An ecological and phytogeographic study of Northern Surinam Savannas
Wentia , Volume 14 - Issue 1 p. 1- 163
Geology and soils in general Surinam is situated at the northern edge of the very old and stable Guiana shield. Six-sevenths of the country’s surface are occupied by formations belonging to the shield and designated together as the basal complex. However, the Roraima formation does not belong to the complex. It was deposited during the Mesozoic (probably the Cretaceous) as a thick layer mainly consisting of sandstone that covered the greater part of the shield. Later on the original sandstone plateau was dissected, a process accelerated by the uplifting of the shield, and finally it disappeared almost completely by erosion. The former surface is now only represented by the flat tops of some table-mountains one of which is found in the interior of Surinam; Tafelberg. See Schols & Cohen (1953). The surface of the northern seventh part of the country is occupied by deposits of Quaternary age. In general may be distinguished (from the south to the north): 1) The ~ ' " ‘ ' – – – – – ..... Zanderij formation, consisting mainly of sands of continental origin; 2) the Coropina formation, comprising the “old coastal plain”; the main parts are (a) the so-called “schols”, i.e. the remnants of an old sea-clay plain, separated by filled-up tidal gullies, and (b) the remnants of the offshore bars that formerly separated the plain from the sea; 3) the Demerara formation, comprising the “young coastal plain”. See Van der Eyk (1954, 1957). Geological-pedological classification of the savannas Savannas are found on the basal complex, the Roraima, the Zanderij and the Coropina formation. Cohen & Van der Eyk (1953) classify them as follows: I Savannas of the Coropina formation 1. Watamalejo-type – on the offshore bars 2. Welgelegen-type – on the “schols” II Savannas of the Zanderij formation a. Kasipora-type — on dry bleached sand soils b. Zanderij-type – on wet bleached sand soils c. Coesewijne-type – on non-bleached soils III Savannas of the Roraima formation; Tafelberg-type IV Savannas of the basal complex 1. Paroe-type – on granitic soils 2. Bosland-type – on schist hills 3. Saban-pasi-type – on subgraywacke hills Savanna soils The climate is characterized by the sequence of a long rainy season (April-July), a long dry season (August-November), a short rainy season (December-January) and a short dry season (February-March). In connection with this periodicity the water-table in many places fluctuates strongly in the course of the year. During the dry seasons the upper layers of the savanna soils are always completely dry, except just after a shower. A soil is called very dry if even during the rainy seasons the upper layers are not influenced by the ground water. A very wet soil, however, at this period is covered by some cm of water; in addition it is characterized by many small hummocks, in Surinam called “kawfoetoes”, which are built up by worms and in which these animals are able to keep their heads above the water. Certain soils occur that in spite of deep watertables are wet, because an impermeable layer in the subsoil impedes drainage of the topsoil. Of course there is a scala between the extremes “very dry” and “very wet”. The texture of the upper layers ranges from bleached and slightly red sand to sandy and silty clay. Object of the investigation The flora and the vegetation of the northern Surinam savannas are the object of this investigation. These savannas do not only represent the types of the Zanderijand the Coropina-formation, but also the Bosland- and the Saban-pasi-type, for these two types are present on the basal complex only near its northern border. The following savannas have been studied. Welgelegen-type: the savannas of Bersaba and Vierkinderen, the Bigi-olo savanna near Hanover and the Fransina savanna near Welgelegen; Kasipora- and Zanderij-type: the white-sandy part of the Lobin savanna near Zanderij; Coesewijne-type: the loamy part of the Lobin savanna, the savanna Mimili Okili near Powaka, the Doti savanna near Wisawini and the Coesewijne savanna near Bigipoika; Saban-pasi-type: the Gros savanna and the De Jong Noord savanna. Data of some other authors pertaining to these and the other types have also been taken into account, some published (Lanjouw, 1936; Maguire c.s., 1948; Heyligers, 1963), some unpublished. The savannas present a marked diversity, among other things with regard to the structure of their vegetation. However, nearly all satisfy this definition: “A savanna (or a campo) is an area with a xeromorphic vegetation comprising an ecologically dominant ground layer consisting mainly of grasses, sometimes together with sedges, and with or without trees and/or shrubs either forming a more or less continuous layer, or in groups, or isolated.” The species have been studied with respect to the relation with the habitat, the means of dispersal and the area of distribution, all in mutual correlation. Vegetationunits have been distinguished and classified; ecological and chorologic aspects have been taken into account. A combination of all data, obtained during this as well as former investigations by others, permits the drawing of a provisional and general picture of the flora and the vegetation of the northern Surinam savannas as far as the present aspects are concerned. The following statements all apply to N. Surinam only, unless mentioned otherwise. Flora Habitat in general. Nearly all plants occurring on the savannas are heliophilous and are able to survive repeated burning. The flora of the open vegetation types consists of about 270 species the majority of which (72 %) is restricted to the open savannas. However, there are species occurring either in other open situations too, partly as weeds (8 %), or on wet savannas and other wet places (3 %), or in savanna rivulets and in swamps (7 %), or in savanna bushes (8 %). Out of ca. 100 species of the savanna bushes only 15 % are restricted to this vegetation type. The other species occur either also in the open savanna (20 %), or along forest borders (8 %), or in savanna wood and forest (23 %) and/or even in rain forest (31 %). A group of 12 % belonging to the last category does not flower or even not grow high in the bushes. Quite apart from this division other groups may be distinguished among the species of the bushes in the following way: occurring also in secondary forest (31 %) in marsh forest (9 %), in swamps (3 %). The trees and shrubs of the savannas support only few epiphytes and (half-) parasites; these belong to 19 different species. In the field nearly all species show some (factual) range with regard to the degree of moistness and the texture of the soil. The texture itself is not necessarily the decisive factor as there is a relation between the texture and some other properties of the soil, e.g. the consistency and the mineral content. This has not been further investigated. The same holds for the species preferring slightly shaded localities. These spots have a microclimate that differs more from that of its surroundings than in light intensity only. The majority of the open-savanna species have diaspores that are not obviously adapted to any agent of dispersal (71 %). The remaining 29 % are distributed over 6 different categories. The diaspores of the species of the bushes belong partly to the non-adapted forms too (35 %), but 50 % of them are fleshy. Generally speaking, the savanna species have a wider geographic distribution than the spieces of the flora of Surinam as a whole. This is particularly true for the opensavanna species. On the basis of similar areas of distribution the species are classed under 6 geographic elements, , viz. the Guianan (G), the northern South-American (N), the northern + eastern South-American (NE), the Middle- and northern + eastern South-American (MNE), the South-American (S) and the American element (A). The distribution of the species of the open-savanna vegetations and of the bushes, respectively, among the geographic elements is as follows (percentages): G 12 ; 26; N 11 : 18; NE 16 : 13; MNE 10 : 3; S 9 : 18; A 42 : 22. It appears from a comparison of these figures too, that the species of the first group in general have a wider distribution. Apart from the geographic elements the Roraima element has been distinguished. It comprises all species collected on one or several of the table-mountains in the Guianan interior. The distribution of these species among the geographic elements does not differ considerably from the one of the savanna flora as a whole. It may have appeared already from the foregoing that the species of the bushes, though presenting a higher percentage of adapted diaspores, nevertheless do not have areas of distribution wider than those of the open-savanna species. The expected correlation is, however, apparent if the two groups are considered separately: the mean area of distribution of the species with adapted diaspores is wider than the one of those with non-adapted diaspores. A comparison of the ecological and the chorologic aspects brings to the fore two focal points within the savanna flora: The elements with a small distribution (G and N) are most numerously represented on wet to very wet sandy (in particular white-sandy) soils, whereas the elements with a wide distribution (MNE, S and A) are concentrated on dry and moist non-bleached sands and loams and on very wet soils and present a preference-top on dry and moist loamy sand. The Roraima species are by far the most numerous on the wet white sand, in general they are more numerous on wet than on dry soils. Vegetation Vegetation-units have been distinguished and classified according to the Braun- Blanquet school. It has been attempted to make the groups of so-called characteristic and differential species correspond with ecological groups in the sense of Duvigneaud (1946, 1949). The latter consist of species with clear, sociological affinities between them because of similar habitat requirements. The open-savanna (and orchard-savanna) vegetation-types have been united into a single class which is defined and divided as follows: Class Leptocoryphio-Trachypogonetea. Principal species: Trachypogon plumosus, Leptocoryphium lanatum, Axonopus pulcher and Rhynchospora barbata. It seems likely that this class and its subdivision up to and including the alliances may be applied to the whole of Guiana. 1. Order Trachypogonetalia plumosi. Principal species: Trachypogon plumosus, Axonopus pulcher and Bulbostylis junciformis. On very dry to moist soils. 1.1. Alliance Cassio (ramosae)-Trachypogonion. Principal species: Trachypogon plumosus, Axonopus pulcher and Bulbostylis conifera. On white sands. There are 3 or 4 associations two of which occur on open patches between so-called muri-bushes (see Bl). Distribution: Kasipora-type; Guiana and adjoining parts of Brazil. 1.2. Alliance Curatello-Trachypogonion. Among the many tens of species the most common ones are Trachypogon plumosus, Axonopus pulcher, Schizachyrium riedelii and Heliconia psittacorum. Usually there is a thin layer of trees mainly consisting of Curatella americana, giving the vegetation the aspect of a type of so-called orchard savanna. A rather large part of the species occurs outside the savannas on other open spots too. The alliance occurs on pure reddish and on loamy sands. On the savannas of the Goesewijne- and the Welgelegen-type 5 associations are present. Similar vegetation types are found throughout Guiana, on the central Venezuelan llanos and far into E. Brazil. 1.3. Alliance Rhynchosporo (barbatae)-Trachvpogonion. Principal species: Axonopus pulcher, Leptocoryphium lanatum, Mesosetum cayennense, Bulbostylis conifera and Rhynchospora barbata var. barbata. On sandy (clay) loam. Two associations on savannas of the Coesewijne-type; they are related to vegetation types in French Guiana and in regions farther to the west, up to the Venezuelan llanos and some of the West Indian Islands. 2. Order Paspaletalia pulchelli. Leptocoryphium lanatum is the only species which is common in all communities of this order. In general the vegetations are not closed. On wet (or even very wet) soils. 2.1. Alliance Syngonantho-Xyridion. Principal species: Paspalum pulchellum, Panicum micranthum, Rhynchospora barbata var. glabra, R. graminea, Xyris guianensis and Abolboda americana. On white sands, wet and very wet. Three associations are found on the savannas of the Zanderij- and the Watamalejo-type. Distribution: Guiana and adjoining parts of Brazil, also on the table-mountains of the Guianan highlands. 2.2. Alliance Bulbostylidion lanatae. Principal species: Trachvpogon plumosus, Paspalum pulchellum, Panicum micranthum, Mesosetum tenuifolium, Rhynchospora barbata var. barbata and R. rhizomatosa. On loamy sand and sandy loam; wet. In northern Surinam 5 associations occur on savannas of the Saban-pasi- and the Watamalejo-type. Distribution: Guiana, probably also on the table-mountains. 2.3. Alliance Imperato (brasiliensis)-Mesosetion (cayennensis). Principal species: Leptocoryphium lanatum, Mesosetum cayennense, Imperata brasiliensis, Rhynchospora barbata var. barbata and R. globosa. On wet sandy loam and heavier soil types. Four associations on savannas of the Coesewijne-, Welgelegen- and Saban-pasi-type. Related vegetation types occur, as far as known, only in regions more to the west, up to the llanos and Guatemala. 3. Order Panicetalia stenodis. Principal species: Leptocoryphium lanatum, Panicum nervosum, Hypogynium virgatum, Heliconia psittacorum and Tibouchina aspera. On very wet soils, in savanna rivulets and small depressions. There are 2 alliances, both showing relationship with vegetation types occurring in regions more to the west, up to the llanos and some West Indian Islands. 3.1. Alliance Axonopodion chrysitis. Principal species: Leptocoryphium lanatum, Panicum nervosum, Rhynchospora globosa and Tibouchina aspera. On very wet soils of sandy loam and heavier. In N. Surinam 3 associations are found on savannas of the same types as alliance 2.3. 3.2. Alliance Mauritio-Hypogynion (virgati). Principal species: Hypogynium virgatum, Leptocoryphium lanatum, Panicum nervosum, Rhynchospora glauca, Heliconia psittacorum and Tibouchina aspera. Typical are the tall palms of Mauritia flexuosa. The alliance has rather many species in common with the communities of swamps, e.g. Blechnum indicum and Rhynchospora cyperoides. There are 3 associations, found in rivulets and depressions on savannas of all types. The different types of savanna-bushes are merely described. A classification or floristic grounds would be justified only if the savanna woods and forests were included in it too. Bl. Ternstroemia-Matayba bushes. See Heyligers (1963). Principal species: Ternstroemia punctata, Clusia fockeana, Licania incana, Humiria balsamifera var. guianensis (“muri”), Pagamea capitata, Matayba opaca and Conomorpha magnoliifolia. On dry white sand. B2. Rapanea bushes. Principal species: Rapanea guianensis, Davilla aspera, Tapirira guianensis, Symplocos guianensis, Miconia rubiginosa, Byrsonima crassifolia, B. coccolobifolia and Curatella americana. On dry loamy sand and dry sandy loam. B3. Cupania bushes. Principal species; Cupania scrobiculata var. frondosa, Davilla aspera, Byrsonima crassifolia, Miconia ciliata, Maprounea guianensis, Protium heptaphyllum, Symplocos guianensis and Curatella americana. On moist loamy sand and moist sandy loam. B4. Clusia-Scleria bushes. See Heyligers (1963). Principal species; Clusia fockeana, Licania incana, Bactris campestris and Scleria pyramidalis. On wet white sand. B5. Marlierea bushes. Principal species: Marlierea montana, Bactris campestris and Licania incana. On wet loamy sand. B6. Roupala-Antonia bushes. Principal species: Roupala montana, Antonia ovata, Davilla aspera, Miconia ciliata, Bactris campestris, Licania incana, Humiria balsamifera div. var., Pagamea guianensis and Marlierea montana. On knolls of pebbles embedded in sandy loam; wet. Existence, origin and maintenance of the savannas There is no type of climate that accounts for a savanna vegetation irrespective of other conditions. However, a climate that permits the existence of savanna vegetations may be called a savanna climate”. The latter is characterized by a certain difference between the precipitation in dry and wet seasons, independent of absolute values. The climate of northern Surinam is a savanna climate in this sense. A savanna vegetation is natural, i.e. determined edaphically, __, if the upper layer of the soil is alternately desiccated and saturated with water, thus in general in wet and very wet localities and in rivulets. As far as known the following savanna types and vegetation types are involved in this situation (the rivulets left out of consideration): Watamalejo (2.1) Welgelegen, partly (2.2), Zanderii (2.1 and B4), Saban-pasi (2.1 and B5, 2.2. and B6) and Bosland (?). A savanna vegetation occurs in dry localities only if fires prevent the formation of a closed layer of trees or shrubs. This is found among the following types: Welgelegen, partly (1.2 and 1.3), Coesewijne (1.2 and B2, 1.3 and B3) and Kasipora (1.1 and Bl). Parts of the savannas of the Welgelegen-, Coesewijne- and Saban-pasi-type occupied now by vegetations of the Imperato-Mesosetion (2.3) and the Axonopodion chrysitis (3.1) would probably be overgrown very slowly by the surrounding forest and only starting from its edges if the fires were stopped. It might be easily assumed that savannas owing their maintenance at present only to deliberate burning, originated from forests as a result of human interference as well. However, the possibility may not be excluded that they came into existence very long ago, either caused by natural fires or in consequence of a water economy of the soil differing from the present one. Final conclusions All available data concerning the flora and the vegetation of the northern Surinam savannas justify the following theories: The wet white-sand savannas of the Zanderij-type have vegetation types (2.1) consisting of species that mainly stem from formerly or still existing savannas on the basal complex and on the Roraima formation, probably chiefly on the latter. These species may have reached the Zanderij formation either directly by means of series of savannas in the interior that still may have been present during the break-down of the Roraima plateau, or indirectly by the way of other sandy regions bordering the edges of the Guiana shield. The vegetations of the savannas belonging to the Saban-pasi-type on wet loamy sand and sandy loam (2.2) consist of species which already for a long time were common to the basal complex and the Roraima plateau or/and which originated from the plateau, and besides of species that developed on the basal complex or migrated from elsewhere to the subgraywacke-area. The savannas of the Watamalejo-type and of the Welgelegen-type N. of the Wane-creek have a flora that may be regarded as a selection from that of the two preceding types. The vegetation types on dry and moist, red, pure and loamy sands belonging to the Coesewijne- and the Welgelegen-type (1.2) have a high percentage of their species in common with the campos of central and eastern Brazil. It seems possible that these species came to N. Surinam from the campos. The species combination of the savanna vegetations from other habitats does not permit a conclusion with regard to their possible origin.
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