Structure, root systems and periodicity of Savanna plants and vegetations in Northern Surinam
Wentia , Volume 17 - Issue 1 p. 1- 162
From July 1958 to May 1959 an investigation was carried out of the relation between physiognomic characteristics of the vegetation and the habitat on some savannas in the vicinity of Zanderij, Surinam. Root systems, structure, periodicity and characteristics of the leaves were considered, both of the species separately and of the vegetation types. Nine vegetation types were studied which were described floristically by Heyligers (1963) and Van Donselaar (1965). The vegetation types differ in structure. The available types of structure are: low and sparse vegetation of tussocks of grasses and sedges; dense vegetation of both low and high tussocks of grasses and sedges; high and closed vegetation of tussocks of grasses and sedges, with or without a continuous but sparse tree layer; bushes with a more or less closed canopy. In the herb layers, apart from the tussocks, some perennial herbs and therophytes, rather many half-shrubs and few shrubs occur. In the area of the investigation the climate shows a long rainy season, a long dry season, a short rainy season and a short dry season. The different vegetation types occupy soil types differing in texture and hydrology. It appears that principally the following factors influence the characteristics of the vegetations mentioned: the alternation of rainy and dry periods in combination with the hydrology of the soil; fires; tree layer with more or less closed canopy; layer of litter. The hydrology of all soil types under consideration is characterised by the alternation of the moist and the bone-dry condition of the upper layers. The moisture content of the upper layers is determined by the rain water taken in by the soil, whether or not in combination with the ground water; the upper level of the latter varies according to the seasons. The run-off and the percolation velocity of the rain water strongly influence the hydrology. The growing of the roots is determined by two mutually opposite factors: desiccation and saturation with water of the soil. In general the roots of the monocotyledons in the studied vegetation types tolerate the water-saturated condition of the soil, but they are not able to protect themselves against the influence of extended drought. The roots of the dicotyledons, on the other hand, do not tolerate a waterlogged soil, but they can withstand extended drought, probably because their corky bark protects them. There are few exceptions, among which the common grass Trachypogon plumosus that behaves just like a dicotyledon species. If the water supply comes from beneath, the root systems of the monocotyledons are narrow and penetrate deeply. If the rain water is the main water source, the root systems spread, both of monocotyledons and dicotyledons. The quicker the rain water penetrates into the soil and sinks down, the larger the horizontal dimensions of the root systems are. In consequence of the demands of the roots and the hydrology of the soil the roots have a certain soil volume at their disposal. This volume may be different for monocotyledons and dicotyledons in the same soil. In soils with a high water table during a part of the year the monocotyledons have narrow and deeply penetrating root systems, whereas those of the dicotyledons are superficial. In soils with a deep water table and above it a layer that is dry for a long time, both monocotyledons and dicotyledons develop their root systems in the uppermost layer which is wetted by the rain water. A few dicotyledon species have a primary root reaching down to the water table, where oxygen becomes the limiting factor. Vegetation structure above ground level is related to the available rooting volume. If the rooting volume is larger, the dimensions of the aerial parts are larger too. Tnen there are more trees and shrubs, the differentiation in layers is greater and the dominating grass layer is higher. It is hard to determine the role of the nutrient content, for this factor and the hydrology of the various soil types show parallel differences, so that their influence on the vegetation can not be distinguished separately. On white sand many species have a root system that is large in comparison with the aerial parts (more euxerophytes than on the other soil types). The distance between the tussocks is correlated with root competition, except on white-sandy soils. If the root systems are narrow and penetrate deeply, the tussocks stand close together, but if the root systems spread in the superficial layers, the bases of the tussocks stand far apart. On white sand, the vegetation is thin, though the layer of roots is not closed. The dry seasons are the cause of a resting period for all species that are not completely woody. Grasses, sedges and perennial forbs survive only underground. Half-shrubs persist with their short, woody stem bases. All completely woody species, however, are evergreen. Only in a more favourable habitat, under a (thin) canopy, or on white sand with ground water available for long periods, non-woody species occur that function throughout the long dry season. Therophytes are present in all vegetation types, but they prefer habitats where the chance of drying up during the growing season is least, i.e. under a canopy or on white sand with ground water available during the greater part of the year. The annual fires destroy completely the aerial dry parts of the plants. Grasses and sedges do not suffer from this, for the fire occurs during their resting period. On the contrary, they seem to profit by it. The aerial parts of half-shrubs are destroyed, but in the soil, a few centimeters below the surface, rejuvenation buds survive on a xylopodium. Shrubs and trees also may have a thickened xylopodium. Such a xylopodium is developed as a result of the repeated forming of new stems after the old ones have been burned. In many cases shrubs have several stems as a result of the fires. The fire does not kill all rejuvenation buds above 2 to 4 m, so that part of the buds of higher trees and shrubs may sprout after the fire. As a result, trees and shrubs are found only in places where the fire has held off for one or several years, or where a period between two subsequent annual fires was long enough for them to grow above the upper limit of the dangerous zone. This last possibility is realised in the vegetation unit with the most favourable hydrology and consequently the largest available rooting volume, i.e. the Polycarpaeo-Trachypogonetum. Trees and high shrubs are observed in all vegetation types, except in those on wet white sand, where the small available rooting volume and the low nutrient content are probably the limiting factors. Even under a thin canopy transpiration may be less in consequence of reduced insolation. Accordingly, characteristics indicating less severe drought are more frequently found here, and the species with these characteristics have a higher degree of cover than elsewhere. There occur more forbs, in particular therophytes (indicating more favourable conditions during the growing season), more species with mesomorphic and hygromorphic leaves and more species that function more or less normally during the long dry season. The decreased amount of light diminishes the role of the grasses. A layer of litter improves the soil; the organic content is higher, evaporation may be less; the temperature of the underlying soil may be lower. As a result, places with a layer of litter are occupied by many shrub species of which the roots restrict themselves to the litter-area. The leaves are scleromorphic as a rule. Species with mesomorphic or hygromorphic leaves are found only in habitats with a continuous water supply during the growing period (wet white sand) or with less evaporation under a canopy. In tussocks with basal leaves the leaf size is related to the height of the stratum. The height is related to the rooting volume. Also the leaves of non-tussock forming species in the herb layers are larger if the layer is higher. Therophytes have small leaves (lepto- and nanophyll). This can be considered in connection with the absence of sclerenchyma. Trees and shrubs usually are micro- and mesophyll. As a rule the size of the leaves is smaller in vegetation types on white sand. Most woody species have leaves with a shiny upper surface, but most other species have more or less hairy leaves. On the basis of these findings the following brief interpretation can be given of the relation between the different vegetation types and their habitats: Panicetum stenodoidis. Silty clay; ground water in the surface during the long rainy season; cracks during the long dry season, so that at the beginning of the rains the soil is wetted quickly and down to a rather great depth. Consequently the rooting volume is comparatively large. The root systems of the monocotyledons are narrow and penetrate deeply, the plants are standing close together and the vegetation is moderately high. Few dicotyledons are present; their roots run very superficially. The annual fire destroys completely the desiccated aerial part of the vegetation. Only a few specimens of treelike Curatella americana were found. Schizachyrio-Rhynchosporetum barbatae. Sandy clay loam; influence of the ground water up to 40 cm below the surface at most; poor percolation; strong run-off; the rain water does not wet the soil further down than 20 to 30 cm. Consequently there is a layer that is dry permanently or at least so for a long time between the ascending capillary ground water and the water in the top layer. The available rooting volume is small for both dicotyledons and monocotyledons; the vegetation is low. The roots spread beyond the projection of the aerial parts (rain water the main water source); above the surface the vegetation is not closed. Few dicotyledons have a primary root penetrating down to the zone influenced by the ground water, where the saturation of the soil with water is restrictive. Trees and high shrubs are rare. The annual fire destroys the vegetation up to 3 m above the surface. Probably trees and shrubs are present only on places where the fire held of for some time. Polycarpaeo-Trachypogonetum. Loamy sand; water table not higher than 80 cm below the surface; no run-off; large available rooting volume; high grass vegetation, with or without a thin tree layer consisting of Curatella americana. All rain water is taken up quickly; the roots spread strongly; at their bases the tussocks stand far apart. In spite of the fierce annual fires in the high and dry tussocks, there may be a tree layer. Probably ~ Curatella is able to outgrow the influence between two of them on this soil type with its relatively favourable hydrologic conditions. Under the tree layer conditions are better; there occur more therophytes and other forbs; more non-woody species without a resting period; more species with mesomorphic and hygromorphic leaves. In the pure white sandy soils the vegetation is not closed, neither above nor below the surface. Quick percolation and ample aeration make the drought extremely effective. It is supposed that because of this situation seedlings have so little chance for survival that the vegetation remains open. Mesoseto-Trachypogonetum. Dry white sand. The roots spread widely, both of monocotyledons and dicotyledons. Only few dicotyledon roots reach down to the layer influenced by the ground water. The root systems are completely arranged so as to catch the water of every shower that may occur occasionally even during the long dry season. In this connection it may be considered that relatively many species are completely woody (dwarf-shrubs) and function normally during the long dry season. The vegetation consists of low and high tussocks and here and there a low shrub. Xyride-Paspaletum pulchelli. Wet white sand; during the greater part of the year the ground water reaches the surface, but during the long dry season the upper decimeters are bone-dry. The alternation of long lasting saturation with water and periodical desiccation of the soil results in a small available rooting volume, also for the monocotyledons. Most dicotyledons have roots that run very near to the surface so that they are subject to extreme drought and high temperatures; the plants remain low. Ternstroemia-Matayba bushes and Clusia-Scleria bushes. The shrubs of some species are able to establish themselves in open, purely sandy soil. They produce a layer of litter that improves the soil. Shrubs of other species that settle themselves now restrict their root systems to the area of the litter layer. The roots of some species are restricted to the area of the litter layer in wet sand, but in dry sand they go beyond this area at a deeper level. This may be caused by the high temperatures of the upper decimeters. Because of the decreased light intensity underneath the canopy the grasses and sedges are limited. On white sands only the vegetations of the Mesoseto-Trachypogonetum „ _ burn every year. The other vegetation types often escape the fires because they are either too thin or do not dry up in the long dry season.
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