7.5.1.1. Change of taxa of dominants (the succession).
7.5.1.2. Change of an old generation of dominants by new one of the same taxon.
From the view of ESPPs, it is important that neither 7.5.1.1., nor 7.5.1.2. do not disturb such
a state of ecosystems both in the categories biomass D-I and D-II. When 7.5.1.1. or 7.5.1.2
operate, total values of the annual stem fall do not exceed the normal level. The change proceeds
gradually over decades. In so doing, values of the annual stem fall become some higher in the
upper story, i.e. in preceding dominants, but they are low in new-coming dominants.
The processes, which take place in the situations embracing by the category 7.5.1.1. “Change
of taxa of dominants (the succession)” are described well. Contrary, the category 7.5.1.2. is
known little.
Consider, two cases, which concern the latter category. Here is a passage of the book by P.S.
Pogrebnyak et al.(1944), the Chapter 23. “General regularities of stability of forest caenoses and
its causes” (pp. 69-72):
“The stable phases of a succession forest and other caenoses are able to exist sometimes on a
very unstable base. As an example, one may use mountain pine forests – in the Crimea
Peninsula, Caucasus, Carpathians and other mountain areas with the shallow stony soils on steep
slopes on the very diverse subsoil. The pine dominants would be changed by deciduous species
if tiny a soil fraction appearing due to soil erosion would stay in situ. But on steep slopes this
fraction is washed away by precipitation. This process, which does not allow increasing of soil
fertility, is called by foresters “rejuvenation of the soil.” In a result, pine stands continue to exist
over unlimited time.” (p. 70).
One more example of maintenance of general stability of a forest ecosystem by means of
potent external forces concerns the pine, Pinus pithyusa Stev. in Caucasus (Georgia) near by the
settlement Pitsunda. This forest is stretched as a belt on the distance of several km along a
seashore usually in a few dozens meters from a line of the surf. The width of the forest belt is
usually several dozens of meters. The ecosystem with the pine as a dominant exists due to strong
gales, which occur once in several decades. Then, the surf floods the belt on all its width and kill
of the understory occupied by shade-tolerant species – Carpinus orientalis Mill. and Buxus
sempervirens L. These species are killed by seawater because they are low salt-tolerant, whereas
P. pythiusa is high salt-tolerant. If the gales would not flood the belt, these species would force
out the pine, which is unable to exist under canopy of them (Ibid., p. 70).
The ecosystem is the only reservation of P. pithyusa over the World.
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
7.5.2. Catastrophic change of the generations (the aged crash of forest)
The concept of the aged crash of forest is based on numerous reports about the decline of
most part of the stock of dominants over a few years or even a few days. Consider these reports.
Here are outbreaks of the mountain pine beetle, Dendroctonus ponderosae LeConte in
ecosystems of the lodgepole pine, Pinus contorta var. latifolia Engel. in North America. These
findings were offered by G.D.Amman (1976, 1977), A.A.Berryman (1975, 1976), W.E. Cole
(1976), W.E. Cole et al, 1976), L. Safranyik (1976a), R.H.Coulson (1979). The outbreaks arise
in the same sites at intervals of 20 to 40 years. “Release of the population from endemic to
epidemic levels occurs, when overall stand resistance is lowered, particularly in more productive
(thick-phloemed) trees” (Berryman, 1975, p. 635). Most of these obviously dominant trees die.
The pine undergrowth, however, survives that is explained by unfitness the trees with thin
phloem for development of the beetle’s brood. At every outbreak, “…from 30 to 60 percent of
the stand has usually been killed” (Ibid., p. 635). The outbreaks undergo a decline, when a stock
of the pines with thick phloem becomes exhausted. Then, the attack density is so high that most
part of the brood dies in the egg stage due to rapid desiccation of phloem that is pierced through
densely by the egg galleries. Nearly all the rest of the brood dies in the larval stage (Cole et al.,
1976).
These outbreaks take place on mountain slopes, where the effective depth of the soil is
shallow. It should to remember that this is “The depth of the portion of the soil that is either
occupied or capable of being occupied by the roots of the tree” (Spurr and Barnes, 1973, p. 266).
The greater the effective depth of the soil, the better conditions for existence of trees.
Some other cases of mass mortality of forest trees might be explained as phenomenon of the
aged crash of forest. Here is the case of outbreaks of needle-eating defoliators, and afterwards
the cerambicyd beetle, Monochamus urussovi Fisch. on the Siberian fir, Abies sibirica Lebed. in
Siberia.
The first observations of this phenomenon were made in 1927 by E.G.Rodd (1930). This
scholar found out complete mortality of fir stands on the area 70-80 thousand hectares
surrounded by a circle of live firs having, however, brown crowns. In the crowns, numerous
branches were girdled mainly in their base on a distance five centimeters and more. Own
observations and reports of inhabitants of this area allowed him to propose the following
scenario of the events: these trees were under the effect of the Siberian pine moth, Dendrolimus
sibiricus Tschetv.7-8 years ago. Died due to moth's feeding, but fresh trees were colonized by
stem borers, nearly the only species - the cerambicyd beetles Monochamus urussovi. When
weakened host-trees became to be in short, the beetles were forced to overcrowd on the rest of
them. In a result of the overcrowding, a part of the larvae died and the rest produced beetles
having underdeveloped gonads. These beetles forced to feed on host-plant branches for
maturation. It is probable that a part of the damaged trees became so weakened to be disposable
for subsequent colonization by these cerambicyds.
Recently, the studies of such outbreaks were analized by A.S.Isaev et al. (1983). The
outbreaks of Monochamus urussovi were preceded by heavy defoliation of fir trees by
Dendrolimus sibiricus (in 1920-ies, 1950-1960-ies) or Ectopis (Boarmia) bistortata Goeze in
1930-ies (in 1920-ies according to E.G.Rodd – the author). After a decline of outbreaks of the
defoliators, the damage by Monochamus urussovi continued 10-15 years, and areas of forest with
signs of a damage by this species increased in 13-15 times comparing the defoliated areas.
In the Middle (Central) Siberia, the habitats, where infestations spots of Monochamus
urussovi take place, are specific. They are fir ecosystems of old age situated on a plateau and
upper parts of slopes mainly of south or west expositions. In particular, in the Enisey Mountain
Ridge, the infestation spots expand on hundreds of kilometers on elevated parts of a relief, but
they do not descent into valleys. These stands were weakened long before affection by
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
herbivores that was demonstrated by presence on 80-90% of the trees the root and stem rots as
well as cancer and necrosis of their branches.
Thus, one might see all the signs the aged crash of forest – old age of trees, the shallow
effective depth of the soil on elevated locations of a relief, the effect of drought, which is worst
on exposed slopes. In such conditions, it operates the Intrinsic selection of the subtype 7.5.2. In
the valleys, in better soil conditions and at less insolation, the change of generations in fir stands
happens on the subtype 7.5.1.
In West Siberia, where relief is plain, localization of the infestation spots of Monochamus
urussovi is not so certain, but in all the cases they are preceded by outbreaks of Dendrolimus
sibiricus.
It should stress that the outbreaks of Monochamus urussovi are stopped by host-tree
Antibiosis rather than natural enemies. The mortality due to subcortial parasites and predators as
well as birds (in the imaginal stage) is insignificant. Woodpeckers kill about 30% of old-instar
larvae, so that the overall survivorship for a generation reaches 15-20% that do not decrease
amount of a population of the beetle in the next generation (A.S. Isaev et al., 1983, p. 276).
A decrease of the population is exerted by host-tree Antibiosis in such a way. On the
defoliated trees, colonization of Monochamus urussovi proceeds freely, and its population thrives
until the density becomes too high to provide the brood by sufficient foodstuff. Then, to reach
sexual maturity, the teneral beetles need to feed on twigs of live host-trees. The optimal ones are
sprouts of 0.3-0.5 cm in a diameter. If amount of the damage is sufficient to weakening of the
trees, they occur vulnerable for colonization by the pest insect. This leads to an increase of areas
of the infestation spots. Nevertheless, the trees with damaged crowns resist to the attacks. In fact,
on the trees with 50% of crown became yellow, 60-70% of the brood die due to affection by
host-tree oleoresin (Ibid., pp. 274-275). In the end, on healthy host-trees, colonization by the
Monochamus urussovi is stopped. It is probable that vast outbreaks of Dendrolimus sibiricus in
Siberia are a result of the aged crash of forest.
It is noteworthy that in the case of outbreaks of Monochamus urussovi in Siberia “One of
necessary conditions at arising of an infestation spot of the cerambicyd beetle is prevalence in a
stand composition of the Siberian fir” (Isaev et al., 1983, p. 277). It seems, as to the fir, the
intraspecific competition is more destructive than the interspecific one. At the above cases of the
aged crash of forest, trees die in standing position.
In the areas close to the Sea of Japan (the Primor’ye Region), where climate is wet,
environmental conditions are unfavorable for the defoliators. Therefore, the aged crash of forest
is accompanied here by stem borers. A.I. Kurentsov (1950) never observed outbreaks of
Dendrolimus sibiricus in the Region, although he saw large areas of Pinus sibirica, which died
out. They were colonized by stem borers, mainly by the bark-beetle Ips sexdentatus Boern.
without preceding outbreaks of defoliators.
Lately, in the Russian Far East, it was found out the stands of Pinus sibirica affected firstly
by Dendrolimus sibiricus, and after that - by stem borers (Malakhova, 1963).. Vast areas of
defoliation by Dendrolimus sibiricus were recorded in divers parts of this region, including the
Sakhalin Island and two southern the Kuril Islands in 1919 - 1922 and in 1951 - 1954 (Ivliev,
1961, p. 11).
In larch stands, the aged crash of forest also takes place. Although it is less expressed than
that in evergreen coniferous species that can be explained by better tolerance of the larch to
drought and defoliation. A.S. Rozhkov (1981, pp. 131-132) offered the review of literature about
the decline of Larix sibirica L. affected by the cerambycid beetle, Xylotrechus altaicus Gebl. In
the Chita Region, it was studied an infestation spots, where over 1936-1938, it was died five
times more larch trees than that over preceding eighteen years. In some s, all the matured stands
died. The amount of died trees was evaluated by several millions of cubic meters. In the Sakhalin
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
Island, it was recorded an outbreak of the resident subspecies of the Siberian pine moth,
Dendrolimus superans albolineatus Mats. After a decline of this outbreak, Xylotrechus altaicus
attacked 47% of the defoliated larch trees. Nearly all they were killed.
Another case of the aged crash of forest on the larch was described by A.S. Isaev and G.I.
Girs (1975) in the Republic Tuva. The affected plots were localized by a narrow zone in a low
part of the Forest biome on its border with the Steppe biome, i.e. in the area of the greatest
damage due to Dendrolimus sibiricus. Here, it took place again repeated defoliation by this
species, after which the trees were colonized by the Xylotrechus altaicus. All the killed trees
concerned the overmatured age category.
Let the cases of the aged crash of forest, when dominants die in standing position at affection
firstly by needle-eating insects and after that by stem borers as well as at affection by stem borers
in the first place be “ the aged crash of forest – 1.”
The scales of the aged crash of forest – 1 are surprising. In this context, it is strange the
impotency of contemporary silviculture to counteract this disaster. In fact, it took place an
arising “ in the middle of 1990-ies on the territory of East Siberia the recurrent destructive
outbreak of the Siberian moth, which spread on the area more than a million hectares” (Isaev et
al., 2001, p 343).
Nevertheless, “Due to an absence of neither a timely forecast, nor necessary control measures
at beginning of the outbreak, about 200.000 hectares of highly-productive coniferous forests
perished only in the Krasnoyarsk Kray (Emergency Program…,1997; Isaev A.S., 1997).”(Ibid.,
p.343).
It seems, the proper methods of forecast and control were developed, but ignored in practice.
In fact, “The most vast works as to realization of the system of ecological monitoring were
conducted in course of the project of the World Bank “The Program of extraordinary measures
on biological control pest insects in forests of the Krasnoyarsk Kray” (1995-1996).” (Ibid., p.
343).
The need in conduction of these works was caused by arising just of the above outbreak of
the Siberian moth.
The content of this vast program was shown in the following passage: “In the limits of
scientific accompaniment of the Program biological control of the Siberian moth, it was
completed the works on gathering of the useful information about population dynamics of the
pest, developed strategy of biological protection of the Taiga forests. It was improved methods
of bacteriological treatments and analysis of a state of the pest. It was developed the unified
system of density control of the Siberian moth with orientation on aims of entomological
monitoring. It was conducted the evaluation of efficacy of biological protection. It was improved
the methods of monitoring and forecast of mass multiplication of the Siberian moth.” (Ibid, p.
343).
It might be at next vast outbreak of the Siberian moth, this big research financing by the
World Bank would be used in practice.
The considering problem is not novel one for Russian science. It was solved long before the
outbreak in 1990-ies. The situation, even more serious than above one, took place in 1950-ies.
In that time, growth of density of Dendrolimus sibiricus on vast areas of Siberia was predicted
on the base of data of deficiency of precipitation in the end of 1940-ies – beginning 1950-ies
(Vorontsov, 1978, p.128 - 129). This scholar reported that as the index of the deficiency was
used lowering of the precipitation on 15-20% during three seasons comparing with the average
values for many years. This index was developed by B.V. Flerov. A modification of the index
was proposed by Yu.P. Kondakov.
Now the effects of droughts are clear. They decrease Antibiosis in needles of evergreen
coniferous species, and provide favorable conditions for hibernation of moth’s caterpillars in
ecosystems with dominance host-trees independently of Antibiosis or Tolerance in their needles..
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G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
In 1950-ies, no problems with conduction of the protective measures took place. V.A.
Lozinsky, who took part in conduction of these measures, told the author that he had available
the official letter signed by the leader of Soviet State Stalin prescribing insecticides for these
control measures.
It was developed the biological method of control of Dendrolimus sibiricus. E.V.Talalayev
(1960) found out a form of Bacillus thuringiensis, which allowed to develop the effective
bioinsecticide dendrobacilline.
However, producing and application of this preparation were not organized. Considering this
problem, A.I. Vorontsov (1978, p. 243) suggested that a cause of failure of this enterprise
consisted in intrigues within the media of engaged persons. Such events took place, when it was
yet absent Stalin, so that the disorder in the Soviet society thrived. It seems, now it operates the
same factor.
As far back 1960-ies, the forest protection service was able to organize conduction of
protective measures in the initial stage of the outbreak, precluding its spread on vast areas. In
fact, “In the period of gradations of 60-ies, conduction of avian chemical works on significant
area (about 190 thousands hectares) allowed to liquidate in time the most of primary and
secondary infestation spots of the moth, to diminish strongly its density, and to prevent mass
migrations of the butterflies from overstocked infestation spots.” (Isaev et al., 2001, pp. 297298).
In 1990-ies, such measures occurred to be impossible despite of financing by the World
Bank.
There exists another kind of the aged crash of forest, when a change of tree generation
proceeds with windthrow (windfall and windbreak) of an upper story. Such the aged crash of
forest is performed over few days. This is common in the stands with a prevalence of the spruce.
The weak resistance of the spruce to the wind is explained by ancestral features of this species.
Ecological optimum of the spruce lies in areas with wet climate, and it is a successful
competitor with other tree species in the conditions of the shallow effective depth of the soil.
Further, the spruce is a very shade-tolerant species. Therefore, it forms heavy crowns, and grows
in high-stocked stands. In a result, the balance between capacity of the roots to keep a tree and
weight of the crown is endangered to be broken at greater extent comparing with other tree
species. The factors, which promote the spruce windthrow are the following: the shallow
effective depth of the soil, the reach soil, high stem density of a stand, old age of trees, pure
composition of a stand. On the rich soils, an over-ground part of a spruce tree reaches a greater
mass, whereas stem density in a stand becomes higher that limits development of a root system.
The studies of dependence of the affection of stands by the wind on characteristics of the
forest and habitats in the East Carpathians Mountains were conducted by the author (Vasechko,
1967,1968). This is an area of dominance of the Norway spruce, Picea excelsa (abies) (L.) Link.,
which grows in pure stands or mixed ones with (in an upper story) the European fir, Abies alba
Mill., the beech, Fagus silvatica L., and the birch, Betula verrucosa Ehrh.
It was shown that the affection by the wind with young age of stands, and grows with an
increase of their age. The distribution of percentage of damaged by the wind areas depending on
the 20-years age groups of the stands was the following: 20-40 - 3-5%; 41-60 - 8-18%; 61-8031-51%; 81-100- 55-88%; 101-120 - 84-96%. The affection by the wind of the stands, which do
not reach the old age might be explained by improper silvicultural practices, namely:
establishing of overstocked spruce monocultures. Such stands become susceptible to the wind in
the age long before maturity. All the affected stands were grown in habitats within an ecological
optimum for the spruce – the rather rich soils, although with a shallow effective depth of it.
Let the cases of the aged crash of forests, when dominants suffer due to windfall and
windbreak be “the aged crash of forest – 2.”
8
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
On the other hand, the spruce stands growing of outside of the optimum for this species on
the poor soil are characterized by mortality of trees in the standing position. Here, the trees are
spiced widely, and their height was rather low. These trees were adapted to affection of the wind.
Nevertheless, they suffered due to the mortality in the age long before an average longevity of
the spruce.
This is characteristic for plains on the shallow infertile soils of the podzol type, where spruce
trees are shorter and thinner than on the rich soils. Therefore, the balance between a stem weight
and size of a root system is better sustained. Here, heavy wind-throws arise seldom. However, in
such conditions, spruce trees often die en masse while standing because of root desiccation at
droughts.
These cases of trees’ mortality in the standing position refer to the aged crash –1.
A maximal resistance to effect of the wind and longevity were recorded in spruce trees
growing from early age on the edge of stands with clearings. They were widely spaced, and had
well-developed pyramidal crowns, that allow them to hold themselves in the soil firmly, and to
minimize a pressure on the part of the wind.
A tree species composition of the stands exerts a not too great effect on resistance to the
wind. Most of the affected stands were of pure spruce ones, but such stands were in abundance
comparing with mixed ones. The mixed stands were among the heavy affected by the wind.
Within them, the spruce and the fir were damaged in the same extent, whereas the beech was
more resistant. Scots pine and beech stands growing in the high mountain area of the Carpathians
Mountains on the limited area occurred more resistant to the wind than ones with a prevalence of
the spruce. Nevertheless, this fact might be explained by different soil conditions, where these
species grow. The soil conditions render greater effect on resistance of stands to the wind than
their species composition.
The destructive effect of the wind took place on an area 700-1400 meters above sea level, i.e.
within the natural range of the spruce, where the effective depth of the soil was shallow nearly
everywhere. On lesser height above sea level, spruce stands occur to be resistant to the wind that
might be explained by the greater effective depth of the soil. In this area, however, spruce stands
were established artificially.
The killed and weakened by the wind trees are colonized by stem borers, which reach so
High density that allow them to overcome resistance of standing host-trees in vicinity of affected
plots. Because success at colonization of healthy trees can be achieved on condition that density
of attacks is High, competition within brood results in decrease and weakening of populations of
stem borers. After exhaust of a stock of affected by the wind trees, populations of stem borers
return to innocuous level during few generations.
On areas of windthrow, light-requiring tree species (the birch, aspen) appear as pioneers.
Then, after cover of these species, shade-tolerant species (the spruce, fir, and beech) become to
regrow, and rise into an upper story.
The areas of windthrow become vast, when the mighty wind is accompanied with a heavy
rain. This is so because in the soil saturated by water, the keeping capacity of the roots becomes
worth, whereas the rain overloads the crowns.
The review offered by R.G.Kiselevs’ky-Babinin and V.M.D’ýakov (1968) shows that in
mountains of the Central Europe, windthrow on vast areas of forest with a prevalence of the
spruce has been recorded from the first part of XIX century. An appearing of them on not too
large areas is common every year, and the large-scale events take place approximately every
decade. Sometimes, the values affected by the wind trees become mighty. Thus, in 1964, in the
Soviet part of the Carpathians Mountains, the wind damaged over five million cubic meters of
wood (Ibid., p. 49).
The windthrow is common on vast areas in north of the Russian Plain on the soggy soils.
Here, the stems of felled by the wind trees serve as sites, where new generations of dominants
9
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
(the spruce, the fir) begin to grow. This is so because the stems are situated above the soil
surface, which over-saturated by moisture and covered by a thick layer of moss. When occurring
in such a surface, seeds of trees are unable to develop, but decaying stems is an admissive media
for grow of young trees. Being older, these trees successfully compete with mosses.
In North America, windthrow is considered as a necessary stage in the change of generations
of the spruce. In particular, in northern parts of the Appalachian Mountains, old stands of Picea
rubens Sarg. are blown down by the wind, but the spruce will appear on the same sites again
(Whittaker, 1975).
Outbreaks of the spruce budworm, Choristoneura fumiferana Clem. in North America have
signs of the aged crash of forest. Indeed, they are characteristic for stands of old host-trees, and
are preceded by droughts. Knowledge of history of endangered areas reveals an operation of the
important factor caused the aged crash of forest – human activity. S.A.Graham (1939, p.172174) stated: “Forest fires, logging operations, and natural succession in the later part of the
nineteenth century resulted in a decided change of forest type from pine and spruce to balsam fir,
so that on vast areas, almost inconceivable in extent, balsam fir became the predominant tree. It
was in these areas, predominantly pure fir, that the great outbreaks had their origin….It is
probable that the forest containing a comparatively small proportion of balsam fir would have
safe from attack had it not been for the adjacent areas of pure or nearly pure balsam fir…it has
been shown that young stands and stands in which the balsam is overtopped for the most part by
other trees are not favorable to building up budworm outbreaks (Graham and Orr, 1939), and
also that general outbreaks are always associated with extensive areas of susceptible forests.
Furthermore, within the areas where budworm outbreaks are likely to occur, there is a sufficient
number of tree species and diversity of forest types, so that the silviculturist can produce a great
variety of conditions by appropriate manipulations.”
Let the case of vast outbreaks of Choristoneura fimiferana be “the aged crash of forest – 3.”
F.G. Craighead (1942, pp. 377-378) has supposed that outbreaks of the spruce budworm are
sequences of inadequate composition of stands, and they serve as means of restoration of the
mixed composition, which provides them with stability. Here are his words: “By the time the fir
and spruce reached maturity they formed a considerable part of the upper crown canopy and thus
made conditions favorable for an outbreak of the spruce budworm. The budworm killed most of
the mature fir and some of the spruce. The eastern spruce beetle was also a factor in killing
overmature spruce. Insects were thus important in the production of the extensive stands of white
pine of early logging days – giant trees 300 or more years of age overtopping a spruce-firhardwood understory.”
Considering population dynamics of the spruce budworm in eastern Canada, J.R.Blais (1983)
has concluded that the causes of the increase in the frequency, extent and severity of the
outbreaks, which takes place in XX century consists in human interference. They are clearcuttings, which favor an appearance of even-aged fir-spruce stands instead the ecosystems divers
in species composition and age structure, fire protection and use of pesticidal control measures
against the budworm.
Likely to stem borers, outbreaks of the budworm arise through uneven periods. In the
Laurentine Park, over three century, these periods were 44; 60; 26; 76 and 37 years (Blais,
1965).
Unlikely to stem borers, outbreaks of the budworm do not lead to complete mortality of
dominants. A part of them survives, and study of their stem increment allows tracing an
appearing of the outbreaks during centuries. J.R. Blais supposes that arising of budworm
outbreaks is an event necessary for a relief of stress at the climax stage in succession of boreal
forest ecosystems. The incomplete tree mortality took place in mixed “primary” ecosystems. In
the even-aged pure “secondary” fir stands, it is possible complete mortality of the dominants.
10
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
S.H.Spurr and B.V. Barnes (1973) were very close to formulation of the concept of the aged
crash of forest. Their reflections are the following: “There are many reasons for overstory
mortality among them being competition, senescence, and death caused by external factors such
as insects, diseases, wind, and lightning”(Ibid., pp. 312-313)… “Sooner or later it will be
weakened to the point that it will be succumb to some insect attack, fungal attack, or of the
external enemy, particularly following some extreme dry spell, wet spell, hot spell, or cold spell.
Winds, too, play a major part in removing the mature forest overstory. In the New England
states, major hurricanes in 1635, 1815, and 1938, as well as many lesser storms, have blown
down many thousands of acres of the toller forest.”(Ibid., p. 316).
Some items of comments to these words might be proposed. It should stress that abovementioned events take place exclusively in the specific soil conditions – at shallow effective
depth of the soil, whereas as the deep one, the change of generations proceeds by means of
gradual displacement of older trees by younger ones. Further, the causes of the forest decline
should by arranged in the true order. In this order, activity of insects and diseases should be
treated as a consequence of all the rest factors. At last, the phenomenon recorded by above-cited
scholars was not denoted by own name. The proposed by the author term is “the aged crash of
forest” (Vasechko, 1983).
7.6. Extrinsic selection (selection on the part of other ecosystems)
It occurred to be situations, when environmental conditions change to such extend that an
ecosystem becomes inappropriate for new circumstances, and it undergoes a radical
modification. Physiological state of its dominants becomes weakened, i.e. their self-protection by
CESPPs 2.1. “Resistance to PPs” has been lost. The former dominants undergo a decline, and it
arises new ones. Causes of the changes concern climatic events or perturbations of water regime
of a terrain. In a result, it takes place a change of an ecosystem by one of other character, which
is more adapted to new-appeared environmental conditions.
This phenomenon was noted by Eu.Warming (1902). He has written that plant communities
fight each other continuously and everywhere; “…every community always trends to intrude in a
habitat of other one, and every minute change in environmental conditions right away breaches
the equilibrium…Often insignificant on the first glance a change in the conditions causes very
potent changes in a vegetation” (Ibid., p. 288). As an example of the effect, it serves persistent
change of water table only on several inches.
To differ the character of this process and the processes in 7.4. and 7.5., it should mention
that the factors causing changes in vegetation at the Selection 7.6. are outer as to an ecosystem,
they are not connected with development of its vegetation. This is not a succession.
Eu. Warming (Ibid., p. 67) explains how the environmental changes exert their effect. They
unnecessarily significantly worsen soil characteristics for former dominants, but they establish
better possibilities for the competitors, which then are forced out their precursors.
Further, Eu. Warming (Ibid., 67) has paid attention on the fact that the cause, why formations
occupy definite habitats, does not consist in unfitness of the soil in the habitats of other
character. The cause is the forcing out of the dominants by competitors more adapted to these
habitats. This suggestion is confirmed by the facts that at assistance on the part of humans, who
remove such competitors, diverse plant species are able to grow successfully in the habitats,
which are improper for them in nature. For example, in botanical gardens, i.e. in approximately
equal soil conditions, the plants from very different habitats with diverse climatic areas are able
to thrive. Thus, presence of a formation on a habitat of a given type is a facultative one. It
determines by its ability to withstand to a competing formation.
The idea of natural selection on the ecosystem level is not conventional. E.R. Pianka (1978,
Ch. 8, The section “Evolution of communities”) in several phrases has offered the view that such
11
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
a phenomenon is little of probable. However, if this scholar would acquainted with the
publication by Eu. Warming, his view would be different one.
This suggestion should be confirmed by factual data about changes of character of formations
under effect of outer factors – climatic, weather of water regime of the soil. Such data are
abundant in the Russian literature on geobotany and forestry. Russian scholars have had a
possibility to observe on the space of their giant country the dynamics of borders of very diverse
biomes – from the Arctic desert to the common desert, as well as interrelations among
ecosystems of forests, meadows and marches. Big attention has been paid to afforestation in
steppe. For solution of the latter problem, it is need to know well regularities of interrelations
between the forest and steppe formations.
It was found out Skythian tumuluses in a forested area, although they always were built in an
open (steppe) area (Tanfil’yev, 1902, p. 361); whereas in the tundra in turf layers, it was
common findings of tree stems (Sukachev et al., 1934, p. 313). These facts are evidences of
moving of the forest formation on the steppe one and the tundra on the forest. Such upheavals of
borders of the biomes take place at the same spacious changes of climate. When climate
bccomes more wet and colder, the forest seizes north areas of the Steppe biome, and the tundra –
north areas of the Forest biome.
Otherwise, it was found out the signs of the opposite trend – an onset of the periods, when
climate got warmer and dryer. A founding of relicts of steppe vegetation evidences that the north
border of this biome was situated hundreds kilometers northern than now. G.I.Tanfil’yev (1894)
supposed that in the past steppes spread nearly to nowadays cities Moscow, Vladimir and Kazan.
A great many data on climatic changes and their effects on vegetation are collected in the
books by Bruks (1950) and S.I.Barash (1989). The changes of climate are so common and
significant that the reflections about a human conditioned cause of the present trend to a
xerotermity seem to be naїve.
The next discourse follows the review by S.I.Barash (1989, p. 12). In 2200-2000 B.C., the
melting of ice-fields was so great that the level of the World Ocean rose on 2-2.5 meters. This
climatic change was called “the First xerotermic period”. In this period, it disappeared large
areas of oak forests that implied advance to the north the Steppe biome. Afterwards, it was the
following periods: wet in 2000-1200 B.C.; dry in 1200-1000 B.C., when areas of lakes became
minimal; heavy cold snap in 1000-700 B.C., when the level of the World Ocean decreased on
nine meters; warm and dry in 700-500 B.C. (the raising of Skythian tumuluses was just in that
time, therefore, they occurred now in a forested area); increase of moistening beginning with 500
B.C.,when the level of the Boden Lake rose on nine meters, large nearby areas of forest
underwent decline due to expansion of turf-fields, fresh climate continued up to beginning of the
new era (Ibid., p. 12).
In the current era, the fluctuations of climate were also great. The available data are
abundant, so that they allow characterizing the changes in separate regions of Europe.
In East Europe, “The centuries from V to VIII were called “the Second xerotermic period”
(Ibid., p. 18)”.
In the west regions of the continent, “After the warm and dry period (with I to V centuries
A.C.), in 650-750, it was observed a weak cold snap…In the early Medieval Age, between 7501300, in the Central Europe, it getting warmer and decreasing of moisture…Average yearly
temperature increased more than 1°C…History of peopling of Iceland by Scandinavians
evidences about the progress in warming.
As early as in X century, Vikings established thriving colonies in the Greenland finding out
here green pastures. Climatic conditions allow them to practice agriculture… The period of XIVXIX centuries is called in climatology “the Little Ice Age”… The average yearly temperature
decreased on 1.4°C… In the East-European Plain, …the Little Ice Age was especially exhibited
in 1550-1700” (Ibid., pp. 39-40).
12
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
At the climatic changes, phenomena of the extrinsic selection are traced both on the borders
of biomes and within them.
As to the latter, at expansion of cold climate, in central areas of the Russian Plain according
to analyses of fossil pollen, vast tracts of spruce forest were changed by the species more tolerant
to low temperatures – the birch and the Scots pine. The beginning of this process is attributed to
XII-XIII centuries (Khotinsky, 1981, p. 26).
Within the biome of Coniferous (Taiga) forests, it takes place the selection of 7.6. type
between ecosystems of forests and marches. The changes are of a great scale, numerous over last
millennia, and happen swift from the view of the geological time. The data were obtained at
studies of stratification of marches in diverse points over the Holoarctic (for review see N.A.
Khotinsky (1981). In turf deposits, it was found out the strata in a state of advanced
decomposition comparing with media above and below them. In addition, the strata contained
numerous remnants of trees.
The strata arose, when marches dried out, and these habitats were seized by forest. It
supposed to be the expansion of forest took place in the periods of cold climate. Then, the level
of the World Ocean became lower, and therefore a water table in marches decreased. In a result,
the effective depth of the soil in these habitats occurred to be sufficient for existence of forest
ecosystems.
The worming of climate uplifted both the Ocean level and water table, so that marches
returned to the habitats. The changes of ecosystems proceeded during a few centuries, i.e. close
to longevity of forest trees. It was found out up to a ten and half of the strata in a march, and the
changes of climate was traced beginning 6-7 millennia B.C. The most expressed periods of cold
climate are supposed to begin in 2300 B.C., 1200 B.C., 600 B.C., 400 A.D., 1200 A.D.
The scale and swift of the selection of the type 7.6. might be illustrated by the following
reports. According to M.I.Neischtadt (1976) “…no whatever balance in nature of the West
Siberia now exists. Marches over last 10.000 years …continuously advance on secadol forests...
destroying huge areas of forest.”(cited in N.A.Khotinsky, 1981, p. 64).
The problem is actual one because the expansion of marched lays obstacles for extraction of
oil and gas in this area. The scholars call upon to practice measures to counteract this process.
Rather that the extraction just intensified it because the pumping out of great amounts of liquid
and gas leads to lowering of the earth surface. This problem was noted by V.Misharin (1989),
who suggested significant increase of expansion of marches in last decades. Thus, the Extrinsic
selection of ecosystems 7.6. proceeds in sight of people.
It was described in detail the events, which take place in borders of biomes, in particular at
changes of climatic conditions.
The southern edge of the Forest biome, if it does not disturbed by humans, consists of a belt
of shrubs (Sukachev et al., 1934, p. 369). Just in such a position, the formation of shrubs is able
to compete successfully with forest and steppe ones. This is so because in winter, this belt
accumulates a mass of snow, which is blown by the wind from open space. In a result, the soil
within the belt becomes less saturated by salts and is moistened on larger depth. Thus, the
effective depth of the soils increases. These changes in the soil are directed to bettering
conditions for growth of trees. As soon as climate becomes more wet and fresh, trees better
compete with the shrubs, and the latter – with grassy vegetation. The area of a forest widens,
whereas the area of a steppe shortens. At a climatic trend in the direction of dryness and worm,
the opposite events take place.
As to the belt of shrubs, it comprises spiny and poisonous species, which are well-protected
against hoofed animals – Caragana frutex C. Koch., Prunus fruticosa Pall., P. spinosa L.,
Amygdalis nana L., Spiraea crenata L. In the forest edge of the Forest-Steppe biome, it is
common Crataegus spp. and Rosa spp. Thus, the belt of shrubs not only makes the soil ready for
13
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
growing of trees, but also protects their regeneration against hoofed herbivores. The CESPPs,
which operates in this case, is 2.1.2.5 “Parasitic antibiosis.”
At onset of the cold and wet periods, on the border of the forest biome with the tundra, it is
described the following events (Sukachev et al., 1934, pp. 312-314). The level of the permafrost
layer becomes nearer to the soil surface. In a result, ever-increasing part of tree roots stays over a
season of vegetation in the frozen media, and does not supply a tree with moisture. A former
generation of trees dies out, a new generation grows at the very limited effective depth of the
soil, and, therefore, reaches only dwarf or tiny size. In depressions within a forest area, waters of
melting snow and rains retain for longer time that induces suffocation of tree roots. In addition, it
begins expansion of the mosses, which have prevalence over trees and grasses in such
conditions. The first invader is the hair moss, Polytrichum spp., which initiates the process of
formation of turf. Then, content of moisture on the soil surface becomes sufficient for thriving of
the bog moss, Sphagnum spp. The latter forms a layer of turf, which covers all the soil surface
interrupting an access of air to tree’s roots, that kills them. The growth of muskegs spreads from
the forest-tundra border area on the Forest (Taiga) biome.
A.I. Vorontsov (1960) revealed in climate of the Russian Plane for the end XVIII – the
middle of XX centuries the periods with different frequency of droughty seasons. Duration of
such periods consists of ten – twenty two years. He supposed that in these periods, the decline of
forest became greater. He postulated that these variations were determined by changing in solar
activity that was not substantiated by facts, but frequency of droughts indeed seemed to be
uneven.
Here is citation from the above scholar (Ibid., p. 233): “The more expressed the period of
droughts and heavier of a drought in an year, the more intensity of forest decline. The significant
deficiency of precipitation and frequency of droughts took place at the end of XVIII and the
beginning of XIX centuries and in 1850-1865. The especially great deficiency of precipitation
and frequent droughts were observed in the period from 1888 to 1910. After that, a number of
years were characterized by large variability of weather conditions with an alternation of wet and
relatively dry seasons.
Notably, that the corresponding fluctuations took place in the long-term cycle of solar
activity. The contemporary state of solar activity being well-expressed after 1931 was
characterized by high intensity and exerted effect on circulation of the troposphere and
fundamental climatic changes (grew warm of the Arctic, lowering of the level of the Caspian
Sea). Our studies in the Volga River Basin showed that during the above period of an increase of
the solar activity, in the new cycle, heavy droughts took place in 1931, 1934, 1936, 1938, 1939,
1946, 1953, 1954, 1955, 1956. During this period, the forest decline was greater, especially as to
oak and pine stands. In those years, the droughts spread far to the north that induced intensive
decline of spruce stands (1936-1939).”
Some cases of the forest decline might be considered as a phenomenon of Extrinsic Selection
7.6. operating in a result of improper silvicultural practices. The cases concern situations in
plantations established in habitats unfit for forest ecosystems.
An example of such a case is provided by N.G.Kolomiets and D.A.Bogdanova (1981). In
southern West Siberia, they studied causes of outbreaks of pest insects in Scots pine stands in
variety of environmental conditions. In them, most serious pest was the large spruce beetle,
Dendroctonus micans Kug., which served as a direct factor of the mortality of trees (Ibid., pp. 45). The decline, which continued over a number of years, took place in the plantations
established in the following conditions:
i)
On the swampy soils with a contrast moistening,
ii)
In flood plains with unstable water table,
iii)
In areas with expressed signs of expansion of steppe vegetation (old field lands, long
fallow, pastures).
14
G.I. Vasechko STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH.
PART II. SUBSTANTIATION OF THE AXIOMS PROPOSED IN THE PART I
iv)
Under a cover of birch groves.
On the other hand, pine stands of natural origin or pine plantations established in common for
this species habitats (the sandy or sandy-clay soils on secadol), were free of damage by
Dendroctonus micans.
In some environmental conditions, forest ecosystems of natural origin undergo natural
selection of the type 7.6. as a common event. It is true, in particular, for flood plains, where
water regime is very unstable. Large deviations in the regime from average values weaken trees,
which die being colonized by stem borers. In a result of the decline of forest, it appears in the
same habitat meadow ecosystems. A stabilizing of the regime over a number of years leads to
return of a forest.
Within every biome, there exists so-called the extrazonal vegetation occupying the habitats,
which are not characteristic for a biome. For example, in the Steppe biome in great ravines, it
grows forest. Somewhere on slopes of the ravines, it is area with the conditions nearly equally
suitable for grassy and woody vegetation. Depending on weather situation, they force out each
other. A retreat of forest is accompanied always affection of trees with PPs, especially stem
borers. It is very common of affection by them of some parts of trees – tops or separate branches.
Human society in its activity often meets phenomena of the Extrinsic Selection 7.6., and
needs solve arising problems. In particular, the problems are complicated at establishing of
articenoses out of borders of biomes native for dominants. This concerns afforestation in the
Steppe biome (shelter-belts, massifs of forest for melioration of the environment), establishing of
woody ecosystems in distinctly arid areas with the aim to counteract desertification, woody
plantings in settlements in the Tundra biome.
The difficulties in this way are great. Nevertheless, achievements of forestry and further
progress in this province allow solving these problems. There exist a number of examples of
brilliant advances as to establishing of stable forest ecosystems outside of the Forest biome. They
will be considered in the next chapter jointly with solution of the problems provoked by human
activity both in articenoses and biocenoses.
15