Savannas I Physiognomy Terminology and Ecotones Why Do Savannas Exist

9.1 Physiognomy and Terminology

Savannas are open habitats typically dominated by grasses and often strongly affected by seasonal changes of rainfall. The term savanna, sabana in Spanish and sa-vana (or campo) in Portuguese, is a West Indian expression of uncertain Caribbean origin (Huber 1987). A classical example of a savanna is the Llanos north of the Orinoco in Venezuela (Fig. 9.1). Alexander von Humboldt vividly described the seasonal contrasts in this large Venezuelan savanna-area:

"When under the vertical rays of the sun, never covered by clouds the combusted layer of grass has fallen into dust, the hardened soil cracks as if it were shaken by mighty earthquakes."1

"The uniform vision of these steppes has some greatness but also some tristesse and depression in it. It is as if the whole nature would be frozen; scarcely every now and then the shade of a small cloud, which hurries across the zenith and announces the near rainy season, falls over the savanna. One hardly can get used to the vision of the Llanos, which offer a picture like the surface of the sea... "2

". ..as a saying goes here: 'The large ocean of greenery' ('los Llanos son como un mar de yerbas')"2

"When after a long drought the beneficial rainy season sets in, the scene in the steppe suddenly changes. When the surface of the earth is just wetted the fragrant steppe covers itself with Kyllingias with highly panicled Paspalum and with a diversity of grasses. Stimulated

1 "Wenn unter dem senkrechten Strahl der niebewölkten Sonne die verkohlte Grasdecke in Staub zerfallen ist, klafft der erhärtete Boden auf, als wäre er von mächtigen Erdstößen erschüttert..."

2 "Der einförmige Anblick dieser Steppen hat etwas Großartiges, aber auch etwas Trauriges und Niederschlagendes. Es ist, als ob die ganze Natur erstarrt wäre, kaum daß hin und wieder der Schatten einer kleinen Wolke, die durch den Zenith eilend die nahe Regenzeit verkündet, auf die Savanne fällt. Nur schwer gewöhnt man sich an den Anblick der Llanos, die... ein Bild der Meeresflächen bieten."

"... wie man hier oft sagen hört, 'dem großen Meer von Grün'... 'los Llanos son como un mar de yerbas'..." (Südamerikanische Reise, A. v. Humboldt 1808/1982 - Humboldt von 1982)

Fig. 9.1A-C The Llanos, Venezuela (A, B) and a savanna in Costa Rica, protected within the borders of the Santa Rosa Park (C)
Fig. 9.2 Physiognomy of savannas with examples from all over the world. Transects of Vareschi (Vareschi 1980, with kind permission of R. Ulmer)

by the light, herbaceous mimosas open their folded dormant leaves and greet the rising sun like the early song of the birds."3

In order to give an impression of the vegetation, Fig. 9.1 shows a picture of the Llanos in Venezuela and a savanna in Central America, and the drawings of transects in Fig. 9.2 present various types of savannas from all over the world. H. Walter (Walter and Breckle 1984) distinguished between savanna and park-land as follows:

• Savanna: homogeneous plant communities with scattered woody plants (trees, shrubs and bushes) in a grass layer closed in a greater or less degree over the soil and with a few herbs in between.

• Park-land: a mosaic of forest islands in an open grassland with few woody plants where the forest is associated with biotopes (e.g. river banks, valley bottoms, hills), which are ecologically different from the grassland.

In contrast, the geographer C. Troll (see Walter and Breckle 1984) considered various types of landscapes as savannas which constitute a macro-mosaic of grassland with different tree-formations (Fig. 9.3). Some of them are illustrated in the accompanying photographs:

gallery-forest savanna gully-forest savanna banco-forest savanna gallery-forest and termite savanna gully-forest and termite savanna banco-isla savanna

Fig. 9.3 Macro-mosaics of plant formations with grassland in landscapes called savannas by C. Troll. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

3 "Tritt endlich nach langer Dürre die wohltätige Regenzeit ein, so verändert sich plötzlich die Szene in der Steppe. Kaum ist die Oberfläche der Erde benetzt, so überzieht sich die duftende Steppe in Kyllingien, mit vielrispigem Paspalum und mit mannigfaltigen Gräsern. Vom Lichte gereizt, entfalten krautige Mimosen ihre gesenkt schlummernden Blätter und begrüßen die aufgehende Sonne, wie der Frühgesang der Vögel." (Ansichten der Natur, A. v. Humboldt 1849/1986 -Humboldt von 1986)

Fig. 9.3 Macro-mosaics of plant formations with grassland in landscapes called savannas by C. Troll. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

Fig. 9.4A, B Gallery forests. A Rio Parupa, Gran Sabana, Venezuela. B In the cerrados near Brasilia (Fazenda Agua Limpa), Brazil
Fig. 9.5A, B Termite savannas. A After a fire, Queensland, Australia. The termite nests scattered over the field are well visible after the vegetation has been burnt. B Acacia wooded savanna, Great Rift Valley, Ethiopia

Fig. 9.6 Savanna with dense stands of the moriche palm, Mauritia flexuosa. Such morichales are restricted to wet marshy parts of savannas. (Gran Sabana, Venezuela.) Alexander von Humboldt has described the morichales as follows: The palm Mauritia flexuosa "at moist places forms magnificent groups of fresh and shiny greenery, which recalls the green of our alder bushes. With their shade these trees maintain the moisture in the soil..."

"Sie bildet an feuchten Orten herrliche Gruppen von frischem glänzendem Grün, das an das Grün unserer Ellergebüsche erinnert. Durch ihren Schatten erhalten die Bäume die Nässe des Bodens

Fig. 9.6 Savanna with dense stands of the moriche palm, Mauritia flexuosa. Such morichales are restricted to wet marshy parts of savannas. (Gran Sabana, Venezuela.) Alexander von Humboldt has described the morichales as follows: The palm Mauritia flexuosa "at moist places forms magnificent groups of fresh and shiny greenery, which recalls the green of our alder bushes. With their shade these trees maintain the moisture in the soil..."

"Sie bildet an feuchten Orten herrliche Gruppen von frischem glänzendem Grün, das an das Grün unserer Ellergebüsche erinnert. Durch ihren Schatten erhalten die Bäume die Nässe des Bodens

(A. v. Humboldt, Ansichten der Natur 1849/1986 - Humboldt von 1986)

This already illustrates the problems inherent in the use of the term savanna, which has been applied to a wide range of habitats.

Attempts to delineate the term more precisely have been made repeatedly (Huber 1982, 1987, 1990). Clearly, in the open habitats of the savannas, the herbaceous ground-layer is the ecologically decisive stratum. There may be shrubs and trees, scattered or in small groups, but they never form a closed canopy. Thus, the major contribution to the input of energy into the whole ecosystem (by capture of solar radiation and primary biomass production) comes from the herbaceous ground-stratum. The herbaceous layer itself may be dominated by grasses and sedges or by broad leaved herbs so that one can distinguish between:

• grass savannas (Fig. 9.1), i.e. the savannas sensu strictu, and

• herb savannas (Fig. 9.7), which as "yerbazal" in Spanish, may not be considered as savannas in a strict sense but then would require use of a rather awkward term in English, e.g. "broad-leaved meadows".

Typical tropical grass savannas are restricted to low and medium elevations not exceeding 1,000-1,200 m, while herb savannas may occur at higher elevations. In

Fig. 9.7 Yerbazal (herb savanna), Sierrania Parti (04°25' N, 65°32' W, 1,250 m a.s.l.), dominated by Stegolepis hitchcockii (broad flat leaves), Brocchinia hechtioides (slender tank-forming bromeliad) and Bonnetia crassa (small shrub)

either case it is essential that we are dealing with tropical or subtropical ecosystems (Fig. 1.3A), where in contrast to the tropical forests with a closed canopy of trees (Chap. 3) the ground stratum of grasses and/or herbs dominates energy turnover (Huber 1982,1987, 1990). This may also occur in ecosystems outside the tropics, which then are distinguished as prairies, pampas or steppes. This strict distinction was not then familiar to Alexander von Humboldt who used "savanna" and "steppe" synonymously. On the other hand, the term savanna has also been used for physiognomic characterization of vegetation outside the tropics (Eiten 1986).

Even in the neighbouring countries of South America, Venezuela and Brazil, classification of savanna-like vegetation has led to different terminologies (Sarmiento 1984; Eiten 1972, respectively). For the vegetation in Brazil the cerrado-concept was developed, which is somewhat narrower than the more general savanna-concept. In Brazil, 20% of the area of the whole country and 40% of the non-Amazonian part are covered by cerrados (Eiten 1972; Gottsberger and Silberbauer-Gottsberger 2006a,b). They are geographically and ecologically intermediate between tropical rainforest and tropical/subtropical desert. The annual precipitation averages between 1200 and 1,600 mm ranging from 800 to 2,000 mm in the driest and wettest parts, respectively. There is strong seasonality with more than 90% of the rain falling in 7 months (October-April). The cerrado soil is very infertile and can vary from less than 5% to over 95% sand, the rest is clay and a little silt (Eiten 1972, 1986).

Table 9.1 attempts a systematic comparison of the terms used to describe Venezuelan and Brazilian savannas and cerrados (see also Figs. 9.1 and 9.8). The density

Table 9.1 Physiognomic description of various types of Venezuelan savannas and Brazilian cerrados. (After Sarmiento 1984; Eiten 1972)

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment