Viktor Masing, Kalevi Kull, Hans Trass, Martin Zobel

Publication data:
Masing, Viktor; Kull, Kalevi, Trass, Hans; Zobel, Martin 1995. Vegetation science in Estonia. In: Aaviksoo, K.; Kull, K.; Paal, J.; Trass, H. (eds.), Consortium Masingii: A Festschrift for Masing. (Scripta Botanica 9.) Tartu: Tartu University, 144-189.

Abstract. The development of vegetation science in Estonia is divided into four periods: before 1918, 1918-44, 1945-68, and after 1968. All periods are characterized, including the main authors, original views, important results, international influences, historiography and local periodicals. Different branches of vegetation science in Estonia are reviewed, such as vegetation mapping, vegetation history, classification problems, succession studies, community ecology and ecophysiology, population studies, mathematical modeling, and theoretical studies.

1. Introduction
2. Historiography
3. Periodization
4. Influences
5. Vegetation mapping
6. Vegetation history
7.1. Forests
7.2. Grasslands
7.3. Wetlands
8. Succession studies
8.1. Forests
8.2. Grasslands
8.3. Wetlands
9. Community ecology and ecophysiology
9.1. Forests
9.2. Grasslands
9.3. Wetlands
10. Plant population studies
11. Mathematical modeling of vegetation structure and dynamics
12. General and theoretical studies
13. Conclusions

1. Introduction
Science is pursued within the framework of a local (national) culture. Therefore, it makes sense to analyse the history of a certain branch on a national (local) basis. In the case of vegetation science, it has a long and rich history in Estonia. As noticed by R.H.Whittaker (1962, p.38), "the magnitude of the contribution of the smaller nations of the Scandinavian and Baltic area (and of Switzerland) is one of the striking features of the history of ecology as a whole".
In this paper, we are trying to list the most important achievements in Estonian vegetation science, dividing them according to the main research areas. We have restricted the topics to research on Estonian vascular plants and vegetation of natural or seminatural communities.

2. Historiography
The papers which have considered the development and status of Estonian vegetation science belong to E.Spohr (1923), G.Vilberg (1929a), T.Lippmaa (1932b, 1938b), K.Eichwald (1958), V.Masing & H.Trass (1963), L.Laasimer (1965, p. 11-17), T.Frey & L.Laasimer (1979), A.Kalda & V.Masing (1982), M.Kask (1982), E.Roosaluste (1987). The monograph on Estonian vegetation by L.Laasimer (1965) also contains an extensive bibliography on Estonian vegetation research. Short reviews have been published on aqatic vegetation research (Trei 1982), forest research (Valk 1960), orchid studies (Jõe 1994), etc. Large bibliographies are included in the monographs on Estonian forests (Valk & Eilart 1974), mires (Valk 1988) and lakes (Mäemets 1968).
Special issues dedicated to International Botanical Congresses (IX, XI, XII, XIII, XIV) contain reviews on the current situation in Estonian botany (Kask & Masing 1959; Laasimer 1969; 1975a; 1981a; Laasimer & Kull 1987). Noteworthy are also the series "Scripta Botanica" (9 volumes) issued by the Institute of Zoology and Botany (Parmasto 1961; Eilart 1963; Kull & Kull 1989; Leht 1991; Palumets 1991), and "Papers on botany" (10 volumes) published by Tartu University as special issues of "Acta et Commentationes Universitatis Tartuensis".
Two important series, containing many papers on vegetation science, have been "Estonian Contributions to the International Biological Programme" with 12 issues altogether (Frey 1977; 1979; Masing 1982a), and "Publications of the Estonian National Committee of the International Programme "Man and Biosphere (MAB)"" (e.g., Zobel 1988b).
From 1954 to 1990, there were 20 conference-excursions of Baltic botanists organized in different regions of the Baltic countries. Conference guides and proceedings contain vegetation descriptions of corresponding regions (e.g., Kuusk et al. 1964; 1984; Ksenofontova 1986; Masing et al. 1990). From five analogous conferences in the period 1929-1935, only the lectures of the last one were published (in "Memoranda Societas pro Fauna et Flora Fennica" 12, 1936-37).
Personal bibliographies of several prominent botanists have also been published: L.Laasimer (Kask 1991), T.Lippmaa (Vaga 1961), V.Masing (Trass 1985a), P.W.Thomson (Kask & Raukas 1992), H.Trass (Masing 1988d), A.Vaga (Eichwald & Trass 1963), G.Vilberg-Vilbaste (Masing 1988a). Bibliography of Tallinn Botanical Gardens (Tarand 1986) contains many works on applied plant ecology.

3. Periodization
First period - from the middle of the 19th century to 1918. This was preceded by a preliminary period, during which botanical work began in Estonia. Tartu University was established in 1632 (Academia Gustaviana), but among the hundreds of dissertations of the 17th and 18th centuries only one was directly botanical - "De Plantis" by A.Arvidi (1647). The first published data about the local flora belongs to the Baltic German authors - J.B.Fischer (1778, 1791) and A.W.Hupel (1777). In the first half of the 19th century, Tartu University was one of the main centers of botany in the Russian empire. But the botanists at Tartu University (C.F.Ledebour, C.A.Meyer, A.Bunge, E.R.Trautvetter, et al.) paid almost no attention to local flora and vegetation (except A.Bunge).
From the 1850s, works appeared in which the first classifications of local vegetation types were described (Wiedemann & Weber 1852; Schmidt 1855; Glehn 1860; Russow 1862; Gruner 1864; et al.).
The main characteristics of this period are: (1) The notion of 'formation' was much less used than in West-European countries, and (2) the elements of new phytosociology as a separate branch appeared 20-30 years later than in the countries of Central and Northern Europe. The only exception was K.R.Kupffer (1909; 1913), the best specialist in Estonian flora and vegetation at the beginning of this century, who already used phytosociological methods and terminology in his papers.

Second period - 1918 to 1944. There were no Estonians mentioned for the first period. They appeared after the Estonian Republic achieved independence (in 1918) and when Tartu University became an Estonian-language university (in 1919).
In the 1920s, vegetation scientists of different nationalities worked side by side - Germans: K.Regel (1921; described grassland communities near Sangaste manor) and P.Thomson (1923; the first analysis of Estonian wooded meadows), Finns: J.G.Granö (1922; review of large-scale vegetation types) and K.Linkola (1929; 1930; application of A.Cajander's forest typology principles in Estonia). In the 1920s, Estonian vegetation science was strongly influenced by the Finnish forest site-type school (Rühl 1927). In the 1930s, this influence diminished (with some exceptions, see Paasio 1939) due to the development of a school under the leadership of T.Lippmaa. As an exception, A.Miljan (1933) used the principles of J.Braun-Blanquet in his classification of grassland, lake and wetland vegetation in Southern Estonia.
In this period, the most influential vegetation scientists in Estonia were G.Vilberg-Vilbaste and T.Lippmaa.
G.Vilbaste (1885-1967), the first professional Estonian phytocoenologist, has been one of the most productive florists, ethnobotanists, and nature protection activists in Estonia (Masing 1988a). He has been influenced mainly by scientists from Finland (K.Linkola), Sweden (G.E.Du Rietz), and Switzerland (H.Brockmann-Jerosch, E.Rübel). Of his works: investigation of alvar vegetation (paying special attention to generative reproduction) in North-Western Estonia (Vilberg 1927; 1929), the first attempt at the classification of Estonian plant formations (Vilberg 1930a), investigation of the vegetation of small islands in the Gulf of Finland (Vilberg 1933) should be mentioned.
In the same period, geographer E.Markus (1889-1971) introduced the notion of a 'nature complex' in the sense of ecosystem and used it to describe forest paludification (Markus 1925a; 1929). This was ten years before the term 'ecosystem' was adopted in biology. His views only began to be recognised three decades later. The complex profiles described by E.Markus (1925b) provided a good example of ecological multifactor analysis of vegetation.
T.Lippmaa (1892-1943) was undoubtedly the most famous Estonian botanist and ecologist in his time (Trass 1991). The evolution of his views was quite complicated (Trass 1955b; 1975; 1976; Trass & Malmer 1973; Vaga 1940; Laasimer 1965). Being an ecophysiologist, plant geographer and nature protection promoter, he influenced the whole of Estonian botany. His methods and views in vegetation science were followed by many local scientists and amateurs, who were classifying vegetation on a synusial basis (Pastak 1935; Rühl 1936; Sirgo 1936; Tomson 1937 (Tomson-Tamsalu); Vaga 1940). T.Lippmaa's works were also internationally influencial - together with H.Gams, G.E.Du Rietz and S.Cain a detailed structural-analytic approach in vegetation science was established. Considering the role of T.Lippmaa and his colleagues, several ecologists have used the term 'Lippmaa school' (Sukatshov 1934) or 'Estonian school' (Whittaker 1962). Nowadays, it could be called 'Lippmaa's tradition'.
The main points of T.Lippmaa's views could be formulated as follows:
(1) Notion of 'plant community'. T.Lippmaa pointed out, that a plant community is, primarily, influenced by ecological or environmental factors, and therefore, these factors (as determining the species composition and community structure) should be included in the description of a plant community as its characteristics (Lippmaa 1932a; 1933; 1935b; 1938a; 1940).
(2) Classification. According to the ecological interpretation of a plant community, the principles of community classification, introduced by T.Lippmaa, are also ecological. In his classification of synusiae of Estonian vegetation, where 150 syntaxa were listed, the unistratal communities were differentiated on the basis of main ecological factors (light, water relations, fertility of soil, etc.) and particular life forms (Lippmaa 1933). An ecological basis for classification has also been used by other Estonian phytocoenologists in their later works (e.g., Laasimer 1965).
(3) Characteristic species. The most important diagnostic value belongs to characteristic species, and not to dominant or constant species as it was considered by many North-European and Russian geobotanists (Lippmaa 1938a). The meaning of 'characteristic species' according to T.Lippmaa is rather different from that of the J.Braun-Blanquet' school - he chose species depending on certain environmental conditions from the whole floristic set.
(4) Synusiae. The notion 'synusia' as the primary functional and structural element of a plant community was introduced by H.Gams in 1918. T.Lippmaa has been one of most active users of this term among vegetation scientists, building on its basis a whole system of vegetation analysis (Lippmaa 1934a; 1935b; 1938a; 1939).
(5) Continuum. This term was adopted in vegetation science until the 1950s, but it should be stressed that T.Lippmaa paid much attention to transitions between plant communities (Lippmaa 1932a; 1933; 1934b; 1935a), and in this sense his views were quite close to the views developed in vegetation science considerably later.

Third period - 1945 to 1968. As a result of World War II, Estonian science suffered great losses. Very few vegetation scientists from the previous period survived (A.Vaga); the main work was done by a new generation (L.Laasimer, V.Masing, H.Trass, M.Kask, K.Pork, H.Karu-Krall, A.Kalda and others). The characteristic topic of this period was the development of ecological-coenological classifications, on the basis of which the local vegetation was described. The culmination of this period was represented by the monograph of L.Laasimer (1965). The first key-book of Estonian plant communities was compiled (Marvet 1970). Vegetation mapping on different scales (Laasimer 1958; Masing 1963) and the description of nature reserves were also considered to be important. The system of financing science in Estonia as a part of the USSR was different from that found in most western countries. Special research grants were usually not used, scientific researchers had a stable income. On one hand, this system gave quite a lot of freedom for scientists to choose topics of investigation. On the other hand, this system did not stimulate publication of results. Also, publishing abroad was impeded for political and bureaucratic reasons. Therefore the majority of scientific papers directed at the broader scientific community outside Estonia were published in Russian periodicals and conference proceedings. Up to 1964, scientific research was largely suppressed by forced lyssenkoism-mitchurinism taken as an official biological ideology in the Russian empire. At the same time, a constituent part of this ideology, which supported only applied research with an agricultural background and advocated the principle of "transformation of nature", provided a great deal of work in some areas of biology, e.g. for telmatologists.

Fourth period - from 1969. The important changes which have taken place since the end of the 1960s could be characterized by: application and development of mathematical methods and models in vegetation research (J.Ross, H.Tooming, T.Möls, T.Frey), using the continuum approach (E.Lõhmus, A.Nilson), new interest in J.Braun-Blanquet' methods (H.-E.Rebassoo). V.Masing was proposing a compromise between the approaches on the basis of system analysis. Intensive population studies were started. Plant ecophysiology (A.Laisk, H.Moldau) and ecomorphology (A.Koppel, J.Frey) deserved attention. Taking part in International Biological Programme brought together many Estonian vegetation scientists, and large, complex study programs were carried out in stationary research areas newly established by T.Frey (Vooremaa Forest Ecology Station). Investigations into human impact on vegetation and pollution influences were also started. Relatively greater political freedoms in comparison to the previous period considerably promoted international contacts and the impact of Western scientific influences. As a result, for the first time in Estonian botany a number of noteworthy papers were published abroad by well-known publishers, nevertheless the majority of scientific papers were published in Estonian and Russian.

The changes taking place in the past few years (since 1988) could be considered as the beginning of a new, fifth period. The representatives of this period are more theoretically-minded and often use mathematical methods in their work (T.Oja, J.Paal, K.Zobel, M.Zobel, O.Kull, K.Kull). The system of financing science has been rearranged (at first it significantly diminished), the research grant system is starting to be more widely applied. Also, the exchange of scientific information has increased considerably; close co-operation with colleagues from other countries, especially from Sweden and Finland, has been established. The main research topics in recent years include theoretical work (measurement of the structural variability of communities, analyzing theories of species coexistence), study of the structure and succession of calcareous grassland vegetation and the impact of air pollution and recreation on forest vegetation. However, the future of vegetation science (as of scientific research in general) will depend much on the economic development of the country.

4. Influences
National sciences are usually strongly influenced (through information exchange or international cooperation) by contemporary leading groups or scientific schools in other countries. There have also been periods when the development of certain theories or methods has been relatively isolated from other groups in the same field (e.g., Scandinavian and American schools up to 1930s, or Soviet geobotany from the 1950s to 80s).
Estonian vegetation science has been influence by different schools from several countries. According to the classification of scientific schools in vegetation science by H.Trass (1976), the main influences are shown in Fig. 1. From this we may conclude, that Scandinavian, Russian, Central-European and American science are all reflected in Estonian vegetation science. The combination and reciprocal enrichment of Western and Eastern experiences could be considered an important feature of Estonian vegetation science.

5. Vegetation mapping
The first phytogeographical division of the Baltic region including Estonia was proposed by K.R.Kupffer (1912; 1925). A forest map was provided by M.v.Sivers (1903). The first detailed description of regional vegetation, including the compiling of a vegetation map, was made by T.Lippmaa for South-Western Estonia (Lippmaa 1931).
One of the most important achievements of Estonian vegetation scientists has been the Estonian vegetation map of scale 1:42,000. This work has its origins in the 5th International Botanical Congress (in Cambridge, 1930), where it was decided to start work on a vegetation map of Europe. In Estonia, the work was started in 1934 under the leadership of T.Lippmaa (1937a; 1937b). By 1940, nearly one half of Estonian territory was mapped. After World War II, the work was continued under the leadership of A.Vaga, and later L.Laasimer. Altogether, 68 Estonian botanists took part in this project. The map itself turned out to be the most capacious European country-level vegetation maps. But because of Soviet cartography policy it was almost inaccessible to other researchers until 1989. Meanwhile, an analysis of Estonian vegetation and a map of scale 1:600,000, on the basis of this material, were made by L.Laasimer (1958; 1965). Vegetation of the different regions of Estonia has been described in several other papers (Lippmaa 1931; Pastak 1935; Jaagomäe 1962).
Later a number of local vegetation maps have been compiled, mainly for the territories of nature reserves (Kalda 1991; Masing 1991b), for example the vegetation map of Matsalu Nature Reserve of scale 1:10,000, compiled under the leadership of K.Pork in 1977-1980, and the vegetation map of Lahemaa National Park of scale 1:25,000 (Kalda 1988).
Several papers concerning the theoretical and methodical problems of large scale vegetation mapping were also published (Eilart & Masing 1961; Masing 1963; Marvet 1968; Kalda 1991).

6. Vegetation history
The history of forest dynamics in the postglacial period was concisely analyzed by P.W.Thomson (1929; 1930). He has also studied the development of Estonian peatlands, and became a founder of palynology in Estonia. Later palynological research in peat and lake sediments was carried out by K.Veber, H.Valk, U.Valk, E.Ilves, H.Mäemets, L.Saarse, T.Koff, S.Veski, A.Poska, and others. Development of certain bog complexes and mire systems was analyzed in a number of papers by H.Allikvee, K.Veber, M.Ilomets and others (Truu et al. 1964; Veber 1974; Orru 1992).
The long-term dynamics of vegetation has been also investigated for several lakes (Koff 1994; Mäemets 1968; Pirrus & Rõuk 1988; Piiper 1988; Saarse & Königsson 1992; Saarse 1994).
The history of Estonian plant communities in the postglacial period has been described by L.Laasimer (1965; 1983), H.Mäemets (1983), T.Koff et al. (1983), and the history of forests in the last few centuries by M.Margus (1974), A.Kalda (1958) and U.Valk & J.Eilart (1974).

7. Classification problems
The problem of classification arose in Estonian vegetation science in the 1930s in connection with vegetation description, but it became more important in the 1950s. Three periods (actually, paradigms) can be seen in its solution: (a) associational, where the main object of study was the community and the main classification unit was association; this approach dominated until the end of the 1960s, but still has supporters; (b) continual, where the main approach concerns mathematical ordination of habitats and communities (E.Lõhmus 1974; 1984a; J.Paal 1987; 1991); (c) systemic, which has developed in parallel to (b), but contrary to the other approaches it proposes to analyze vegetation as a multi-level system and in the place of a common classification it proposes separate classifications for each level (Masing 1958; 1984; 1991; Masing & Trass 1963). Instead of the whole community, its more or less self-regulating parts (clones, populations, microcoenoses) become more important together with the problem of their classification, ordination and dynamics.

7.1. Forests
The forest vegetation of Estonia has been classified mainly according to A.Cajander's site type system (see Frey 1973) and later using Sukachev's approach. Different versions of forest typology have been produced by E.Schabak (1922), A.Rühl (1927; 1932; 1936b), A.Ilves (1953), A.Karu & L.Muiste (1958), A.Katus & E.Tappo (1964) and V.Masing (1969). A new approach was introduced by E.Lõhmus (1974, see also Frey 1973), who ordinated forest stands, representing all the site types of A.Karu & L.Muiste (1958), by means of Wisconsin polar ordination. The last version of this 'ordinated site type system' (Lõhmus 1984a) also included detailed characteristics of both plant communities and soil conditions of each site type. The classification of plant communities in clearcut forest areas was also given by E.Lõhmus (1970). Besides, forest vegetation was also classified on a floristic basis (e.g. Kalda 1962). Forest floor vegetation was analyzed by H.Rebane (1967). Forest vegetation in different regions of Estonia has been described by several authors (e.g., Kaar & Kalda 1970). The analysis of broad-leaved forest vegetation is provided by A.Kalda (1960). The survey of forest associations was made by L.Laasimer (1965).
There have been several attempts to use numerical methods to ordinate and classify the forest communities of certain areas, but only papers on the forests of Canada (Frey & van Groenewoud 1972), Karelia (Paal et al. 1989) and the Far East (Paal 1991) have been published.

7.2. Grasslands
With some exceptions (sea-shore and flood-plain grasslands, some alvars) grassland vegetation in Estonia is seminatural. Changes in technology and the economic system have resulted in a strong decrease in the area of seminatural grasslands (among them, mainly, wooded meadows), which was estimated to be 1,570,000 ha in 1939 (Krall et al. 1980) and 303,000 ha in 1981 (Aug & Kokk 1983). Despite this, grassland vegetation is still receiving constant attention from vegetation scientists.
A detailed description and classification of grassland associations of southern Estonia was provided by A.Miljan (1933). The first ecological typology of Estonian grasslands was elaborated by R.Toomre et al. (1957), and H.Trass (1957). A complete floristic classification of grassland vegetation was made by L.Laasimer (1965). The classification units were further elaborated for particular vegetation types such as alvar vegetation (Akkel 1967; Zobel 1984; 1987b), flood-plain meadows (Pork 1964), and sea-shore vegetation (Rebassoo 1975). A new classification of grassland vegetation was elaborated by H.Krall et al. (1980), where two different approaches - site type classification and community classification on the floristic basis - were combined. Site types according to forest typology (using mainly soil characteristics) were recognized as bases for primary (larger) taxa, then grassland plant associations (both natural and seminatural) within site types were described.
Grasslands in different regions of Estonia were described by several authors (Krall & Pork 1970a; see Pork 1981 for a review).

7.3. Wetlands
The classification of mire vegetation (peat-forming ecosystems) has been considered by A.Vaga (1953), E.Varep (1953), V.Masing and H.Trass (Masing & Trass 1955; Masing 1958; Trass 1958; 1963a; 1963b; 1994). The classification system in which both community structure and species composition were considered was developed by V.Masing (1975a; 1982c; 1984; cf. also Botch & Masing 1979). Site types and forest communities of drained peatlands were classified by E.Lõhmus (1981), plant associations of spring fens were elaborated by E.Roosaluste (1988). Vegetation of wetlands other than mires (reedbeds, salt marshes, inundated grasslands) has been described by L.Laasimer (1965), H.-E.Rebassoo (1974), H.Krall et al. (1980). The monograph by M.Kask (1965) can serve as a case study of mire vegetation.
The wet meadows of Matsalu area were studied and described by K.Pork and P.Vissak (Vissak 1991), the aquatic vegetation by T.Trei (1991). The aquatic vegetation of the Estonian waterbodies has been studied by A.Miljan (1933; 1958), A.Mäemets (1968; 1974), and others.
One of the favorite topics in wetland studies has been the description of the surface topography and vegetation structure of Sphagnum bogs (Allikvee 1974; Masing 1974; 1982a; 1984; 1994b; Zobel & Masing 1987). Species composition of bog plant communities and their relation to structural features and ecologi-cal conditions within them were characterized by M.Ilomets (1988) and A.Loopmann (1988).

8. Succession studies
8.1. Forests
The general description of successional pathways in forest communities can be found in numerous papers by forestry specialists (e.g., Valk & Eilart 1974), botanically more exactly by A.Ilves (1953) and L.Laasimer (1965). More detailed investigations of forest succession were carried out at Järvselja forest reserve, in southeastern Estonia. The successional dynamics of understorey species (Zobel & Zobel 1988; Zobel 1989), plant communities (Masing & Rebane 1987) and soil (Zobel 1990b) were described there. M.Zobel (1993) described pine forest succession in clearcut areas under different edaphic conditions. Succession in clearcut areas was also studied by G.Vilberg (1930b). Successional studies also include descriptions of the early stages of revegetation of abandoned oil-shale quarries (Laasimer 1973) and allogenic succession of forest communities due to trampling (Roosaluste 1983).

8.2. Grasslands
Many efforts were made to study grassland succession, especially changes in community composition and productivity due to fertili-zation (Adojaan 1961; Pork 1979; 1981). The influence of trampling on meadow communities has been analyzed by L.Truus (1983). Natural successions in floodplain meadows have been described by K.Pork (1964). Also, detailed data about the changes in the field layer structure (e.g., Akkel & Hein 1973; Hein 1979) were pub-lished. K.Annuk has studied the seed bank in meadows (Annuk 1979). Grassland and shrub vegetation in the land uplift area in the coastal zone of western Estonia was described by H.-E. Rebassoo (1972; 1974; 1988) and M.Zobel (1984). The interrelations of alvar community succession and soil development were described (M.Zobel 1985; Zobel & Kont 1991). M.Pärtel & M.Zobel (1995) described the changes in species turnover and species richness during the experimental restoration of an alvar community from overgrown land.
Theoretical analysis of anthropogenous successions in relation to life strategies of grassland plants is provided by H.Krall & H.Rand (1983).

8.3. Wetlands
Due to the possibility of reconstructing community development on the basis of stratigraphic records, peatlands are especially suitable objects for succession studies. The main trends of mire community succession were generalized in a type of a transition probability matrix by K.Aavik-soo et al. (1984; 1994). Earlier successional stages (fen communities) were characterized by multidirectional transitions, whereas during the later stages only transitions in certain directions were possi-ble. Recent dynamics of wetland landscapes were analyzed by K.Aaviksoo (1988) and K.Aaviksoo & H.Kadarik (1989). The dynamics of bog water bodies have been studied by V.Masing (1982d; 1984). Fine-scale analysis of community succession and hummock-hollow dynamics in bogs was presented by E.Karofeld (1986) and M.Ilomets (1988). V.Masing (1988c) analyzed the role of trees in creating the vegetation pattern of bogs. In addition to autogenic succession, anthropogenous changes in wetland vegetation were also investigated: the topics considered include succession in burnt areas (Masing 1960), post-drainage succession in bogs (Masing 1953; Masing & Valk 1968) and cal-careous spring fens (Trass 1955a; Roosaluste 1984), impact of trampling on bog communities (Roosaluste 1982), regeneration succession on peat fields (Kristian & Roosaluste 1988), and influence of atmospheric pollution (Karofeld 1994). K.Aaviksoo (1993) studied the vegetation dynamics of mire systems on the landscape level.
Gradient analyses in wetland areas include the study of interrelations between soil oxygen conditions and plant community composition (M.Zobel 1986; 1987a; 1990a; Zobel & Toom 1990). It was demonstrated that soil oxygen conditions in forest sites which represent the first stages of paludification succession were sometimes even less favorable than, for example, in transitional bogs. Literature on community succession in boreal mires was reviewed by M.Zobel (1988a).
Succession of reed communities during the last 30 years in connection with progressive eutrophication was described by T.Ksenofontova (1989).
Trends and relationships in lake vegetation successions have been studied in detail and generalised by A.Mäemets (1974).

9. Community ecology and ecophysiology
9.1. Forests
Early investigations in the mathematical analysis of phenological data collected for trees were initiated by A.Oettingen (1879).
In the 1930s, several ecological factors were investigated, e.g., soil acidity, water and nitrate content in plants by A.Rühl (1936a; 1937). In the 1950s, consistent research into the ecology of certain forest sites began, especially on peat soils (Valk & Eilart 1974; Valk 1988).
Many authors described relations between soil conditions and vegetation (species composition, stand productivity, structure of root systems etc.) in various forest site types (Pork et al. 1977; E.Lõhmus 1983; 1984b; 1985; K.Lõhmus et al. 1986; 1989; Reintam et al. 1987). Quantitative analysis of roots was carried out for forest herbs (Kukk 1967) and trees (K.Lõhmus). Within the framework of IBP, stationary investigations of forest productivity and nutrient cycling were carried out - the results were presented for example by R.Kõlli (1971), R.Kõlli & M.Ingermaa (1970), R.Kõlli & R.Kährik (1970a; 1970b) and T.Frey (1981), later also by L.Reintam & M.Zobel (1990). A survey on biomass partitioning in spruce stands has been compiled by J.Palumets (1991).
The phenology of nemoral forest vegetation was studied by A.Kalda (1964) and L.Kannukene (1979). The aerobiological studies and regular monitoring of pollen amounts in the air in Tartu region have been carried out by M.Saar during the last decade.
The first complex ecological forest research areas were created in 1937 on Abruka island and in 1939 in the Rangu alvar forest, and studied by T.Lippmaa (1940) and L.Laasimer (1946). Unfortunately, World War II cut off these studies.
Important forest ecology studies in a permanent research area have been carried out in Vooremaa Forest Ecology Station, organized by T.Frey and A.Koppel (Frey 1977; 1979).
The problems of mineral cycling in pine stands have been analyzed by M.Mandre (1992). Forest litter decomposition was investigated by A.Nõmmik (1939). J.-M. Punning (1994) published case studies about the structure, dynamics and pollution impact of forested ecosystems in north-east Estonia. A.Kont et al. (1994) have investigated complex influences on vegetation using landscape transects.
The growth analysis of forests using dendrochronological methods and tree ring measurements have been provided by E.Lõhmus, A.Läänelaid et al.
Special attention has been paid to forest radiation regime (Ross et al. 1977). Light reflectance parameters were measured by U.Peterson (Peterson 1989; 1992; Peterson & Ross 1988).
The parameters of leaves of almost all woody species of the Estonian flora have been measured by Ü.Niinemets & K.Kull (1994).
The vertical structure and the mechanisms which account for the appearance of layers in forests have been investigated by O.Kull (Kull & Koppel 1987; Kull & Niinemets 1993).

9.2. Grasslands
Earlier attempts to measure grassland productivity are reviewed by K.Pork (1981). Approximate values of primary productivity were given by H.Krall et al. (1980). Organizers of grassland productivity research have also been A.Sau and others (Loid 1986). A climatic productivity atlas has been compiled by P.Karing (1980).
An attempt at the statistical analysis of grassland vegetation was made by K.Regel (1921). Recruitment of alvar vegetation was studied by G.Vilberg (1929b). The phenology of calcareous grassland vegetation was described by K.Pork (1963) and V.Hein (1970). There are examples of the study of allelopathic interactions between species (Pork 1975) and the investigation of mineral content in different species of meadow plants (Kalmet & Michelson 1969). The ecomorphology of meadow plants was studied by H.Krall & H.Rand (1979). The architecture of grassland communities has been described by V.Ross et al. (1986) and V.Hein (1986). K.Kull & M.Zobel (1991) analyzed the pattern of species richness in a calcareous grassland, and found the site of maximum species richness. K.Zobel et al. (1994) studied small-scale species turnover in a calcareous grassland.
H.Krall and K.Pork (1970b) compiled, on the basis of Ellenberg indicator values, an improved table of ecological indicator values for 351 grassland species. Theoretical aspects of meadow community functioning were analyzed by H.Krall (1979).
Ecological work on permanent plots has been carried out at Laelatu Biological Station since 1961, organized by K.Pork, K.Kull and M.Zobel, and at several less durable research areas by K.Pork, J.Liiv, V.Hein, H.Krall, and others.

9.3. Wetlands
There are many wetland reserves in Estonia, and complex ecological studies have been carried out in several of them - Endla, Nigula, Matsalu, Viidumäe. Data concerning the growth rate of Sphagnum mosses, net primary productivity and peat increment in different structural features of bogs have been measured (Ilomets 1982; Tint 1982).
The ecology of reedbed communities has been studied mainly in Matsalu, which is a protected wetland area of international importance (Ramsar area). Published material is available about the morphology and productivity of Phragmites australis (Ksenofontova 1988a), nutrient cycling and primary production of reed communities (Ksenofontova 1988b).

10. Plant population studies
A study on the reproduction biology of Populus tremula was made by P.Reim (1930). Detailed study of Picea abies populations in Estonia has been carried out by I.Etverk. Demography of bog pines (Pinus sylvestris) was investigated by A.Läänelaid & V.Masing (1974).
The population ecology of some forest species of special interest (forest berries, medicinal plants, rare species) has been studied more profoundly. The distribution along soil gradient, productivity and regeneration of Arctostaphylos uva-ursi (Pihlik 1988; 1989) and Vaccinium vitis-idaea populations (Pihlik 1991) was studied in boreal forest site types. The time needed for regeneration after harvesting the berries or shoots was also estimated. A general description of the population structure and productivity of Vaccinium vitis-idaea in Karelian forests is presented by T.Paal (1988; Paal & Paal 1989). Demographic studies of Cypripedium calceolus were carried out by T.Kull (1987; T.Kull & K.Kull 1991; Kull & Tuulik 1994), Cephalanthera longifolia by Ü.Püttsepp (1994), and Rubus chamaemorus by Ü.Reier (1984). Populations of Cuscuta epilinum (Ratt 1940), Agropyron repens (Ennvere 1947), Dactylorhiza ruthei (Kuusk 1994), Orchis ustulata (Tali 1994), Cephalanthera rubra, Epipactis atrorubens, and Neottia nidus-avis (T.Kull & Tuulik 1994; T.Kull 1994) were investigated in smaller studies.

11. Mathematical modelling of vegetation structure and dynamics
Since the 1960s, the application of mathematical methods in vegetation science has become popular in Estonia (Frey 1969).
J.Ross (1964; 1967; 1976; 1981; Myneni & Ross 1991) has been a beginner and an Estonian leader in compiling mathematical models of vegetation. His influence through numerous pupils and followers has been noticeably strong not only in Estonian science, and it has been called the Ross' (or Estonian) school in mathematical modelling of vegetation productivity processes. Initially, the main work was done in agrocoenoses (Tooming 1984), but later different natural communities were also included. Special attention has been paid to the theory of light penetration in the plant canopy (Tooming, 1968; Ross 1972; T.Nilson 1977; T.Nilson et al. 1977; Tammets 1980), the application of optimality principles in the modelling of plant growth (Tooming & Kallis 1972), and the using of meteorological data for predictions of vegetation growth (Sepp 1983). A number of scientists belonging to the J.Ross' school have worked on plant physiological models (A.Laisk, H.Moldau) and are therefore outside the scope of this survey.
A model of forest stand dynamics was developed by T.Oja (Paal et al. 1989). Other models of forest analysis have been developed by T.Nilson, J.Ross (Hari et al. 1985), K.Kull and O.Kull (1989), A.Nilson, and others.
Plant population dynamics and mechanisms of species co-existence were analyzed using mathematical models by K.Kull (1986; 1987; 1993). Much attention has been paid to elaborating new approaches in numerical ecology and the application of statistical models. An original version of agglomerative cluster analysis was elaborated by T.Frey (1971; Frey & Võhandu 1966). J.Paal (1987; Paal et al. 1989) proposed several numerical methods for measuring the continuity of vegetation patterns, he also dealt with optimal planning of vegetation monitoring (Paal 1984). Methodological research also included fitting of log-linear models to contingency tables with a presence/absence (frequency) scale as an additional dimension, with application to nemoral forest understorey vegetation (Zobel & Zobel 1988b). K.Zobel & M.Zobel (1988) offered a null hypothesis, a so-called independent random distribution hypothesis, to construct a randomized community. The model was tested to predict the ecological mechanisms behind successional changes of forest communities (K.Zobel et al. 1993).

12. General and theoretical studies
After T.Lippmaa, one of leading theoreticians was A.Vaga, his review paper on the notion of phytocoenosis (1940) was remarkable at the time. After 1940, during the period of national calamity, the development of theoretical science was inhibited for many years. The new generation of vegetation scientists grew up isolated from science in other countries besides the USSR. The first theoretical works after the World War II concerned vegetation structure and dealt mainly with the further development of the ideas of T.Lippmaa (1933; 1938a; 1939; 1940) about the synusial structure of vegetation (Laasimer 1946; 1961; Trass 1955b). From the 60s onwards, the exchange of scientific papers with the rest of the world gradually started. Theoretical papers, discussing the continuity of vegetation (Trass 1966) and the proposal to use special approaches for different levels of vegetation organization instead of creating a single 'natural classification' (Masing 1958; 1974; Masing & Trass 1963) were published.
One traditional topic of interest for Estonian vegetation scientists has been the study of vegetation structure, with emphasis on spatial relations. The general framework of struc-tural units was offered by T.Lippmaa (cf. Laasimer 1961), H.Trass (1970, infra-coenotic structure), and V.Masing (1965; 1988b, five structural levels of the world vegetation). V.Masing (1963; 1973; 1982b; 1982c; 1984; 1994a; 1994b; Masing & Läänelaid 1976; Zobel & Masing 1987) elaborated the system of structural features of bog vegetation and surface topography. According to this system, six structural levels (microsite, microform, compound microform, mire facies, mire complex, mire system) were distinguished. Community succession (directed changes) and oscillation (cyclic changes) can be observed in each structural level at various time scales. If the temporal or spatial scale is changed by one step only, the process under investigation can express new qualitative features. V.Masing (1975b; 1981) also studied the evolutionary development and functional structure of ecosystems, concentrating on consortia (systems of heterotrophic organisms and epiphytes connected with certain plant species) as elementary units of functional structure. This concept has been applied to Sphagnum bogs. Functional structure has also been considered by M.Zobel & K.Zobel (1989) who made an attempt to distinguish guilds within nemoral forest plant commu-nities - life forms, height and vegetative mobility were used as functional characteristics of species. K.Kull & M.Zobel (1991) described the structure of a species-rich grassland community and analyzed the fitness of different explanations of species co-existence. M.Zobel (1992b) reviewed the theories explaining the coexistence of plant species and offered an approach which takes into account evolutionary and historical factors. Using a species removal experiment, K.Zobel et al. (1994) analyzed the existence of specific mechanisms of diversity maintenance.
Theoretical research also produced accounts concerning the vegetation of Estonia and the USSR, and review papers. The history, distribution and ecological syntaxonomy of the vegetation of Estonia was characterised by L.Laasimer (1965). Later, monographs about lakes (Mäemets 1968), forests (Valk & Eilart 1974) and mires (Valk 1988) were published. M.Botch & V.Masing (1979; 1983) gave an overview of the mire vegetation of the whole USSR, and V.Masing et al. (1990) of the wetlands in the biosphere.
Theoretical reviews also considered the history and basic concepts of vegetation science. The development of vegetation science was described by H.Trass (1976), V.Masing (1994a), K.Kull & M.Zobel 1994), more detailed review papers dealt with vegetation science in northern countries (Trass & Malmer 1973; Trass 1975), the Finnish forest site-type system (Frey 1973), and the development of vegetation science in the Soviet Union (Masing 1991a). The future of vegetation science was discussed by K.Kull (1988).
Vegetation scientists have also argued the need to protect plant communities - the principles of community protection have been considered by several authors (Laasimer 1975b; 1981b; Masing 1974; Masing & Botch 1973; Botch & Masing 1979; Trass 1985b; 1987; Zobel 1992a).
As an important result of the work, several original university textbooks have been published (Kalda 1970; Masing 1979; Raudsepp 1982).

13. Conclusions
Estonian vegetation science has been quite diverse and chequered. Among other branches of biology in Estonia, it has been one of most continual and influential. In trying to point out its most important and original achievements, we would like to emphasize the following -
(1) works of E.Markus about nature complexes and their succession anticipating the ecosystem approach;
(2) works of T.Lippmaa and his school about synusiae and their classification, which were developed by A.Vaga, H.Trass, and others;
(3) vegetation maps of Estonia of scale 1:42,000 and 1:600,000 and, as a generalization, a comprehensive survey of Estonian vegetation (L.Laasimer);
(4) works of V.Masing on structural levels of vegetation on various space and time scales;
(5) works of the the Estonian school of mathematical modelling on vegetation architecture and production processes (J.Ross and his followers);
(6) complex vegetation investigations on permanent research areas: Abruka and Rangu (T.Lippmaa, L.Laasimer), Avaste (L.Laasimer, M.Kask), Vooremaa (T.Frey, A.Koppel), Laelatu (K.Pork, K.Kull, M.Zobel);
(7) investigations of the mechanisms of community structure and species co-existence (K.&M.Zobel, O.&K.Kull).

Acknowledgements. We are thankful to Toomas Kukk, Olevi Kull, Erich Lõhmus, and Jaanus Paal for valuable comments.

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