Main investigation results on the forest radioecology in the Kyshtym and Chernobyl accident zones

the Science of the Total Environment ELSEVIER

The Science of the Total Environment 157 (1994) 45-57

Main investigation results on the forest radioecology in the Kyshtym and Chernobyl accident zones F.A. Tikhomirov*, A.I.

Shcheglov

Moscow State University, Soil Science Faculty, Leninskie gory, 119899 Moscow, Russian Federation

Abstract As a result of the long-term studies of radionuclide migration in forest ecosystems in zones of radioactive contamination after the Kyshtym and Chernobyl accidents, the following trends were revealed: (1) High retention capacity of stand canopy with respect to radioactive fallout. This leads to high doses absorbed by apical and leaf meristems, /3-radiation giving the main part of the dose; (2) Fast self-decontamination of crowns during the growth period and relatively slow decontamination in the phase of physiological rest, regardless of amount of atmospheric precipitation. The rate of crown decontamination determines the value and duration of radiation stress on woody plants; (3) Accumulation not less than 95% of the total radionuclide amount in the forest litter 1-2 years after the cessation of radioactive fallout; (4) Relatively slow migration of strontium and cesium radionuclides along the forest soil profile; (5) High capacity of the forest when serving as a biogeochemical barrier to the routes of horizontal and vertical radionuclide migration and export out of the zone of initial contamination, including migration into the river water; (6) Considerable difference between strontium and cesium when migrating in forest soils and in the soil-plant system; (7) Broad variations in transfer factors for uptake of cesium-137 from soil into forest plants depending on the plant species and soil type. The primary radiobiological effects connected with irradiation of organisms are considered and secondary disturbances due to changes of ecological bonds between the components of irradiated forest ecosystem are discussed. Keywords: Forest; Radioactive contamination; Radionuclides migration; Radioecological effects

I. Introduction The foundation of radioecology as a separate branch of science goes back to the end of the 50s. In Russia, the impetus for the development of this field was given by the large-scale forest contamination produced by the Kyshtym radiation accident in the Urals. Studies on the diverse

* Corresponding author. Elsevier Science BV. SSDI 0048-9697(94)04266-P

problems regarding forest ecosystems at this natural radioactive research ground were performed by a number of scientists (R.M. Alexakhin, A.I. Ilyenko, R.T. Karaban, D.A. Krivolutsky, N.N. Mishenkov, A.A. Molchanov, V.M. Plestsov, B.S. Prister, E.G. Smirnov, F.A. Tikhomirov, G.F. Hilmy, V.P. Julanov et al.). Some of the results of these studies were published revealing the most important conformities to natural laws in radionuclide distribution and migration and radiation effects in the forest ecosystem [1-6].

46

F.et. Tikhomirov. A.I. Shcheglov / Sci. Total Environ. 157 (1994) 45-57

At the same time, A.H. Sparrow with coworkers researching the gamma field managed to demonstrate a phenomenon of extremely high radiosensitivity of pine, and further, of coniferous woody species in general. This initiated the

I R~dioactive

large-scale experiments aimed at investigating the effect of nuclear irradiation on forest ecosystems using powerful point sources of y-radiation and neutrons [7,8]. Further progress in studies of different aspects

fallout from atmosphere

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and migration .

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of radionuclides .

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in forest ecosystems: .

Modeling of radioecological processes and long-term radiation prognosis

Primary radiation I effects on plants and animals

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i. Primary radionuclide distribution between the components of forest ecosystem 2. Decontamination of forest canopy 3. Redistribution of radionuclides between the structural parts of woody stands 4. Migration of radionuclides in the soil profile 5. Radionuclide transfer from soil to plants 6. Migration of radionuclides through food chains 7. Biogeochemical cycles of radionuclides 8. Redistribution of radionuclides between the geochemically connected forest landscapes

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I Ecological dosimetry I (irradiation doses for plants and animals)

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Countermeasures: i. Forest decontamination 2. Restrictions of forestry management 3. Dosimetric and medical control of personnel 4. Afforestation

Irradiation doses for peoples: I. External 2. Internal

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Postradiation recovery of forest ecosystem

Secondary effects following primary effects

Long-term

gical

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radioecolo-

effects

Fig. 1. Major topics of forest radioecology and their interrelations.

F.A. Tikhomirov, A.I. Shcheglov / Sci. Total Environ. 157 (1994) 45-57

of forest radioecology was associated with largescale radioactive contamination of territories in many European countries resulting from the accident at the Chernobyl nuclear power plant. The present report is an attempt to analyse and summarize some of the results from investigations on the most important aspects of forest radioecology using our results and other published data known to us. The schematic layout of these aspects and their interrelations are presented in Fig. 1. 2. Role of the forest in the distribution of radioactive fallout

There are three known paths for radionuclide transfer from atmosphere into the soil and canopy: (1) Gravitational precipitation of large particles (with size exceeding 0.01 mm); (2) washout of radioactive aerosols by atmospheric precipitation; (3) dry fallout of small particles due to turbulent diffusion in the atmosphere. The intensities of gravitational precipitation and washout by rainfall should not directly depend on the type of ground surface. In contrast, the speed of dry fallout of small particles onto the forest, as shown in Ronneau et al. [9], is higher by an order of magnitude compared to that onto open areas. This, evidently, results from the more turbulent atmosphere over the forest, as well as from higher efficiency of particle absorption by the tree phytomass. But due to the extremely high efficiency of washout of aerosols, the latter mechanism dominates over dry fallout during wet weather. This causes hot spot formation along the radioactive cloud trace. This is the explanation for local zones with high contamination levels registered in the territory of the former USSR and in European countries after the Chernobyl fallout. The results of radiation monitoring carried out by our laboratory in these zones in the territory of Russia did not reveal any notable difference in contamination density of forests and neighbouring meadows and pastures. Similar results are reported for the forests of West European countries [10,11]. In accordance with these results, the average contamination density of

47

forests is found to be not more than 25-30% higher than that of adjoining open field plots. Similar results were noted by us during radioecological studies of forests in the 30-km zone surrounding the Chernobyl Nuclear Power Plant, where there was no rainfall during and after the accident, with mainly large size particles precipitating. In the northern part of the radioactive trace (Byelarus), 50-100 km away from Chernobyl, there was no rain either, but smaller particles still precipitated, at a lower speed. For this part of the zone of radioactive contamination, elevated levels of radioactive fallout are reported in the forests in comparison with open field areas. Another important phenomenon discovered in contaminated forests close to the radioactive ejection source in the zones of the Kyshtym and Chernobyl accidents is a so-called forest edge effect. A two- to fivefold elevated radionuclide precipitation is observed at the forest edges facing the ejection source and as far as 20-50 m deep into the forest [1,13]. The degree of this effect varies, supposedly, on the ejection height, distance, meteorologic conditions and the type of tree stand. From the available data, a conclusion can be made that the effect of a forest on the intensity of radioactive fallout depends on the radioactive cloud parameters and meteorologic conditions. However, the role of forests discussed should not be postulated as a decisive one in total distribution of radioactive fallout along the territory. 3. Distribution and migration of radionuclides between the components of forest ecosystems

The data on primary distribution and further migration of radionuclides in forest ecosystems plays a key role in solving major theoretical and applied problems of radioecology, i.e. developing a scientific basis for forestry management in conditions of radioactive contamination, estimating absorbed doses received by humans as a result of their residence in the forest and consumption of forest products as well as those received by forest plants and animals. The same data can be used for implementing countermeasures aimed at re-

F.A. Tikhomirov. A.I. Shcheglov / Sci. Total Environ. 157 (1994) 45-57

48

ducing the negative consequences of radioactive forest contamination.

3.1. Primary distribution The studies performed in the vicinity of the Kyshtym and Chernobyl radiation accidents in Russia and Ukraine [12] and during model field experiments [13] demonstrated that 60-90% of radionuclides falling on the forest were initially intercepted by the tree crowns. Similar results were obtained in Sweden in May 1986 [14]. There, the crowns of birch and beech stands before the spring blooming retained up to 40% of radioactive fallout of Chernobyl origin, those of pine forest, up to 80% and of dense spruce stands, up to 90%. For aged spruce forest in Germany, this share made up about 70% [11]. In this connection, the contamination of forests during the initial post-accident period could, during a strong wind, act as a source of generation of radioactive aerosols spreading in the atmosphere and thus as zones of secondary wind transfer to the neighbouring territories [15]. After a definite time, the

main part of radionuclides migrated from the canopy into the forest litter. According to published data, the physiological status of woody plants becomes the main factor determining the rate of natural self-decontamination [16]. In spring, i.e. during active growth, the deactivation of trees was considerably accelerated, the half-loss time for canopies varying from 3-4 weeks [13,17] to 3 months [2], depending on the type and age of stands. In the phase of physiological rest (in autumn and winter) the speed of radionuclide loss is diminished, the half-loss time expanding to 4-6 months [13,16]. This indicates that the radionuclides migrate to the forest litter presumably by means of biogenic transfer associated with shedding of epidermal leaf and bud scales and bark scales. In general, the processes of radionuclide migration and redistribution between the components of forest ecosystems can be divided into two stages (Fig. 2). In the first stage, lasting 2-4 years, contamination of different stand components results

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F.A. Tikhornirol;, A.I. Shcheglou / Sci. Total Environ. 157 (1994) 45-57

mainly from primary aerosol precipitation of radionuclides on the tree canopy, the highest contamination levels being characteristic of open surface components, i.e. branches, bark and needles. The content of radionuclides in this phytomass is identical to that in the fallen radionuclide mixture. It does not depend on soil properties. In that period, much lower contamination levels are registered for structures shielded from radioactive fallout (wood, bast). Cesium radionuclides, capable of incorporation into assimilating organs and of further transfer to other structural parts of trees, including bast, wood and roots, prevail in the latter. In contrast, Sr, Zr, Ru and Ce radionuclides, characterized by low ability for basipetal transport under air contamination, do not enter these organs. Later, along with radionuclide transfer from the phytomass into the forest litter and then into the root-inhabited soil layer, the second stage begins, the root pathway of radionuclide uptake dominating. Therefore, the dynamics of radionuclide distribution between the forest components is de-

49

termined by the difference between two major processes - - those of deactivation and of root absorption. In the initial period, deactivation dominates, with an observed decrease in contamination levels for structural parts of trees. Later, a point of compensation is achieved followed by a possible enhancement in radionuclide content in the above-ground phytomass until some quasisteady state is reached. In these conditions, the annual transfer of radionuclides from soil exceeds its return with foliage fall only by the content of annual accretion of phytomass. The period for achieving such an equilibrium is 10-15 years (Fig. 2). In quantitative calculations, the dynamics of radionuclide distribution in the wood components depends on biological availability of the radionuclide and on the species structure in the forest. On the soils providing high availability, the point of compensation is achieved faster, and a quasisteady state is established at a higher level (Fig. 3). The quasi-steady strontium-90 content in the overground tree phytomass makes up about 1% of the total for pine forests in the Kyshtym acci-

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50

FLA. Tikhomirov. A.I. Shcheglot; / Sci. Total Environ. 157 (1994) 45-57

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dent zone and about 5% for birch forests. In the 30-km zone around Chernobyl, the corresponding values in 1991 made up 3% on automorphic forest landscapes (sandy soddy-podzolic soil) and about 11% on hydromorphic landscapes with sandy soils characterized by the presence of a rather thick holorganic upper layer (Table 1).

3.2. Radionuclides in forest soils The upper layer of the forest soils, including forest litter, plays the role of a radionuclide pool for a protracted period. These soils are characterized by a comparatively slow rate of radionuclide migration along the vertical profile (Table 2) and by low washout of activity by infiltrating waters. For example, in the Chernobyl zone forests with intra soil outflow, not more than 0.1% of strontium-90 and 0.01% of cesium-137 are washed out annually to the depth below 30 cm. The horizontal radionuclide migration with surface water flow is practically absent [19,20]. In the forests in the Kyshtym zone, the annual washout coefficient for strontium-90 made up about 0.001% [3]. This demonstrates the effectiveness of forest ecosystems as a biogeochemical barrier on the pathways of radionuclide migration and export from the zone of initial contamination.

3.3. Radionuclides in forestry production The most crucial role in the radioactive contamination of the main forestry products is played by the radionuclides of strontium (Kyshtym accident) and cesium (Chernobyl accident). To characterize the level of production contamination a so-called transfer factor (TF) is used, with dimensions of m2/kg, equal to the ratio of the radionuclide content in the product (Bq/kg) to contamination density (Bq/m2). The data presented in Table 3 indicate that different types of forestry products may vary in cesium-137 content by several orders of magnitude. Wood and its processing products. During the first post-accident year, the soil properties in the Chernobyl zone were unimportant as a factor affecting the radionuclide content in wood. The role of soil type became pronounced during the next period, cesium-134, and -137 transfer factors into the wood decreasing two- to fivefold on automorphic soils and increasing approximately the same way on hydromorphic soils which are characterized by a thick holorganic upper layer. The greater the age of the tree stands, the lower is the level of wood contamination. The peripheral trunk part is contaminated most, therefore untruncated

F~A. Tikhomirov, A.L Shcheglov / Sci. Total Environ. 157 (1994) 45-57

51

Table 1 Dynamics of cesium-137 distribution (% of total) in automorphic and hydromorphic forest landscapes of Chernobyl zone in 1986-91

Ecosystem component Automorphic landscape Leaves + needles Timber Bark

Branches All overground

phytomass Soil (0-15 cm) Hydromorphic landscape Leaves + needles Timber

1986

1987

1988

1989

1990

6.7 1.72 4.9 4.3 17.6

0.87 1.48 2.95 2.85 8.2

0.92 0.77 2.9 1.4 6.0

0.51 0.71 3.5 1.2 5.9

0.22 0.41 3.0 1.04 4.7

82.4

All overground

6.0 1.7 4.9 4.3 16.9

phytomass Soil (0-15 cm)

83.1

Bark

Branches

91.8 0.48 1.3 3.9 2.0 7.7 92.3

94.0 0.50 0.84 2.9 0.9 5.1 94.9

94.1

95.3

0.35 0.64 3.4 0.7 5.1

1991

0.09 0.35 2.17 0.4 3.3 96.7

0.69 1.7 3.8 1.4 7.6

94.9

92.4

1.11 2.6 5.4 1.75 10.9 89.1

Figures are % of total; mixed birch-oak-pine forest, 55-60 years old; soddy podzolic sandy soil; sampling time is August annually (Data from ReL 18).

wood contains about twice as much radionuclides as truncated materials. Such products as turpentine, tar, pitch and alcohol contain practically no radionuclides. Higher contamination levels are typical for pine gum and charcoal. The data on radionuclide transfer from wood into cellulose pulp are not available in CIS. Food products. The food products of forest origin are typically characterized by a higher radionuclide content compared to that of agricultural origin. The levels of contamination are highly variable depending on production type and ecological conditions. The highest radionuclide transfer factors are observed for hydromorphic landscapes, their maximal values being characteristic of definite species of mycorrhizal mushrooms. Less contaminated are saprophytic lignicolous parasitic mushrooms. The radionuclide content in forest berries varies within narrower limits. 4. Doses of irradiation of forestry personnel An important factor, limiting forestry management, are the absorbed doses for the personnel. Results of dosimetric monitoring carried out indicate that the forestry personnel - - guards, woodcutters and other professionals, working in the forest - - form a group at professional risk [21,22].

In 1990, the irradiation doses for persons in this group were 1.5-3 times higher compared to those for other groups working in the same area (Table 4), the main share of the dose (60-90%) being due to external irradiation. This may be explained by the y-irradiation dose in the forest being several times higher compared to that on ploughed and arable land. The average value for the total annual dose in the most contaminated forest regions even for the higher risk professional groups was less than 5 mSv per year in 1990, its maximum value being 14 mSv per year. As a rule, the Table 2 Distribution of cesium-137 (% of total) in the profile of forest soddy podzolic sandy soil Soil layer (cml

0 - 3 (holorganic) 3-4 4 5 5 6 6-7 7 8 8-9 9-10 10-15

Year 1987

1988

1989

199(/

1991

95.0 2.3 1.2 0.6 0.3 0.2 0.1 0.06 0.24

89.0 5.6 2.4 1.3 0.7 0.4 0.2 0.1 0.3

85.3 6.7 3.5 1.9 1.0 0.6 0.3 0.2 0.5

80,8 8,7 4,6 2,5 1,3 /),7 0.4 0.3 0.7

76.6 10.6 5.6 3.0 1.6 0.9 0.5 0.3 0.9

Data from Ref. 18.

F.4. Tikhomirov. A.1. Shcheglov / Sci. Total En~ron. 157 (1994) 45-57

52

Table 3 Transfer factors ( x 10 -3 m2/kg) for the uptake of cesium-137 to the forestry production in Ukrainian Polesye in 1986-1991 Product

Barked wood, dry mass Pine Birch Oak Charcoal Pine gum Turpentine Pitch Tar Cereal grasses Fresh mushrooms of of different species Fresh wild berries Bilberries Strawberries Raspberries

Rubus saxatilus

Eluvial landscape, automorphic soils (1986 ~ 1991)

Accumulative landscape, hydromorphic soils (1986 ~ 1991)

0.8 ~ 0.3 1.52 ~ 0.3 0.7 ~ 0.3 7.6 (1989) 0.3 ~ 0.8 (1989-90) < 0.01 (1987-89) < 0.01 < 0.01 5-6 (1987-90) 1-25 (1987-90)

0.8 ~ 0.5 1.5 ~ 4.6 0.7 --* 1.3 -----75-170 (1987-90) 3-250 (1987-90)

-1.6 (1989) -1.0 (1989)

8.0 (1989) -13 (1989) 13 (1989)

Soddy-podzolic sandy soils; Data from Refs. 13 and 18.

highest doses are received by personnel keeping privately-owned farms. 5. Doses received by organisms in forest ecosystems under radioactive contamination

In conditions of radioactive fallout, the doses absorbed by plants and animals originate mainly from external /3- and y-irradiation. The proportion of these two radiation types in the absorbed dose varies with time and is determined by the

following factors: the dynamics of the vertical radionuclide distribution in the canopy, speed of radioactive decay, morphology of the given living species and its habitat in the ecosystem. For the organisms, woody plants first of all, with vital organs non-shielded from radioactive precipitation and of relatively small size, comparable to the /3-particles path length, external /3-radiation considerably contributes to the absorbed dose. The main part of the energy of/3-irradiation from radionuclides precipitated on the crowns is absor-

Table 4 The average doses of internal and external irradiation of forestry workers in 1990 Profession

Bryansk oblast, Klinzy region

Bryansk oblast, Zlynka region

Gytomir oblast, Ovruch region

Total External Internal Total External Internal Total External Internal (mSv per year) (%) (%) (mSv per year) (%) (%) (mSv per year) (%) (%) Forest guards, 4.00 forest workers Mill workers, 1.84 drivers Engineering & 1.6 technical personnel

62.5

37.5

3.3

82.6

17.4

4.6

60.9

39.1

87.0

13.0

3.5

88.0

12.0

4.2

54.8

45.2

92.0

8.0

2.7

92.6

7.4

3.2

71.9

28.1

Cesium-137 contamination density is 0.37-1.1 MBq/m2; data from Ref. 21.

F.A. TikhomiroL; A.I. Shcheglov / Sci. Total Era#on. 157 (1994) 45 57

bed by assimilative organs and apical meristem tissues. In these conditions, the absorbed /3-radiation dose accumulated in these organs during the residence of the main radionuclides in the canopy is about an order of magnitude higher compared to the y-radiation dose. After radionuclide migration from the canopy onto the soil litter surface, the crucial part of the forest ecosystem, subjected to highest irradiation, is also represented by the inhabitants of forest litter. These are, first of all, the soil invertebrates and small rodents, with sizes comparable to the /3 particles path length. Seeds of dendrarious and herbaceous plants, winter buds of perennial plants (hemicryptophytes and chamephytes), located near the soil surface belong to this group too. For such biological objects, the contribution of external /3-radiation becomes comparable to and even exceeds that of y-radiation (Table 5). In contrast to the above-mentioned, no significant danger is presented by external /3-radiation to large animals with a body size much greater than the path length of /3-particles. In this case, the /3-radiation is absorbed mainly by skin and

53

Table 5 Dynamics of absorbed dose rate received by biological structures from external /3- and y-radiation in the forest ecosystems in the post-accident period in the Chernobyl accident zone Irradiation object Dose rate a 0 10 days (average) Leaves, needles Populations of forest litter Plant roots and soil animals

After 30 days After 1 year

100 (90 + 1(I) 20 (10 + 101

20 (16 + 4) 12 (8 6 4)

1 (0.5 + 0.5) 3 (2.5 + 0.5)

3

1

0.02

~'100 is the initial absorbed dose rate in leaves and needles; figures in parenthesis are (/3 + V) dose rates.

fur, making its contribution to the total dose much lower than that of external -/-radiation. A significant contribution to absorbed dose in the post-accident period is also made by irradiation from incorporated radionuclides. The dose from internal irradiation for particular organisms is determined by their ability to accumulate certain radionuclides, their inhabitation, type of nutrition, seasonal conditions, etc.

Table 6 The effects produced by irradiation of the forest canopy in contaminated forests at the Kyshtym (1959) and Chernobyl (1987) accident zones Absorbed doses (Gy)

Effects of irradiati,m

Chernobyl

Kyshtym Needles, leaves

Bud meristem

Needles, leaves

Bud meristem

>_ 200

> 1(I(I

> 500

>_ 100

311- 50

15 25

100

25

25 3(1

12-15

20-100

15-25

10 20

5 10

20-50

5-10

10-20

3-5

Data from Ref. 13.

Severe crown damage in deciduous trees; die-back of pines Die-back of pine forests. Morphological disturbances in deciduous species Severe damage to pine forests: partial shrinkage of forest stand; up to 90% damage of crowns for remaining trees; hardly any growth Moderate damage to pine forests. Damage to needles in lower part of crown. Growth processes associated with awakening of lateral dormant buds restricted growth; changes in shape and dimensions of needles and shoots. Complete suppression of reproductive capability. Light damage to pines: reproductive capability inhibited in post-accident year; increased frequency of gene mutations

54

FM. Tikhomirov. A.I. Shcheglov / ScL Total Ent~ron. 157 (1994) 45-57

Comparatively low radiation damage is suffered by organisms and their structural parts protected from irradiation, for instance by the upper soil layer (Table 5). Roots and buds located in the soil depth belong to this biological group. In general, the radiation situation in the forests in the Chernobyl zone evolved in a manner resulting in the main part of dose being absorbed by a majority of forest species during the initial, rather short, post-accident period (1-2 months). After that, we registered low doses of permanent irradiation with a gradual decrease of dose over time. 6. Radiation effects in the forest ecosystems The highest sensitivity for radioactive contamination is typical for coniferous woody plants (Pinus sylvestris, Picea abies) (Table 6). This results, on the one hand, from the high radiosensitivity of coniferous trees, and on the other, from the high capacity of their crowns for interception and retention of radioactive fallout. This results in high irradiation doses for assimilative organs and apical meristem tissues, originating mainly from the action of /3-radiation (Table 5). The deciduous woody species are more resistant against irradiation on average by an order of magnitude (judging by the value of lethal dose) compared to conifers. As a result of radiation, after the absorbed dose reaches 5-10 Gy, there is a decrease in the population density of mesofauna and other invertebrates inhabiting the soil litter and the upper soil layer (Tachinidae - - the stage of chrysalis; Lymantria dispar - - the stage of egg; representatives of mesofauna - - worms, myriapods). This effect is observed in contaminated zones over a long period (several years), where radiation damage of woody plants was initially registered [2]. In the group of forest mammals, mice receive the highest doses of radiation. They are exposed to high levels of fl-radiation not only from external irradiation, but from incorporated radionuclides, contained in the plant food. According to Ilyenko [5], the effect of protracted irradiation from incorporated strontium-90 and yttrium-90 on the populations of rodents Clethrionomus ru-

tilis and Microtus agrestis under a contamination density of 0.1 G B q / m 2 (corresponding to a daily absorbed dose of 10 mSv) results in a reduction in reproductive ability and duration of the reproduction period. There is also an elevated mortality of animals during the winter season, the radiation effect being combined with severe living conditions and a lack of food. 7. Secondary ecosystem shifts Irradiation of plants and animals in lethal and sublethal doses (primary effect) results in the disruption of ecological relations between the components of forest ecosystems and in further (secondary) disturbances. The disturbance of ecological interrelations are induced by the following factors: (1) changes in microclimatic and edaphic conditions under the canopy damaged by radiation (enhanced heat, light and water entering in soil); (2) disturbances in the synchronism of seasonal phases in the development of ecologically connected groups of organisms (for example, between the time of leaves blossoming and the eggs of leafworms hatching); (3) shifts in food interrelations between consumers and producers (decrease in food resources under the action of radiation); (4) changes in biological pressure from parasites and predators as a result of the selective action of radiation. The role of certain ecological factors modifying the radiation effects in natural conditions was revealed during our observations of populations of Lymantria dispar in respect of its trophic relations with birch leaves and parasites of the Tachinidae family. Chronic irradiation of eggs of Lymantria dispar in doses of 1-3 Gy was demonstrated to stimulate the hatchability of caterpillars. Higher doses (5-30 Gy) induced a decrease in hatchability. In contrast, at the caterpillar stage, survival increases in parallel with contamination density, resulting from the decrease in the Tachinidae population under the action of radiation (Fig. 4). The higher sensitivity of the latter towards radioactive contamination in comparison with Lymantria dispar caterpillars is explained by

F.,4. Tikhomirou, A.I. Shcheglou / Sci. Total Environ. 157 (1994) 45-57

the fact that the Tachinidae chrysalis spends the winter in the forest litter receiving high doses of irradiation (up to 0.3 Gy/day). Similarly, though not so distinct, a correlation is observed between the extent of infection with parasites and survival of the 'hosts', on the one hand, and contamination density, on the other, in the Lymantriadispar population at the chrysalis stage (Fig. 4). In addition, the radiation damage of crowns leading to better warming of eggs results in rather early worm hatching under conditions of high contamination density. On the other hand, the blossoming of birch leaves on these plots was delayed by 5-10 days during the initial 4 years post-accident. In this situation at the moment of

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hatching of worms, their usual food was not available for them in the forest. The common effect of radiation on the populations of Lymantria dispar and its parasites, on fodder plants, and, possibly, some other radioecological factors resulted in a reduced population density of this forest vermin with an increase in contamination density. It was determined that the degree of canopy damage by worms under high contamination levels was 1.5-2 times less than that for neighbouring less contaminated forest plots. The effect discovered was not determined by food resources (quantity or quality of food), but because single egg-laying is much less in the first case compared to the second.

B

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Fig. 4. The effects of radioactive contamination on the populations of Lymantria dispar and its parasites (Tachinidae) - - Kyshtym zone (data from Ref. 23) (A) Hatchability of caterpillars depending on irradiation dose; (B) Infection of caterpillars by Tachinidae (@) and survival of caterpillars (O) depending on strontium-90 contamination density; (C) Infection of Lymantria dispar chrysalis by Tachinidae; (D) Hatchability of Lymantria dispar butterflies depending on strontium-90 contamination density; @, O, A, C, D: experimental data for two subsequent generations of Lymantria dispar (1960 and 1961).

56

F.A. Tikhomirov. A.I. Shcheglov / Sci. Total Ent~ron. 157 (1994) 45-57

8. Conclusion

The analysis of data available on the radioecology of forest ecosystems enables us to draw the conclusion that the conformities discussed are non-specific, i.e. applicable not only under conditions of radioactive contamination. To a considerable extent, these conformities may be extrapolated for situations of forest contamination by other hazardous agents, for instance of chemical origin. This is explained by the similarity in the ways and forms that radioactive and chemical compounds enter the forest ecosystems, the sources being mainly atmospheric ejection in aerosol and gaseous forms. The conformities of migration and distribution of these compounds along the ecosystem components are also similar. The sensitivity of particular species (coniferous woody plants, primarily) towards different types of effects was also found to be non-specific: as a rule, the radiosensitive species are susceptible to chemical damage too. All this enables us to expand considerably the sphere of application of the results of radioecological studies to the explanation and forecast of the consequences of chemical contamination of the environment. References [1] R.M. Alexakhin and M.A. Naryshkin, Migration of Radionuclides in Forest Biogeocenosis (in Russian), NAUKA, Moscow, 1977, 144 pp. [2] D.A. Krivolutsky, F.A. Tikhomirov, E.A. Fedorov, D.A. Pokarzhevsky and A.I. Taskaev, The Effects of Ionizing Radiation on the Biogeocenosis (in Russian), NAUKA, Moscow, 1985, 240 pp. [3] G.N. Romanov, D.A. Spirin and R.M. Alexakhin, The Kyshtym large scale accident: the behaviour of radioactive substances in environment (in Russian). Priroda (Nature), 5 (1990) 53-58. [4] D.A. Spirin, E.G. Smirnov, L.I. Suvorova and F.A, Tikhomirov, The effects of radioactive contamination on living nature (in Russian). Ibid. Priroda 5, 1990, 58-63. [5] A.I. Ilyenko, Accumulation of Radionuclides by Animals and their Effect on the Populations (in Russian), NAUKA, Moscow, 1974, 168 pp. [6] F.A. Tikhomirov, R.M. Alexakhin and E.A. Fedorov, Radionuclide migration in forests and radiation effects on woody plants. In: Peaceful Uses of Atomic Energy, IAEA, Vienna, 1972, Vol. 11, pp. 675-684.

[7] A.H. Sparrow and G.M. Woodwell, Prediction of the sensitivity of plants to chronic y-irradiation. Radiat. Bot., 2(1) (1962) 9-26. [8] G.M. Woodwell, The ecological effects of radiation. Sci. Am., 208 (1963) 1913, p. 40-50. [9] C, Ronneau, I. Cara and D. Apers, The deposition of radionuclides from Chernobyl to a forest in Belgium. Atmos. Environ., 21 (1987) 1467-1468. [10] O. Guillitte, M. Koziol, A. Debauche and J. Andolina, Plant-cover influence on the spatial distribution of radiocesium deposits in forest ecosystems. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of Radionuclides in Natural and Semi-natural Environments, Elsevier Applied Science, Barking, UK, 1990, pp. 441-449. [11] K. Bunzl, W. Schimack, K. Krentzer and R. Schierl, Interception and retention of Chernobyl-derived Cs-134, 137 and Ru-106 in spruce stand. Sci. Total Environ., 78 (1989) 77-87. [12] L.P. Kharitonov, The behaviour of Sr-90 in pine stand of north Taiga (in Russian). In: Materials of All-Union Symposium. Theoretical and Practical Aspects of Influence of Small Doses of Ionizing Radiation, Syktyvkar, 4-6 September 1973, p. 142. [13] F.A. Tikhomirov and A.I. Shcheglov, The radioecological consequences of the Kyshtym and Chernobyl accidents for forest ecosystems (in Russian). Proceedings of Seminar on Comparative Assessment of the Environment. Impact of Radionuclides Realised during Three Major Nuclear Accidents: Kyshtym, Windscale, Chernobyl. Luxemburg, 1-5 October 1990, Rep. EUR 13574, Vol. 2, pp. 867-887. [14] J. Melin and L. Wallberg, Distribution and retention of cesium in Swedish boreal forest ecosystems. In: Radiation Fallout in Sweden after Chernobyl Accident. (in press). [15] A.I. Burnazyan (Ed.), Investigation Results and Experience of Liquidation of Accident Consequences on the Territory Contaminated by Products of Uran Fission (in Russian), Energoatomizdat, Moscow 1990, 144 pp. [16] F.A. Tikhomirov, R.T. Karaban and V.P. Yulanov, Damage of woody plants by /3-irradiation from Sr-89 (in Russian). Lesovedenye (Forest Science), 5 (1972) 142. [17] L.M. Sombr6, M. Vannouche, Y. Thiry, C. Ronneau, J.M. Lambotte and C. Myttenaere, Transfer of radiocesium in forest ecosystems resulting from a nuclear accident. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of Radionuclides in Natural and Semi-natural Environments, Elsevier Applied Science, Barking, UK, 1990, pp. 74-83. [18] A.I. Shcheglov, F.A. Tikhomirov, O.B. Tsvetnova, S.V. Mamikhin, A.L. Klyashtorin and G.I. Agapkina, Distribution and Migration of Radionuclides in Forest Ecosystems (in Russian), No. 1656-B91, Dep. in VINITI, Moscow, 1991, 176 pp. [19] F.A. Tikhomirov, A.I. Shcheglov, O.B. Tsvetnova and A.L Klyashtorin, Geochemical migration of radionu-

FLA. Tikhomirov, A.L Shcheglov / Sci. Total En~ron. 157 (1994) 45-57 clides in forest ecosystems in Chernobyl contaminated zone (in Russian). Pochvovedenye (Soil Science), 10 (1990) 41-50. [20] B.S. Pryster, N.L. Omelyanenko and L.V. Perepelyatnikova, Migration of radionuclides and their transfer into plants in the Chernobyl accident zone (in Russian). Pochvovedenye (Soil Science), 10 (1990) 51-59. [211 F.A. Tikhomirov, V.P. Sidorov, R.M. Barkhudarov, N.D. Kuchma and A.V. Panfilov, Elaboration of the Safety Rules for the Forestry Workers in the Contaminated Areas of Chernobyl Accident Zone (in Russian). Report

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of Radioecology Laboratory, Soil Science Faculty, Moscow State University, 1991, 110 pp. [22] V.E. Shevchuk and A.M. Skryabin, The doses of external and internal irradiation of forestry workers at Belorussia in 1990 (in Russian). Repub. Conference on Scientific-practical Aspects of Health Care of Population Subjected to Irradiation as a Result of Chemobyl Accident, 12-14 March 1991, Minsk, p. 18. [23] F.A. Tikhomirov, The effect of ionizing radiation on the population of Lymantria dispar (in Russian). Ecologia, 5 (1973) 15-21.