Impact of forest disturbance on the pollen influx in lake sediments during the last century

Review of Palaeobotany and Palynology 111 (2000) 19–29 www.elsevier.nl/locate/revpalbo

Impact of forest disturbance on the pollen influx in lake sediments during the last century T. Koff *, J.-M. Punning, M. Kangur Institute of Ecology, Tallinn University of Educational Sciences, Kevade 2, Tallinn 10137, Estonia Received 13 July 1999; accepted for publication 2 February 2000

Abstract The pollen accumulation rates of four lakes in different regions of Estonia were estimated in order to study the relationship between pollen influx and the character and intensity of disturbances in the pollen catchment area. The pollen influx data obtained are in accordance with model calculations on the size of the pollen source areas. The influx of arboreal pollen and that of the dominant taxa (mainly Pinus) in the lakes investigated shows that, in the case of small lakes (area 3–6 ha) in a forested landscapes, the bulk of the pollen originates from an area within 100– 200 m around the lake. The distribution patterns of influx from two lakes situated close to each other but at different distances from forest fires show that past disturbances can be reliably detected when the disturbance occurred in the immediate vicinity of the lake and at least 25% of the local pollen source area was involved. In the case of a large lake (137 ha) only fires embracing thousands of hectares can be detected in the pollen diagrams. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Estonia; forest disturbances; pollen influx; recent lake sediments

1. Introduction In biostratigraphical studies pollen analysis is a widely applied method for describing past vegetation changes in terms of increases or declines in percentage values of certain pollen types. During the last decades attention has been focused on obtaining precise information on absolute fluctuations of pollen abundance with time, that is, pollen influx (Pennington and Bonny, 1970; Tolonen, 1978; Huttunen, 1980). Use of this method allows for a more precise history of individual tree taxa (Hicks, 1992), including density of the forest (Odgaard, 1994) and range of tree limits ( Van der * Corresponding author. Tel.: +372-2-451-634; fax: +372-2-453-748. E-mail address: [email protected] ( T. Koff )

Knaap and Van Leeuwen, 1998). For this the density of each pollen type within the sediment and the rate of sedimentation expressed as the number of pollen grains settling on a given area of surface sediment (cm2) in a given time (yr) is needed. Hicks (1994) found that pollen influx is more informative in the cases when the changes in the vegetation are studied in unforested landscapes where also the pollen productivity of the local vegetation is rather low or when considerable changes had occurred in forest edge dynamics. The influx reflects changes in species abundance or density, only one must be sure that the influx calculations themselves are reliable and not affected by changes in the sedimentation rate. The main problem in the use of pollen data in palaeoecological and geographical studies is the establishment of the pollen source area. The pollen

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source area of a site is a composite concept and, in reality, each taxon in the diagram will have its own pollen source area according to its pollen production and dispersal abilities, depending at the same time on the size of the site. The nature of pollen representation has been subject of much discussion over several decades ( Tauber, 1965; Andersen, 1970; Janssen, 1973; Kabailene, 1979) which has provided material for different models (Jacobson and Bradshaw, 1981; Bradshaw and Webb, 1985; Prentice, 1985; Jackson 1990; Sugita, 1994, 1998) for establishing the pollen source area. There have been numerous theoretical studies but comparatively little empirical data with which to evaluate the models. Many models have been calculated in regions with a homogeneous forest composition in rather densely forested areas. Estonia provides good possibilities to test these results in a mosaic of vegetation types. The need for such studies has increased because it is not enough just to determine the broad regional trends in vegetation development. Changes at a local level are more important for predicting the resistance and resilience of the ecosystem under different loads and therefore it is important to estimate the limits of pollen analysis in investigations of that kind. Natural and human disturbances are important factors affecting the development of ecosystems. Fossil pollen can potentially provide crucial empirical data over thousands of years and the study of the history of human impact based on pollen data is important. However, to interpret fossil pollen records connected with a disturbance we have to know the effect of the lake size, its catchment area and the size of the disturbed patch on the pollen representation of the disturbance.

The main aims of the current research were: (1) to estimate the relationship between pollen influx and the size of the lake; and (2) to determine the potential limits of detecting the disturbances in the vegetation on the catchment on the basis of pollen influx values. For these purposes four lakes in different parts of Estonia with different sizes and disturbance regimes but with quite similar catchments and feeding types were selected.

2. Study sites All the lakes selected ( Table 1) are dystrophic or semidystrophic closed lakes situated in forested landscapes ( Fig. 1) where, according to historical data, only small disturbances (with the exception of Lake Ta¨navja¨rv) have occurred influencing the lake and its surroundings. In the case of Lake Matsima¨e the disturbances include some agricultural activity and clear-cutting in the forest when a gravel quarry was established in the vicinity of ˜ dre clear-cutting of the lake. In the case of Lake O forest occurred at various distances from the lake and in the case of lakes Ta¨navja¨rv and Mustja¨rv forest fires were present. The studied sediment sequences were formed during the 20th Century in a rather even accumulation. Lithologically all the studied sediments were homogeneous brownish gyttja. 2.1. Lake Ta¨navja¨rv Lake Ta¨navja¨rv (59°10∞N, 23°48∞E) (Fig. 1b; Table 1) is situated in western Estonia. The lake

Table 1 Characterisation of the study sites Characteristic

˜ dre O

Matsima¨e

Ta¨navja¨rv

Mustja¨rv

Elevation (m a.s.l.) Length (m) Width (m) Area (ha) Maximum depth (m)

95 260 140 3.0 9.1

77 340 230 5.5 8.1

19 2300 830 136.9 2.5

19 300 270 4.8 2.7

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Fig. 1. Location of study sites (a), main vegetation units and disturbances around the studied lakes: (b) lakes Ta¨navja¨rv and Mustja¨rv; ˜ dre. In (b) the areas disturbed by forest fires in 1952 (—), 1980, 1992 and 1997 ( · · · ) are shown. (c) Lake Matsima¨e; (d) Lake O

is elongated in the northeastern–southwestern direction. The lake is semidystrophic and precipitation-fed with no inlets or noteworthy outlets. The eastern and western shores of the lake are sandy. The lake is surrounded by a big mire system and the main vegetation type here is the pine forest. Around the lake sandy soils, poor in nutrients predominate presenting favourable conditions only for Pinus sylvestris. Betula spp. also grows on the shores of the lake. The nearest settled areas and fields are located ca. 7–8 km to the NW and SW from the lake. Using air reconnais-

sance and forestry maps we determined that the surroundings of the lake have suffered from several forest fires during the last century. The last fire was in 1997 destroying the forest near the northern shore of the lake. In the same area (within a radius of 10 km) fires have also occurred due to the very flammable and dry pine forest; in 1992 ca. 460 ha was burnt and in 1980 ca. 100 ha. The most extensive fire occurred in 1952 when an area of ca. 2000 ha of the forest was burnt. A fire can last for months because of the peat cover.

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2.2. Lake Mustja¨rv Lake Mustja¨rv (59°10∞N, 23°48∞E) (Fig. 1b; Table 1) is situated only 800 m to the south of Lake Ta¨navja¨rv and therefore the surrounding vegetation of both lakes is rather similar. The lake has swampy shores, surrounded with pine trees and birches towards the north and west with an open mire landscape towards the east. There is no historical evidence of any fire in the immediate vicinity of the lake. The distance to the biggest forest fire area close to Lake Ta¨navja¨rv is 2.5 km to the north.

the southeastern and northwestern swampy shores birch grows. The main economic activity in the area is forestry and thus the main disturbance affecting the vegetation around the lake could be clear-cutting of the forest. On forestry maps it was possible to distinguish that a mature pine forest had stood around the lake before 1953. On the map from 1963 this area is marked as a young pine forest, indicating that in the meantime large areas of the forest had been cut down. After the establishment of a National Park in 1979 clearcutting ceased. The nearest agriculturally used area is situated 1–2 km to the south from the lake.

2.3. Lake Matsima¨e 3. Methods Lake Matsima¨e (59°04∞N, 25°31∞E ) ( Fig. 1c; Table 1) is situated in central Estonia. The lake is almost circular in shape. Sediments are evenly distributed in its deeper central part and close to the eastern shore. The lake is surrounded by an open stretch of bog where Pinus sylvestris grows. Along the western shore is an esker covered with Betula pendula, Alnus glutinosa and Picea abies. Within a distance of ca. 10 km from the lake large bogs and swamps unsuitable for agricultural activities predominate. Favourable places for farming are found only on some eskers on the limnoglacial plain. On one esker, close to the western shore of the lake, a single farm with a garden and a fallow field is located. Older maps give evidence of fields ca. 20 ha in size at the end of the 19th Century. For 5 yr from 1961 onward, the esker was excavated as a gravel quarry (covering 8.2 ha). Use of land for agricultural purposes ceased and later P. sylvestris was planted on the quarry. At the end of the 1950s a swimming pool was built in the lake and since then the lake has been intensively used for recreational purposes. ˜ dre 2.4. Lake O ˜ dre (57°45∞N, 26°27∞E) (Fig. 1d; Lake O Table 1) is situated in southern Estonia, in the northern part of the Karula National Park. It is a typical woodland lake, shaped by the mosaic hills (105–110 m a.s.l.) of the Karula Upland. The main vegetation type around the lake is pine forest; on

Cores (50–60 cm) from the upper unconsolidated surface of sediment were taken using a modified Livingstone–Vallentyne piston corer (diameter 7 cm) in the deepest part of the lake. The lithology of the sediment was described directly at the study site. Sampling was continuous, with the sampled layer 1 cm thick. Samples were kept in the refrigerator prior to analysis. Samples were dried at 105°C. From the same sample different analyses were performed for the upper 25 cm part of the cores, which covered approximately the last 90–100 years. For pollen analysis a 50 mg dried sample from the sediment core was boiled in 10% KOH and treated according to standard acetolysis (Moore and Webb, 1978). Three to six tablets with a known content of Lycopodium spores were added to each sample at the beginning of laboratory treatment to calculate the pollen concentration (Stockmarr, 1971). In general, at least 500 arboreal pollen (AP) grains were identified under the microscope. From all the pollen slides also charcoal pieces 100 mm2 were counted, and their concentration and total surface were calculated. For age estimation various methods were used. The core from Lake Matsima¨e was dated in the most detailed way. Here we applied the 210Pb method and as the reference levels the layers with maximum 137Cs activity (ref. yr=1986) were separated. The appearance of spherical fly-ash particles (SFAP) was correlated with the year 1945.

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Considering the history of high temperature combustion of fossil fuels in Europe, the sediment layers with a significant increase in the concentration of SFAP must have accumulated at the end of the 1940s. The age of individual layers and the deposition rate were calculated on the basis of the mean annual sedimentation rate between their reference layer and the sediment surface and using the measured dry matter content in individual layers (Punning and Alliksaar, 2000). The cores from the other lakes were dated using SFAP distribution and in the case of Lakes Ta¨navja¨rv and Mustja¨rv charcoal maximums were also used. To reconstruct the impact history we used historical sources such as old forestry maps, land-use maps, as well as oral information. For the calculation of the pollen influx I (pollen grains cm−2 yr−1) the following equation was used: I=

P×LYC×M lyc×m×S×A

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where P number of pollen grains counted from the sample, LYC total number of added Lycopodium spores, lyc number of Lycopodium spores counted, M dry weight of the whole sample (g), m dry weight of the analysed sample (g), S surface area of the sample (cm−2) and A time of the formation of the layer (yr).

4. Results The content of dry matter in the studied cores ˜ dre where it ( Fig. 2) was the lowest in Lake O varied from 0.01 g cm−3 on the surface up to 0.03 g cm−3 at a depth of 18 cm and then increased rapidly. The dry matter content records from lakes Matsima¨e, Ta¨navja¨rv and Mustja¨rv are quite similar (Fig. 2). No increase in the dry matter content was observed in Lake Matsima¨e sediment due to the erosional processes caused by the exploitation of the quarry. In Lake Ta¨navja¨rv the dry matter

˜ dre; —, Lake Matsima¨e; Fig. 2. Dry matter content (g cm−3) (a) and age–depth curve of the studied lake sediments (b). +, Lake O - - -, Lake Ta¨navja¨rv; $, Lake Mustja¨rv.

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˜ dre; —, Lake Matsima¨e; - - -, Lake Ta¨navja¨rv; Fig. 3. Pollen influx (grains cm−2 yr−1) curves in the studied lake sediments: +, Lake O $, Lake Mustja¨rv.

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content has a peak at a depth of 9 cm, which is most probably caused by an increase in minerogenic matter due to wind erosion from nearby sandy dunes after a big forest fire. The top 5 cm of the lake deposits in all lakes is a loose flocculent layer. According to the age–depth curve (Fig. 2) the mean accumulation rate in different lakes varies from 1.7 (in Lake Matsima¨e) to 2.7 mm yr−1 (in Lake Mustja¨rv). The pollen influx values of selected pollen types for the studied lakes are given from the beginning of the 20th Century until the present and are presented in Fig. 3 and Table 2. The average AP influx values as well as the influxes of individual pollen types in the studied lakes differ greatly. The average of AP influxes was the lowest in Lake ˜ dre — 7200 pollen grains cm−2 yr−1 ( Table 2). O The curve is almost vertical (and therefore most stable) up to the 1970s (Fig. 3) followed by a slight trend towards an increase. In Lake Matsima¨e the average AP influx values are 10 700 pollen

grains cm−2 yr−1 over the last century with a short period of decrease in the 1960–1970s when the minimum values fell to 5700 pollen grains cm−2 yr−1 ( Table 2). In Lake Mustja¨rv the AP influx has on average greater values than those ˜ dre and Matsima¨e with a tendency in lakes O towards decrease in the 1960s. The greatest AP values and their variation were established in the Lake Ta¨navja¨rv core. Here a sharp decrease in the influx started around the 1930s (Fig. 3) and this trend continued practically till the 1950s when the influx was similar to that of Lake Mustja¨rv. Since the 1970s the Lake Ta¨navja¨rv core shows sharp fluctuations until present day. Pinus is the dominant type in the pollen spectra in all the studied lakes, comprising on average ca. 44% of the AP ( Table 2). The maximum relative values of the Pinus content within the lakes investigated are rather even. However, the minimum percentage content shows some differences between the sites. The relative content of Pinus (as well as

Table 2 The minimum, maximum and average values of the pollen influx (pollen grains cm−2 yr−1) and percentages of main pollen types in the total AP in the analysed sediment sequences of different lakes ˜ dre O Influx

Matsima¨e %

Influx

Mustja¨rv %

Influx

Ta¨navja¨rv %

Influx

%

AP Minimum Average Maximum

3800 7200 12900

Pinus Minimum Average Maximum

900 3300 6000

19 45 64

2300 4400 6600

20 44 68

6800 11400 16000

34 45 61

1200 20300 79500

14 42 58

Betula Minimum Average Maximum

1400 2500 3900

26 35 64

1400 3200 5200

18 39 74

3700 7500 13600

20 31 39

5900 15800 50900

17 33 72

Alnus Minimum Average Maximum

400 1200 3000

6 15 25

200 2300 3300

2 19 27

1800 3700 7700

11 16 24

700 8200 32900

9 17 27

Picea Minimum Average Maximum

60 300 900

1 4 10

300 700 1200

2 7 20

600 1400 1900

3 6 8

200 2700 8800

1 6 11

5700 10700 14000

18700 24400 35700

8500 47800 176600

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of other pollen types) is the most stable in the Lake Mustja¨rv core. In the case of lakes Matsima¨e, ˜ dre and Mustja¨rv the fluctuations in the absolute O (influx) as well in the relative (percentages) values of different pollen types have significantly lower frequencies than in the Lake Ta¨navja¨rve core. The relative average content of Betula and Alnus pollen in the total AP was 34 and 17%, respectively. Betula and Alnus grow mainly close to the shores of the lakes and therefore their impact on the influx is quite direct and depends notably on the variation of the humidity regime, fluctuation of the water level and disturbances along the lake shore. This statement is verified by more essential variations in the content of Betula and Alnus pollen in the diagrams for lakes Matsima¨e and Ta¨navja¨rv. In these lakes the influxes of both these pollen types started to decrease around the 1940s and had a continuous trend till present day with maximum values in the 1950s (Fig. 3). The influxes of Picea pollen are much lower than those of Betula and Alnus in all the studied lakes. The average proportion of Picea in the AP is ca. 6% ( Table 2). In Lake Matsima¨e a sharp decrease can be observed around the 1970s (Fig. 3). In the case of Lake Ta¨navja¨rv a strong trend can be observed from the 1940s ( Table 2). ˜ dre the Picea influx record shows Only in Lake O a continuous increasing trend from the beginning of the 20th Century to the present time ( Fig. 3). The differences in the influxes of the pollen of broad-leaved trees (Quercus, Tilia, Ulmus) in the lakes investigated are great ( Fig. 3). The pollen of Tilia and Quercus is transported from a distance of at least 7–8 km where a few trees are growing near old farmhouses. Due to the small number of pollen of these taxa in the counted samples the statistical uncertainty is higher and thus we should be very careful in interpreting these influx values. The same tendencies of high variation were observed also in the influxes of pollen considered as indicators of human impact. However, note that for example Cerealia-type pollen is present in all the studied cores irrespective of whether the nearest fields are located 1–2 ( lakes Matsima¨e and ˜ dre) or 7–8 km ( lakes Ta¨navja¨rv and Mustja¨rv) O from the lake.

5. Discussion There are several problematical points in the determination and interpretation of the pollen influx values. Dating accuracy is the most crucial issue for the calculation of pollen influx values. Also the geomorphological location of a lake in a landscape causes differences in pollen distribution and redeposition. Openness of the landscape can contribute to an increase in the share of longdistance transported pollen. Also important is the age structure of the forest and its closeness to the water table. In addition, differences in the pollen productivity of different species and the fluctuation of the most productive flowering periods connected with climatic variations should be considered. The predominant local pollen source is the vegetation in an area within some 200 m around the lake, as most authors have demonstrated (Jackson and Wong, 1992; Sugita, 1994; Punning and Koff, 1997). Van der Knaap and Van Leeuwen (1998) observed that pollen of trees growing within 0.3 km from the shore is several times more strongly over-represented in pollen spectra than that of trees growing outside this range. As the lake area (S ) increases, also the area coverage of l the vegetation band (S ) around the lake will b increase, but its area coverage relative to the lake area will decrease (when expressed as a percentage of this) (Fig. 4). Thus, the proportion of the influx of local pollen (from the source area S ) into the b lake should fall as the lake area increases. At the

Fig. 4. Changes in pollen source areas in dependence of the lake size. If we presume that the dominating local pollen source area (S ) consists of ca. 200–300 m around the lake and/or opening b area (S ), the proportion of this local pollen will decrease with l increasing of S . Thus, the proportion of local pollen influx into l the lake should fall as the lake area increases.

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same time we should consider not only the size of the lake but also the size of the ‘opening in the forest’. A lake surrounded by a treeless mire will act like a bigger lake in the sense of Jacobson and Bradshaw (1981) with the mire area effectively increasing the ‘lake’ area S . In catchment terms it l is the distance from the sampling point to the forest edge that is significant. The ratio of the pollen source area and deposition area obtained by a geometrical approach should be the measure of the proportion of local pollen in the total influx. This approach should help specify the approximate disturbed area to understand signals in the influx curves. Using the obtained influx values it is possible to calculate the approximate amount of pollen produced by ˜ dre the vegetation in S . The ratio S /S for Lake O b b l is 8.3, for Lake Mustja¨rv 5.9, Lake Matsima¨e 5.5 and Lake Ta¨navja¨rv 0.7. Naturally, such assumptions are correct only if the distribution of pollen deposition is even all over the lake surface and the width of the S ‘zone’ is always some 200 m. As b our earlier sediment trap experiment on Lake Matsima¨e showed, in real situations the pollen concentrations near the shores are higher than those in the central part of a lake ( Koff, 1998). When comparing the sedimentation rate of dry matter and pollen grains calculated from sediment traps with those of sediment cores, the effect of focusing, that is, accumulation into a smaller deposition zone than the lake area must be taken into account. Hurley and Armstrong (1991) showed that the ratio of the surface area to that of the sediment depositional area in lakes similar to such lakes as lakes Matsima¨e, Mustja¨rv and ˜ dre is 0.6–0.7. In the case of Lake Ta¨navja¨rv, O where post-depositional migration of sediments is of great importance (Saarse et al., 1989), the overall focusing correction factor might be smaller. The shoreline coefficient, K, which reflects the shape of the lake, must also be taken into account. K is the ratio of the shoreline length to the circumference of the circle with an area equal to ˜ dre, which that of the lake. Lakes Matsima¨e and O are almost circular in shape (K=1.05) compared to a K value of 1.23 for Lake Mustja¨rv. The elongated shape of Lake Ta¨navja¨rv increases its K value to 1.38. The higher the K value, the greater

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is the impact of the edge effect on pollen influx. Because of the edge effect the greatest amount of pollen is deposited directly at the edge of the forest. If the forest reaches the shores of a lake, pollen may be deposited also directly into the lake. If, however, the forest edge moves for some reason further from the shoreline, this deforested area will be seemingly added to the area of the water table. Though the approach shown above is rather approximate, our calculations demonstrated that in the case of the small lakes — lakes Matsima¨e, ˜ dre (surface area 3–6 ha) — the Mustja¨rv and O influx values were mainly determined by the vegetation in their immediate vicinity and only disturbances in that area could be detected. The Lake Matsima¨e records show clearly a marked decrease in the influx of Betula, Alnus and Picea around the 1960s (Fig. 3). The esker near Lake Matsima¨e was and is also nowadays a suitable habitat for Picea. The establishment of the gravel quarry led to the clear-cutting of the forest on the esker and consequently to a decrease in the local pollen source area, which is reflected, for example, in a 6–8-fold decline of the Picea, Betula and Alnus pollen influxes. As Pinus is growing on ca. 75% of the area of the bog surrounding Lake Matsima¨e the influence of quarrying on the western shore of the lake was not as important on its pollen influx. The close dependence of the influx data on disturbances in the vegetation and changes in the quantity and density of trees growing around the lake is vividly documented also in the influx ˜ dre. Here the increase in the records of Lake O total AP and in the pollen of single types from the 1960s onwards reflects a halt in clear-cutting around the lake and the growing number and density of trees. Correlation of the decrease in the influx of pollen from Betula and Alnus (growing mainly on the shores of the lakes) with those of the total AP and Pinus in the case of Lake Ta¨navja¨rv indicates that some part of the pollen of these taxa originates from a local (some hundred metres from the lake) source. Comparison of the influx values with the area of Lake Ta¨navja¨rv showed, that in the case of large lakes a major part of the pollen influx originates from outside the 200 m vegetation band around the lake. This

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suggests that a disturbance covering a much larger area ought to be detectable. Changes in the influx values of different pollen types are the most remarkable in the case of Lake Ta¨navja¨rv. As the time of forest fires and their extent are well documented [Fig. 1(b)], it is possible to draw some conclusions about the spatial extent of a detectable disturbance around the lake. In detecting disturbances Sugita et al. (1997) regarded a value of 10% change in the pollen loading or percentage after the disturbance as the threshold for detectable change. Proceeding from this value they concluded that disturbances of any size only 100 m away from a lake of 3 ha in area do not create detectable changes in the pollen loading of any of the taxa. Our data show that a disturbance patch covering ca. 30% of the area within a 100 m wide band around a lake of ca 6 ha (Lake Matsima¨e) in area is detectable in pollen influx. When a disturbance occurred outside ˜ dre, Lake Mustja¨rv) the this distance (Lake O changes in the influxes could not be determined unambiguously. In the case of a sufficiently large lake, a detectable disturbance must embrace a much larger area, depending on the distance from the lake. When very large disturbances (forest fires affecting up to 2000 ha) occurred in the vicinity of Lake Ta¨navja¨rv, the decline in the pollen loading was up to a factor of 10. According to Sugita (1998), such a disturbance will cause less than a 10% decline in the pollen loading when it occurs only 100 m or a few hundred meters away from the lake. At the same time there was no detectable impact of these fires on the influx into Lake Mustja¨rv situated only ca. 800 m from Lake Ta¨navja¨rv. It is clear that in the case of a small lake the main pollen source area is situated in its immediate vicinity and a disturbance outside this belt, even when it embraces a large area, does not show a significant record in the pollen spectra. This example shows that in the case of small lakes the detection of a disturbance is possible only, if it occurred in the vicinity of the lake and covered a sizeable part of the pollen source area. A disturbance that increases the openness of the landscape around the lake creates more possibilities for an increase in ‘long-transported’ pollen. This has been demonstrated in some earlier studies

( Koff et al., 1998). The pollen of Quercus, Tilia and Ulmus in the studied cores originates fully from outside the immediate vicinity of the lakes. These species can be found around farms some 7– 8 km from all the studied lakes. Thus it seems that there is some background value of broad-leaved tree pollen in the atmosphere and fluctuations in the pollen spectra from the upper part of sediment cores reflect more statistical uncertainties than those in the forest composition. Direct indicators of long-distance transportation of pollen are also Cerealia-type pollen, which was found in all the studied sites. This is clear evidence of the presence of long-transported pollen as the studied lakes are situated rather far from the nearest fields ( lakes Ta¨navja¨rv and Mustja¨rv ˜ dre and Matsima¨e 1–2 km). The 7–8 km, lakes O influx of long-transported pollen to a certain lake (especially larger than 3–6 ha) of course also depends greatly on the openness of the landscape, dominant atmospheric circulation mechanisms, aboveground turbulence, etc. Statistically irregular fluctuations in the longtransported (not closer than 7–8 km) pollen of broad-leaved trees and Cerealia-type pollen and a comparison of the influx data from the cores of lakes Ta¨navja¨rv and Mustja¨rv show that pollen diagrams allow for a reliable detection of human impact in the immediate vicinity of lakes if these disturbances embraced some 25–30% of a 100– 200 m band area.

6. Conclusions The obtained results demonstrate how complex the reconstruction of the history of vegetation disturbance is. Pollen influx data seem to provide a promising tool for this. In practical work a correct selection of study objects relevant to that aim is very important. The threshold of detection is lower if the disturbance concerns a small lake (some 3–6 ha). In the influx curves of significantly larger lakes (such as in this study Lake Ta¨navja¨rv, area 136 ha) only disturbances (forest fires) that took place repeatedly and embraced large territories (up to 2000 ha) close to the lake are recorded. The distance from the disturbance to the accumula-

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tion area ( lake) is of great importance as is suggested by the absence of practically any disturbance signals in the sediments of Lake Mustja¨rv, located 800 m away from Lake Ta¨navja¨rv.

Acknowledgements The research was supported by the Estonian Science Foundation Grants Nos 2873 and 3773 and Research Project 8/97-2. The authors thank Dr S. Hicks and Professor R. Janssen for valuable comments on an earlier draft of the manuscript.

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