Radiocesium in a Danish pine forest ecosystem

the Science of the Total Environment ELSEVIER

The Science of the Total Environment 157 (1994) 125 - 132

Radiocesium in a Danish pine forest ecosystem Morten Strandberg Rise National Laboratory, DK-4000 Roskilde, Denmark

Abstract During the autumn of 1991, a Scots pine forest, Tisvilde Hegn, was investigated with respect to the distribution of radiocesium on compartments in the forest ecosystem. The sandy acidic soil is poor, with a ~ 5-cm thick layer of organic soil, and clay content is very low, between 0 and 2%. Cesium from Chernobyl is still totally in the upper 5 cm, while almost half of the fallout cesium has penetrated to depths lower than 5 cm. More than 95% of the total amount of 137Cs is in the soil compartment. The rest is mainly in the trees (3.4%) and vegetation (0.4%), moss and lichen included. The concentrations of radiocesium are highest in the endshoots of the pine trees, and lowest in the hardwood. There are indications that the Chernobyl cesium is mainly distributed in the parts of the trees that have been formed since 1986. Observed Ratios (OR) were used to characterize the ability of the different components of the forest ecosystem to accumulate radiocesium. OR is defined as the ratio between the content of ~37Cs kg ~ (dry wt.) and the deposition per meter square. In vascular plants, mosses and lichens, OR varied between 0.01 and 0.1 m2/kg. In fungi, it varied between 0.05 and 4.5 m2/kg, though generally it was between 0.2 and 1 m2/kg. OR (137Cs kg-L/dry wt. of meat × 137Cs m -2) levels in three roe deer samples varied between 0.016 and 0.21 kg -~/dry wt. With an annual harvest of around 70 000 animals, this might be the most important pathway of this radionuclide to man from semi-natural ecosystems in Denmark.

Keywords: Radiocesium; Distribution; Forest; Pinus siltestris; Ecosystem

1. Introduction The aim of this project has been to produce data on the radiocesium distribution and cesium observed ratios in a Danish forest ecosystem. In the rest of Scandinavia, forest ecosystems have been and are in the process of being m o r e thoroughly investigated with respect to radiocesium (Bergman et al., 1988, 1991). In Europe, the contamination of forest ecosystems with radiocesium have also been studied from a great variety of angles. Many organisms have been mentioned, but among the most thoroughly investigated are fungi and roe deer (Capreolus capreolus). ComElsevier Science BV.

SSDI 0 0 4 8 - 9 6 9 7 ( 9 4 ) 0 4 2 7 3 - P

mon to many fungi and roe deer is that 137Cs levels did not decrease in the years after the Chernobyl accident (Karldn et al., 1991). In Denmark, the occurrence of radiocesium in forest ecosystems has only been investigated once (Roos et al., 1990), probably because of the supposed minor radiologigal importance of this pathway to man compared to agricultural pathways. This is of course true, but nevertheless there is good reason to improve our knowledge about, and understanding of, this part of Danish radioecology, especially when studying the specific group of people that increasingly use more wildlife products, i.e. hunters and m u s h r o o m pickers. In

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the case of future releases, studies of this kind will be of help when estimates of the consequences are to be made. Studies like this make possible a comparison of radioecological data from different forest ecosystems, not least because the study site is similar in type to the coniferous forests on sandy ground, that is the dominating forest type in the Chernobyl evacuation zone (Stukin, 1991; Davydchuk, 1992).

1.1. The locality 'Tisvilde Hegn' Tisvilde Hegn is a near-shore forest on sandy ground. The sampling site is old seafloor, giving a very flat topography. The composition of trees are mainly Scots pine (Pinus silvestris), Norway spruce (Pices abies), beech (Fagus silvaticus) and oak (Quercus robur). Birch (Betula pendula) is common as upgrowth wherever light conditions are present. The coniferous parts of the forest are very Scandinavian as regards composition of understorey species e.g. Linnaea borealis, Goodyera repens and Vaccinium vitis-idaea. Also, fungi are widely distributed in Scandinavian coniferous forests, e.g. Rozites caperata and Russula decolorans. The podsolic soil is mainly sand covered by a ~ 5-cm thick layer of decomposed organic matter. The clay content is low, between 0 and 2%, and is ~ pH 5 (Nielsen and Strandberg, 1989). The soil and topography at the study site represent a particularly simple ecosystem, well suited for general investigations. The sampling area was mainly under Scots pine, but some samples of fungi were collected in other forest types. 2. Material and methods

Sampling was generally done with three replicates collected within the sampling site which is about 5.5 ha. The stand of Scots pine (Pinus silvestris) is from 1890 with a mean height of 15.4 m and a mean diameter of stem of 29.0 cm measured 130 cm above ground (Mortensen, 1992). There are 141 m 3 of wood per hectare. The replicates were counted separately, in order to get estimates of the variation. Some of the fungi samples have been taken in similar areas outside the sampling area in order to get three replicates.

The deer were shot near the sampling area. Generally, samples were dried at 100°C, but most of the fungi only at 50°C. Some samples were ashed at 400°C or 500°C in order to concentrate the cesium. The dry weight was the weight obtained after drying at 50°C or 100°C. Soil samples were collected with a 10 x 10 cm square-formed stainless steel frame, which was driven down into the soil to get a 20-cm high column. This column was divided into three parts: the upper 5-cm fraction, the 5- to 10-cm fraction and the 10- to 20-cm fraction. Above the soil sample, the layer of litter and decomposing litter was sampled in a 25 x 25 cm area. This sampling was not sufficient to include all global fallout 137Cs, but the fraction in the soil below 20 cm was minor compared to that in soil above 20 cm. Vegetation was sampled by cutting approximately 1 cm above ground. Leaves and twigs were not separated. When large areas of homogenous vegetation were available, 1-m2 samples were taken, in order to estimate the uptake per unit area. This was done with Calluna vulgaris, Vac-

cinium vitis-idaea, Empetrum nigrum, Deschampsia flexuosa, Cladina portentosa and Hylocomium splendens (mixed with Pleurozium schreberi and Scleropodium purum). Other vegetation sampling was done without estimates of the area. Samples of Scots pine (Pinus silvestris) were taken from three different parts of the tree: year-shoots with needles, wood from the outer parts of the stem, formed after the Chernobyl accident in 1986 and wood from the inner parts of the stem formed before 1986. This was done in order to assess the amount of cesium in the tree compartment and to investigate the translocation inside the tree. The distinction between old and new wood was made by counting the annual rings. Leaves of birch (Betula pendula) were sampled from young trees standing as upgrowth between the pines, no wood samples were taken. Fungi were sampled as if they were to be eaten by man, that is, taken up and cleaned, not washed, and their bases were removed. Generally, a sample represents specimens from a single myeelia. Necks of Roe deer were sampled the day after they were shot, the meat was removed from the

M. Strandberg / Sci. Total Environ. 157 (1994) 125-132

bone, dried and measured for radiocesium and potassium. Samples were measured for 134Cs, 137Cs and 4°K using a Ge (Li) detector. Both 604- and 795-keV lines were used for determination of 134Cs and thereby a weighted mean of the 134Cs determination was obtained. Standards for determination of isotopes of cesium were made from solutions of cesium chloride. The correction for variation in density between samples was made after a model, presented by Lippert (1983), describing this problem for the Ge (Li) detector. The efficiencies for the detectors used for the measurements varied between 25 and 38%. samples that were expected to have only minor 134Cs activity were placed in the most efficient detectors. Samples and standards were measured in 200ml cylindrical plastic containers for a maximum of 24 h. If no value was obtained in this interval of time, no result is indicated in the tables. The statistical counting error on measurement of 134Cs varied between 1 and 10%, and if necessary the period of counting could be prolonged in order to obtain measurements with less than 10% counting error. Results were decay-corrected to date of sampling; the main part of the sampling was conducted during September and October 1991 (for exact dates, see Tables). 3. Results and discussion

All results are compiled in Table 1.

127

and 80% in the upper 5 cm of the soil profile. The distribution of fallout cesium in the soil profile shows that 3% is in the litter layer, 56% is in the upper 5 cm, 27% in the 5- to 10-cm layer and 14% in the 10- to 20-cm layer (Fig. 1). 3.2. Trees

The investigation of the distribution of cesium in Scots pine shows that concentrations are highest in the endshoots and decrease by a factor 30 to the new wood and by a factor three from the new wood to the old wood. For potassium, the corresponding factors are 10 and three. The picture is the same but the concentration fall is not so high for potassium from endshoots to new wood as it is for cesium (Fig. 2.). From the results in Table 1, it is seen that the concentration of cesium in endshoots of Scots pine is four times higher than it is in leaves of birch. The samples of pine and birch were taken very close to each other in a mixed stand. This indicates a higher uptake rate for Scots pine than for birch. A comparison of observed ratios (OR) for the plant samples are given in Table 2. 3.3. Grasses and shrubs

From observed ratios given in Table 2, it is obvious that grasses generally take up less cesium than shrubs, except Empetrum which is on the same level as the grasses. The observed ratios for Calluna and Vaccinium ~tis-idaea are, respectively seven and five times higher than the grasses. Observed ratios (soil-plant) for grasses and shrubs range between 0.008 and 0.068.

3.1. Soil and litter

When the original ratio 134Cs//137Cs in the Chernobyl debris is known, the Cs-contribution from Chernobyl can be calculated, because 134Cs did not occur in Danish soil before the Chernobyl accident. According to data from Aarkrog et al., (1988), the original ratio 134Cs/137Cs in the Chernobyl debris was 0.54. In the autumn of 1991, there was a total in soil and litter of 3135 Bq 137Cs/m2, 923 Bq 137Cs/m2 deriving from Chernobyl and 2212 Bq ~37Cs/m2 deriving from weapon testing (Table 1). Per hectare, this is 9 MBg and 22 MBq, respectively. The Chernobyl cesium is distributed with 20% in the litter layer

3.4. Mosses and lichens

OR 137Cs for mosses range from 0.033 to 0.037 mZ/kg. It is higher for the lichens where it ranges from 0.033 to 0.080 m2/kg (Table 2); OR 134Cs is approximately three times that of OR J37Cs. The measurement of the observed ratio for lichens should not be confused with the transfer from the soil, because lichens mainly take up Cs from the air, direct from rain, by dry deposition or via resuspension. This may be true for mosses as well. One should be careful interpreting observed ratios as a measure of the transfer from soil to plant.

M. Strandberg/ Sci. Total Ent~ron. 157 (1994) 125-132

128

The takeup route could be the explanation of the 134Cs//137Cs ratio observed in these species (Table 1). The cesium in the investigated moss and lichen samples is around 90% Chernobyl and only 10% fall-out; this also demonstrates that cesium is not taken up from the soil in mosses and lichens.

3.5. Fungi The total levels of radiocesium and potassium in fungi is shown in Fig. 3. It is seen that there is no obvious connection between the concentrations of potassium and cesium, e.g. Cantharellus cibarius has a high concentration of potassium, but a very low concentration of cesium. When

Table 1 Primary results on the occurrence of radiocesium in soil and biota from a Scots pine plantation (Tisvilde Hegn, Denmark) Species

Date

N

(day-month-year)

134Cs

137Cs

134Cs/137Cs

Bq kg- 1

Bq m - 2

Bq kg- 1

Bq m - 2

15.4(21) 109 (60)

156(59) 109 (60) 24(126) 2.6(104)

235 (49) 1989(32) 602(61) 309(105)

K (g kg- i )

Soil

Litter Soil 0-5 cm Soil 5-10 cm Soil 10-20 cm

0.071 (25) 0.038(39)

3.28 (29) 8.06 (74) 11.48(32) 12.14(11)

105.5 (62) 3.1 (18) 1.1 (29)

0.050 (8)

5.78 (10) 0.56 (30) 0.19 (21)

27,4 (18)

0.029 (39)

6.13 (33)

55.8 (42) 16.8 (25) 3.7 7.5 (27)

0.058 (17) 0.066 (15) 0.082 0.076 (5)

4.05 (24) 5.04 (8) 5.12 4.45 (18)

4.5 (52)

0.051 (43) 0.082 (23)

13.47(18) 8.13 (38)

50.4 (17)

0.082 (18) 0.088

4.53 (29) 5.82

93.6 (43)

0.090 (7) 0,090(2) 0.090

2.05 (27) 3.02 (23) 2.60

13 343 (7) 5241 (37) 892 (27) 1300 (15) 2205 (55) 212 (33) 1366 (36)

0.065 11 0.050 (8) 0.063 (16) 0.054 (19) 0.072 (6) 0.071 (10) 0.057 (12)

45.57 (2) 52.00 (13) 25.79 (4) 33.61(10) 30.00 (10) 47.46 (19) 39.60 (15)

49.25 66.65 627.2

0.076 0.059 0.050

12.60 8.24 10.98

11-10-1991 11-10-1991 11-10-1991 11-10-1991

3 3 3 3

10.0 (31) 4.75 (96)

04-10-1991 04-10-1991 04-10-1991

3 3 3

5.39 (63)

02-10-1991

3

0.85 (57)

23-09-1991 19-08-1991 25-09-1991 11-10-1991

3 2 1 3

13 (53) 9.5 (11) 4.6 1.8 (26)

3.4 (53) 1,1 (11) 0.3 0.6 (26)

213.9 (42) 147.7 (25) 56.8 23.7 (27)

06-09-1991 27-09-1991

3 3

1.8 (6) 3.9 (64)

0.38 (63)

34 (50) 44 (52)

Hylocomium splendens 17-09-1991 Polytrichum commune 18-09-1991

3 1

8.7 (34) 10.3

4.18 (34)

105 (17) 117

17-09-1991 04-10-1991 23-09-1991

4 3 1

17.6 (45) 22.5 (16) 9.3

8,40 (45)

196 (43) 250 (15) 103

18-09-1991 14-10-1991 22-10-1991 22-10-1991 23-09-1991 02-10-1991 29-10-1991

3 3 3 3 3 2 4

869 (15) 258 (30) 55 (27) 70 (4) 159 (55) < 14 (26) 76 (24)

10-10-1991 28-10-1991 28-10-1991

1 1 1

3.72 3.96 31.05

Trees

Pinus silvestris Year-shoot Wood after 1 9 8 6

Wood before 1986 Betula pendula Leaves Shrubs

Calluna culgaris Vaccinium vitis-idaea Vaccinium uliginosum Empetrum nigrum Grasses

Molinea caerulea Deschampsiaflexuosa Mosses

Lichens

Cladinaportentosa Hypogymniaphysodes Cetraria islandica Fungi

Rozites caperata Corr. albot~olaceus Suillus variegatus Boletus badius Lactarius r u f u s Cantharellus cibarius Canth. tubaeformis Roe deer

C. capreolus (female) C. capreolus (kid) C. capreolus (male)

Values in parenthesis are % S.D,

M. Strandberg / Sci. Total En~ron. 157 (1994) 125-132

129

Table 2 Observed ratios for plant samples Sample type

n

134Cs

137Cs

( B q / m 2)

Litter Soil 0-5 cm Soil 5-10 cm Soil 10-20 cm

3 3 3 3

n

15.4 69.0 0.0 0.0

Ratio

(Bq/m 2)

235 1989 602 309

1346s

137Cs

(Bq/kg dry wt.)

(Bq/kg dry wt.) 106 3.6 1.0

Pinus siivestris Year-shoot New wooda Old wood a Grasses

3 3 3

5.39 0.17 >0

M. caerulea D. flexuosa

3 3

1.81 3.90

3 3 2

Soil 134Cs %

0.071 0.038 0 0

Ratio

18.2 81.8 0 0 OR

134Cs

Soil

Chernobyl

%

%

7.5 63.4 19.2 9.9

75 40 0 0

137Cs

OR

137Cstota I

137Cs

Chernobyl 137Cs %

0.051 0.047 >0

0.064 0.002 0.000

0.034 0.001 0.0003

53 50 > (I

34.1 44.0

0.051 0.082

0.021 0.046

0.011 0.014

54 87

12.97 1.80 9.52

213.9 23.7 147.7

0.058 0.076 0.066

0.154 0.021 0.113

0.068 0.008 0,046

62 81 70

4 3 1

17.59 22.46 9.32

196.3 250.3 103.3

0.090 0.090 0.090

0.208 0.266 0.110

0,063 0.080 0.033

95 95 96

3 1

8.72 10.31

104.8 116.8

0.083 0,088

0,103 0.123

0.033 0.037

88 93

1 1 1

3.72 3.96 31.05

49.3 66.7 627.2

0.076 0.059 0.050

0.044 0.047 0,370

0.016 0.021 0.200

76 60 51

Shrubs

C. culgaris E. nigrum V. vitis-idaea Lichens

C. portentosa H. physodes C. islandica Mosses

H. splendens P. commune Roedeer Female Kid Male

aNew wood is formed after Chernobyl, old wood before.

calculating the OR for the fungi, the radiocesium content in the litter layer is included in the calculation because this layer is available to fungi (Table 1). The selection of fungi illustrates the high uptake that is known for this group. Table 3 gives observed ratios for the investigated fungi. The highest values are seen in Rozites caperata and in Cortinarius alboviolaceus. In the group of fungi, observed ratios range between 0.07-4.26 m2/kg. The lowest OR is seen in the Chanterelle, while the Trumpet Chanterelle has a considerable uptake with 0.4-0.5 m2/kg as mean OR. Values of OR as low as 0.002 mZ/kg have been observed in Lactarius vellereus and as high as 6.55

m2/kg in a species of Cortinarius (Strandberg, 1992). Most of the mycorrhizal fungi lie in the range from 0.2-1.0 m2/kg. In another investigation (Strandberg, 1992), the saprophytes ranged between 0.004 and 0.11 m2/kg. This indicates that the symbiotic ectomycorrhizal fungi generally have a higher uptake of cesium than the saprophytes. 3.6. Distribution of cesium-137 in Tisvilde Hegn The distribution of 137Cs in the Tisvilde Hegn ecosystem is described below (Table 4). Of course, the information gained from a 1-year study is not satisfying for a full and complete understanding and explanation of the ecosystem.

M. Strandberg / Sci. Total Ent#on. 157 (1994)125-132

130

Table 3 Observed Ratios (fungi/soil) for fungi in Tisvilde Hegn in the autumn of 1991 Species

137Cs (Bq/kg)

Rozites caperata Cortinarius albot~olaceus Lactariusrufus Cantharellus tubaeformis Boletus badius Suillus variegatus Cantharellus cibarius

13 343 5241 2205 1366 1300 892 212

ORarea I (m2/kg)

OR weight

4.26 1.67 0.70 0.44 0.42 0.29 0.07

122.4 48.1 20.2 12.5 11.9 8.2 1.9

Deposition ( B q / m 2)

Concentration
3135

109

24.8 (in litter) 59.7 (soil 0-5 cm) 100 (soil 5-10 cm) 100 (soil 10-20 cm) 71 (total)

has no dimensions. It is calculated from the ratio between concentration of The ORweight 137Cs concentration of in the upper 5 cm of the soil column.

From Table 1, it is calculated that there is 31 M B q / h a t37Cs in the soil compartment. In the Scots pine trees, there is 1.1 MBq 137Cs/ha, calculated from Table 1 and Mortensen, Danborg and Heding (pers. commun.). By combining information from Table 1 with estimates of biomass/ha of the different parts of the vegetation, it can be calculated that grasses, shrubs, moss and lichens contribute around 0.4 MBq t37Cs/ha. In the fungi, there is around 2500 Bq t37Cs/ha, calculated from Table 1, Knudsen (1973), Knudsen and Petersen (personal communications). In the animal compartment, there is 35 Bq 137Cs/ha, calculated from the median value from Table 1, and assuming approximately 0.2 roe deer/ha (Mortensen, personal communication). 1009080 70 6050 4030 20 100

Litter

>5

>0

~.~_C!!r~obyl..R

Fa!lout

~

>10<20 Potassium

]

Fig. 1. The distribution of 137Cs and K in the soil layers in Tisvilde Hegn in the autumn of 1991.

1O0 ff

Fallout (%)

137Cs in the fruitbody and the

__

- - q

90 80 70 60 50 40 30 20 10 0

'

Endshoots [[~0s134

New wood' mOs137

Old wood [[~Potassium

' J

Fig. 2. The distribution of concentrations of 134Cs, 137Csand K in different parts of Scots pine in the autumn of 1991.

3. 7. Transfer to man from Danish forest ecosystems In Denmark, the annual yield of roe deer is around 70000 animals. If the three roe deer presented in Table 1 are representative of Danish roe deer, it can be calculated that between 8 and 116 MBq were transferred to man from roe deer in 1991. The real figure is likely to be much closer to 8 MBq than 116 MBq, because of the supposedly higher radioecological sensitivity of Tisvilde Hegn compared with the majority of Danish forests. A level of 8 MBq represents 2% of the transfer of 137Csfrom diet to man in Denmark in 1991 (Aarkrog et al., 1991). The transfer from other products representing natural and seminatural ecosystems in Denmark is not very well investigated. Aarkrog (personal

M. Strandberg / Sci. Total Environ. 157 (1994) 125-132 Table 4 The distribution of laVCs in the soil, vegetation and roe deer compartment in Tisvilde Hegn in the autumn of 1991 Compartment

Bq/ha

% of total

Roe deer Scots pine Understorey vegetation Fungi fruitbodies Soil (including soil biota)

35 1 100 000 400 000

0.0004 3.4 1.2

2500

0.008

31000 000

95.4

communication) estimates that around 10% of the total transfer to man in Denmark in 1991 can be ascribed to consumption of products from seminatural and natural ecosystems. Limnic fish and game (roe deer) are believed to be the two most important single sources of radiocesium to the diet from natural ecosystems (Aarkrog, personal communication). 4. Conclusion

In 1991, both Chernobyl and fallout soil cesium were still concentrated in the upper soil layers with 95% of the total cesium in the soil and only 5% in above ground parts. Chernobyl cesium is entirely in the upper 5 cm. More than 50% of the fallout cesium is still in the upper 5 cm. In the upper 10 cm, 86% of the fallout cesium is found

C.cib ~ S.var. f

]

J

B.bad. ~ \ ~

'--

L ruf C.alb.

10

100

I~

cSi34

1000 10000 Bq or gK/kg dry weight ~ 1 (~s137

~

100000

Potassium i

Fig. 3. Levels of radiocesium and potassium in the fungi investigated.

131

and only 14% has penetrated to lower levels. The observation of reduced penetration of cesium in sandy coniferous forest soils is in agreement with observations made in a similar forest ecosystem by R6mmelt et al. (1990). The observed low penetration in the soil layers is of course due to some kind of fixation or binding to soil compounds. Since clay is almost completely absent, it is reasonable to believe that humus compounds are responsible for the binding of cesium. The relative high bio-availability indicates that this binding is reversible to some degree and thereby cesium becomes available for uptake by roots, rhizoids and mycelia of plants, mosses and fungi. Andolina and Guillitte (1990) found that lignin was important in the retention of cesium in organic forest soils, and reasoned /that this explained why fungi had a higher uptake than autotrophic plants. The observation of an OR of L~4Cs roughly twice that of 137Cs indicates a higher availability of Chernobyl cesium than of fallout. A likely explanation is given by the long timescale for fixation and transfer processes of Cs radionuclides in the order of several years. Differences in cesium and potassium distribution in the soil profile explain some of the reasons for the greater uptake of cesium in the forest ecosystem, but the whole answer to this question requires further, more detailed studies of cesium and potassium distribution in the soil profile as well as studies of the feeding depths of the plant species investigated. In plants, mosses and lichens, observed ratios are rarely higher than 0.1 m2/kg - - while in the fungi, OR is often higher than 1.0 m2/kg and up to 4.26 m2/kg in Rozites caperatus and for an individual sample of Cortinarius anomalus, as high as 6.5 m2/kg. Most often OR is between 0.2 and 1.0 m2/kg in symbiontic basidiomycetes. Why do we observe these differences? Do fungi have some specialised physiological capability of reversing cesium fixation to soil compounds like clay or other than clay? Or is the cesium situated in the mycelia of the fungi, as proposed by some authors, for example, R6mmelt et al. (1990)? The high uptake of potassium seen in many

132

M. Strandberg / Sci. Total Environ. 157 (1994)125-132

fungi of course explains part of the higher levels of cesium seen, but at the same time, it raise new questions. How do some fungi with a high potassium uptake avoid taking up cesium e.g. Agaricus, Russula xerampelina (Rantavaara, 1987), and Lactarius vellereus. The relatively high levels of 137Cs found in roe deer are interesting and are on such a high level that, in future, it ought to be taken into account when assessing total doses. The explanation for the cesium transfer from soil through food to roe deer needs more investigation to be answered. From observations of what is undoubtedly roe deer grazing of fungi in Danish forests, it is very likely that fungi play a role in the higher levels observed in roe deer in the autumn (Johansson et al., 1990). References Andolina, J. and O. Guillitte, 1990. Radiocesium availability and retention sites in forest soils. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of Radionuclides in Natural and Semi-natural Environments. Elsevier Applied Science, Barking, UK. Aarkrog, A., Personal communication on transfer from natural ecosystems to man in Denmark. A. Aarkrog, Ris¢~ National Laboratories, Roskilde, Denmark. Aarkrog, A., L. B0tter-Jensen, C. Qing Jiang, H. Dahlgaaid, H. Hansen, E. Holm, B. Lauridsen, S.P. Nielson and J. S0egaard-Hansen, 1988. Environmental radioactivity in Denmark in 1986. RISO-R-549. Aarkrog, A., L. BOtter-Jensen, C. Qing Jiang, H. Dahlgaard, H. Hansen, E. Holm, B. Lauridsen, S.P. Nielsen and J. Soegaard-Hansen, 1991. Environmental Radioactivity in Denmark in 1988 and 1989. Rise-R-570, Riso National Laboratories, Roskilde, Denmark. Bergman, R., K. Danell, A. Ericsson, H. Grip, L. Johansson, P. Nelin and T. NylEn, 1988. Upptag, omlagring och transport av radioaktiva nuklider inom ett barrskogsekosystem. FOA rapport E 40040, September 1988, ISSN 0281-9945. Bergman, R., T. NylEn, T. Palo and K. Lidstr6m, 1991. The behaviour of Radioactive Caesium in a Boreal Forest Ecosystem. FOA report A 40066-4.3, October 1991, ISSN 0281-0220. Danborg, F., 1992. Personal communication on density of dried Scots pine. Frede Danborg, Royal University of Veterinary and Agriculture of Copenhagen, Copenhagen, Denmark. Davydchuk, V., 1992. Series of vegetation and soiltype maps of the Chernobyl evacuation area made for ECP-4 participants. Laboratory of Landscape-Ecological problems of

Chernobyl, Institute of Geography, Academy of Sciences of Ukraine, Ukraine. Heding, N., 1992. Personal communication on weight distribution of above ground parts of Scots pine. Niels Heding, Danish Forest and Landscape Research Institute, Denmark. Johansson, K.J., R. Bergstr6m, S. von Bothmer and G. Kari6n, 1990. Radiocaesium in wildlife of a forest ecosystem in central Sweden. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of Radionuclides in Natural and Seminatural Environments. Elsevier Applied Science, Barking, UK. Karl6n, G., K.J. Johansson and R. Bergstr6m, 1991. Seasonal variation in concentration and daily intake of Cs-137 in Swedish roe deer. J. Environ. Radioact., 14: 91-103. Knudsen, H., 1973. Studier over faenologi og ¢kologi hos storsvampe i fyrreskov. Kg~benhavns Universitet (unpublished). Knudsen, H., 1992. Personal communication on fungi densities in Tisvilde Hegn. Henning Knudsen, Botanical Museum, Copenhagen, Denmark. Lippert, J., 1983. Detector-efficiency calculation based on point source measurement. J. Appl. Radiat. Isot., 34: 1097-1103. Mortensen, K., 1992. Personal communication on tree and animal densities in Tisvilde. Skovfoged Kaj Mortensen, Tisvilde Hegn, Denmark. Nielsen, B. and Strandberg, M., 1989. Caesium i graesningsf~dekaeden. Specialerapport fra Forskningscenter Ris¢~ (MIL-ECO), Denmark. Petersen, P.M., 1992. Personal communication on fungal densities in forests generally. Peter Milan Petersen, Institute of Ecological Botany, University of Copenhagen, Denmark. Rantavaara, A., 1987. Radioactivity of Vegetables and Mushrooms in Finland after the Chernobyl Accident in 1986. (Supplement 4 to annual report STUK-A55) STUKA59, Helsinki, Finland. Roos, N., M.N. Jensen and N.L. Jensen, 1990. Radioaktivt cesium i svampe. Rapport, RUC 90, Roskilde Universitets Center, Roskilde, Denmark. R/Smmelt, R., L. Hiersche, G. Schaller and E. Wirth, 1990. Influence of soil fungi (basidiomycetes) on the migration of Cs-134 + Cs-137 and Sr-90 in coniferous forest soils. In: G. Desmet, P. Nassimbeni and M. Belli (Eds.), Transfer of Radionuclides in Natural and Semi-natural Environments. Elsevier Applied Science, Barking, UK. Strandberg, M., 1992. Investigation of radiocesium in fungi in a Danish Scots pine forest ecosystem. Presentation at the 6th Nordic Radioecological Seminar, Thorshavn, Faroe Islands, 14-18 June 1992. Stukin, E.D., 1991. Characteristics of primary and secondary caesium-radionuclide contamination of the countryside following the Chernobyl NPP accident. In: Proceedings of Seminar on Comparative Assessment of the Environmental Impact of Radionuclides Released during Three Major Nuclear Accidents: Kyshtym, Windscale, Chernobyl. CECreport EUR 13574, Luxembourg 1-5 October 1990.