Inselbergs

11.1 Physiognomy

Inselbergs1 are isolated rock outcrops in the palaeotropics and the neotropics coming up out of different vegetation of savannas or cerrados (Fig. 11.1A) or rainforests (Fig. 11.1B). In savannas with a certain savanna affinity of their flora they also have been described as "rock savanna". They consist of monolithic blocks, mostly of granite or gneiss, of a considerable geological age, i.e. 10 x 106 years at least and 40-50 x 106 years on average. More rarely inselbergs may also consist of sandstone. They range from several tens or hundreds of meters high, the highest one found in French Guiana being 740 m high (Schnell 1987). "Shield-type" inselbergs may have extensions of several square kilometers. In arid regions and deserts they may have been eroded to heaps of rather small rocks (Fig. 11.1D). Inselbergs evolve by deep and intensive weathering (Fig. 11.2A). Thus, inselbergs are really islands separated from the surrounding savanna or forest vegetation (Figs. 11.1A, B and 11.2B; Barthlott et al. 1993; Porembski and Barthlott 2000a). They provide very different ecological conditions.

In the analysis of inselberg vegetation, one tries to determine whether the occupation of these isolated habitats is deterministic or stochastic, and whether there are climax states or dynamic oscillations and if there is a coexistence of equilibrium and non-equilibrium communities (Barthlott et al. 1993; Porembski et al. 2000b). In fact, the inselbergs themselves are again fragmented in a number of sub-islands and smaller ecological units (Porembski et al. 2000a; Fig. 11.3), and therefore their j-diversity (see Sect. 3.3.1). is high. Besides a large spectrum of higher plant life forms, cryptogamic life forms are particularly important. The rock itself is rarely free of life, usually being covered by dense cryptogamic crusts of cyanobacteria, lichens and mosses. Interestingly, depending on the respective dominance of lichens or cyanobacteria, one may distinguish lichen and cyanobacteria inselbergs. The former are more abundant in Africa, the latter in South America (Barthlott et al. 1993).

1 The term inselberg from the German Insel = island and Berg = mountain, coined by Bornhardt (1900), is somewhat more specific than "rock outcrop" and has been widely adopted in scientific writing.

Fig. 11.1A-D Inselbergs coming up out of a savanna along the Rio Orinoco near Puerto Ayacucho at the Southern rim of the Llanos, Territoria Federal Amazonas, Venezuela (A) and out of a rainforest with a rain storm moving up at Les Nouragues, French Guiana (B). Individual inselberg near Puerto Ayacucho (C), eroded inselbergs, fragmented to heaps of small rocks, in the savanna, near Caicara del Orinoco, Venezuela (D)

Fig. 11.1A-D Inselbergs coming up out of a savanna along the Rio Orinoco near Puerto Ayacucho at the Southern rim of the Llanos, Territoria Federal Amazonas, Venezuela (A) and out of a rainforest with a rain storm moving up at Les Nouragues, French Guiana (B). Individual inselberg near Puerto Ayacucho (C), eroded inselbergs, fragmented to heaps of small rocks, in the savanna, near Caicara del Orinoco, Venezuela (D)

Sub-islands on the rock are formed by individual plants or small groups of plants growing in humus in cracks, gaps and hollows (Fig. 11.3). Xerophilic plants, such as succulents and epiphytes, grow saxicolously. Patches of shrubbery originate from the vegetation islands (Fig. 11.4A,B) and small forests often form on top of the inselbergs (Fig. 11.4C). In South America these forests comprise the only deciduous vegetation units occurring in the area around the Guayana shield, with trees like Pseudobombax chrysati, Tabebuia orinocensis and Yacaranda filicifolia (Fig. 10.4), and also palms (Syagrus orinocensis, Fig. 11.4C). While most sites on inselbergs are extremely dry and hot in the dry season (see Sects. 11.2 and 11.3), there are also wetter sections (Fig. 11.5) and even rock pools (Porembski et al. 2000a), which seasonally keep small ponds of water and harbour aquatic plants. Floristic diversity of inselbergs in various regions of the world is documented in detail in Porembski

Fig. 11.1 (Continued)

and Barthlott (2000a). It is very high and inselbergs are extraordinarily rich in endemic species. Huber (1980) listed the natural habitats of the inselbergs along the Orinoco in Venezuela as belonging to the "flower paradises and botanical gardens of the earth".

Phytogeography, plant sociology and vegetation analysis are advancing to ask the pertinent questions for providing a deeper understanding of these exciting sites (see Barthlott et al. 1993). Ecophysiology of this fascinating vegetation is still less developed. One exception are studies of desiccation tolerance in inselberg plants (Sect. 11.4). Where there is strong seasonality, pools will dry out in the dry season. The rock surface during midday may easily heat up to temperatures around 60 °C (Fig. 11.6). Not only cryptogams but also many angiosperms of the inselbergs have been shown to be able to overcome such dry periods by ecophysiological adaptations, particularly by desiccation tolerance (Sect. 11.4).

Fig. 11.2 A Deep and intensive weathering leading to the formation of inselbergs (after Bremer and Sander 2000). B Scheme of the vegetation of a granitic inselberg in South America (French Guiana) (Schnell 1987, with kind permission of Masson S. A. Paris)

11.2 Cryptogams 11.2.1 Cyanobacteria 11.2.1.1 Ubiquity

A conspicuous feature of the inselberg rocks is their superficial appearance of dark coloration. This was first noted in Venezuela by Alexander von Humboldt (Humboldt von 1849) who described the black surface of the rocks in the riverbed of the Orinoco and also of the rock outcrops further away:

"In the Orinoco, especially in the cataracts of Maypures and Atures, all granite blocks and even white pieces of quartz to the extent they are touched by the water of the Orinoco

Fig. 11.3 Isolated plants and sub islands on inselbergs: The bromeliad Pitcairnia pruinosa on the inselberg at Galipero near Puerto Ay-acucho, Venezuela (A), the bromeliad Tillandsia arau-jei (B) and the orchid Maxil-laria sp. (C) on the inselberg Pedra Grande at Atibaia, S.P., Brazil, and the Velloziaceae Pleurostima gounelleana (D) on rock outcrops at Itatiaia, R.J., Brazil

Fig. 11.3 Isolated plants and sub islands on inselbergs: The bromeliad Pitcairnia pruinosa on the inselberg at Galipero near Puerto Ay-acucho, Venezuela (A), the bromeliad Tillandsia arau-jei (B) and the orchid Maxil-laria sp. (C) on the inselberg Pedra Grande at Atibaia, S.P., Brazil, and the Velloziaceae Pleurostima gounelleana (D) on rock outcrops at Itatiaia, R.J., Brazil

develop a greyish-black cover which does not penetrate more than 0.01 line2 into the interior of the rocks. One might think to see basalt or fossils stained by granite. In fact the sheath appears to contain manganese oxide and carbon."3

3 "Im Orinoco, besonders in den Cataracten von Maypures und Atures, ... nehmen alle Granitblöcke, ja selbst weiße Quarzstücke, so weit sie das Orinoco-Wasser berührt, einen graulichschwarzen Überzug an, der nicht 0,01 Linie ins Innere des Gesteins eindringt. Man glaubt Basalt oder mit Granit gefärbte Fossilien zu sehen. Auch scheint die Rinde in der That braunstein- und kohlenstoffhaltig zu sein."

Regarding the dark colour of the rock outcrops at a greater distance from the current river bed of the Orinoco he assumed that the river had extended much further in earlier times:

"This assumption is supported by several observations. One sees black caves 150 to 180 feet above the present water level. Their existence teaches ... that the streams, whose size presently excites our admiration, are only humble remains of the enormous amounts of water in archaic times . . . These simple observations even did not escape the rough natives. Everywhere they drew our attention to the old waterlevel."4

The finding of prehistoric petroglyphs in the rocks of the inselbergs (Fig. 11.8A) also played a role in Humboldt's discussion, since he asked how the Indians might have found access to the steep walls of rock for carving them:

"Between Encaramada and Caycara on the banks of the Orinoco one frequently finds ... hieroglyphic pictures in considerable height on the rock faces, which now would only be accessible by means of extraordinarily high scaffolding. If one asks the natives how these pictures could have been carved, they reply with a smile, as if they were telling a story which

4 "Diese Vermutung wird durch mehrere Umstände bestätigt ... man sieht schwarze Höhlungen 150 bis 180 Fuß über dem heutigen Wasserstand erhaben. Ihre Existenz lehrt ..., daß die Ströme, deren Größe jetzt unsere Bewunderung erregt, nur schwache Überreste von der ungeheuren Wassermenge der Vorzeit sind."

"Selbst den rohen Eingeborenen ... sind diese einfachen Bemerkungen nicht entgangen. Überall machten uns die Indianer auf die Spuren des alten Wasserstandes aufmerksam."

Fig. 11.4 Shrubs (A, B) with Clusia criuva (B) on the inselberg Pedra Grande at Atibaia, S.P., Brazil and deciduous forest (C) with the palm Sya-grus orinocensis on top of the inselberg at Galipero near Puerto Ayacucho, Venezuela

Fig. 11.4 Shrubs (A, B) with Clusia criuva (B) on the inselberg Pedra Grande at Atibaia, S.P., Brazil and deciduous forest (C) with the palm Sya-grus orinocensis on top of the inselberg at Galipero near Puerto Ayacucho, Venezuela

Fig. 11.5 Wetter sections and pools on the inselberg Galipero near Puerto Ayacucho, Venezuela, where seasonally water is flowing and small ponds with water are formed in hollows

Fig. 11.6 Temperature of the air above the inselberg at Galipero (°) and the rock surface (•) during two consecutive days (15-16 March 1991) (Pto. Ayacucho, Venezuela)

only the white man may not know: that in the days of the extended waters their forefathers were boating in canoes in such a height. This is a geological dream for the solution of the problem of a long vanished civilization."5

5 "Noch mehr: zwischen Encaramada und Caycara an den Ufern des Orinoco befinden sich häufig hieroglyphische Figuren in bedeutender Höhe auf Felsenwänden, die jetzt nur mittels außerordentlich hoher Gerüste zugänglich sein würden. Fragt man die Eingebornen, wie diese Figuren haben eingehauen werden können, dann antworten sie lächelnd, als erzählten sie eine Sache, die nur ein Weißer nicht wissen könne: 'daß in den Tagen der großen Wasser ihre Väter auf Canots in solcher Höhe gefahren seien'. Dies ist ein geologischer Traum, der zur Lösung des Problems von einer längst vergangenen Civilisation dient."

Fig. 11.7 Black surface of the rock of the inselberg at Galipero. A small section was removed to show that only the very surface is coloured

Furthermore, regarding the extensive distribution of the petroglyphs, Humboldt alludes to his correspondence with Sir Robert Schomburgk:

"I may be permitted to include a remark, which I take from a letter of the distinguished traveller Sir Robert Schomburgk: 'The hieroglyphic pictures have a much wider distribution than you might have assumed' ... the symbolic figures, which Robert Schomburgk found engraved in the river valley of the Essequibo at the rapids of Waraputa according to his observation resemble the truly Caribbean ones on one of the small Virgin Islands (St. John)6, however, notwithstanding the wide expansion of the invasions of Caribbean tribes and the ancient power of this beautiful human race, I cannot believe that this whole immense belt of carved rocks which cuts across a large part of South America from west to east is the work of the Caribs. They are rather traces of an ancient civilization, which possibly belongs to an epoch, when the tribes which we distinguish nowadays were still unknown by name and relationship. Even the reverence, which everywhere is deferred to these rough sculptures of the forefathers, proves that the present Indians have no idea of the creation of such works."7

7 "Es sei mir erlaubt, hier noch eine Bemerkung einzuschalten, welche ich einem Briefe des ausgezeichneten Reisenden Sir Robert Schomburgk an mich entnehme: 'Die hieroglyphischen Figuren haben eine viel größere Ausbreitung, als Sie vielleicht vermutet haben'. ... Die symbolischen Zeichen, welche Robert Schomburgk in dem Flußtal des Essequibo bei den Stromschnellen ... von Waraputa eingegraben fand, gleichen zwar nach seiner Bemerkung den ächt caraibischen auf einer der kleinen Jungferninseln (St. John); aber ungeachtet der weiten Ausdehnung, welche die Einfälle der Caraiben-Stämme erlangten, und der alten Macht dieses schönen Menschenschlages, kann ich doch nicht glauben, daß dieser ganze ungeheure Gürtel von eingehauenen Felsen, der einen großen Theil Südamerikas von Westen nach Osten durchschneidet, das Werk der Caraiben sein sollte. Es sind vielmehr Spuren einer alten Civilisation, die vielleicht einer Epoche angehört, wo die Racen, die wir heut zu Tage unterscheiden, nach Namen und Verwandtschaft noch unbekannt[cont. p.389]

Fig. 11.8A-C ► Petroglyphs of an inselberg at Pintado near Puerto Ayacucho, Venezuela (A) and on granite rocks near a small pond in the secondary forest covering the US Virgin Island St. John, Lesser Antilles (B,C)

However, there are also already some slight reservations in Humboldt's writings regarding the inorganic nature of the black sheets on the rocks. He stressed that it appears to be manganese oxide and carbon:

"I say, it appears; because the phenomenon has not been investigated diligently enough. At the Orinoco these leadlike coloured rocks if wetted emit harmful emanations. One believes their proximity to be a cause of fevers."8

A biological cause is suggested by the observation that the black coloration is associated with only the organically rich white-water rivers and not the more sterile black-water rivers:

"It is also noteworthy that the rivers with black water, aguas negras, the coffee-brown or wine-yellow waters, in South America do not stain the granite rocks black."9

Indeed, an elemental analysis of the black cover of the inselbergs along the Orinoco using X-ray fluorescence spectroscopy shows that there are traces of manganese only (Table 11.1). There is only one exception, and these are the rocks directly in the riverbed of the Orinoco (Table 11.1), where the analysis actually indicates the dominant presence of manganese oxide. The high levels of Al and Si in all cases, of course, are due to the bed rock. The dominance of elements like S, K and Ca is consistent with the occurrence of life in these crusts. Indeed, lichens and small mosses are often readily discerned on the rock surfaces (Sect. 11.2.2). However, microscopic inspection shows that even the smooth black covers of these rocks result from living organisms. They are mainly composed of epilithic and en-dolithic cyanobacteria, predominantly of the genera Gloeocapsa, Stigonema and Scytonema (Figs. 11.9 and 11.10). Similar coverings of cyanobacteria are found on rocks throughout the tropics and the diversity of species is quite large (Büdel 1999). The phenomenon has also been described for sandstone rocks, e.g. near Cumana in eastern Venezuela (Golubic 1967), and examples are given in Fig. 11.11 of the granite rocks of Sierra Maigualida and sandstone rocks of Tepuis (Sierrania Paru) in the Guayana highlands of Venezuela. They resemble the "Tintenstrich" ("ink strip") formation frequently found in the European Alps, particularly on calcareous rocks (Jaag 1945).

In the tropics almost every free surface on rocks is covered by cyanobacterial mats and crusts. Based on the large extension of supporting rocks these cyanobacteria overall must constitute an enormous biomass in the tropics.

waren. Selbst die Ehrfurcht, welche man überall gegen diese rohen Sculpturen der Altvorderen hegt, beweist, daß die heutigen Indianer keinen Begriff von der Ausführung solcher Werke haben."

8 "Ich sage: sie scheint; denn das Phänomen ist noch nicht fleißig genug untersucht... Am Orinoco geben diese bleifarbigen Steine, befeuchtet, schädliche Ausdünstungen. Man hält ihre Nähe für eine fiebererregende Ursache."

9 "Auffallend ist es auch, daß die Flüsse mit schwarzen Wassern, aguas negras, die caffeebraunen oder weingelben, in Südamerika die Granitfelsen nicht schwarz färben."

Fig. 11.9A, B Closeup photographs of rocks covered with cyanobacteria on the inselberg at Galipero (A) and on the granite in the Sierra Maigualida, Guayana Highlands (05° 30' N, 65° 15' W, 2,040 m a.s.l.) Venezuela (B)

Fig. 11.10A-C ► Cyanobacteria composing the black crusts of the inselbergs along the Orinoco, Venezuela. A Stigonema ocellatum. B Scytonema crassum. C Gloeocapsa sanguínea. (Courtesy B. Büdel, Kaiserslautern; see Büdel et al. 1994)

Fig. 11.11A-C Black granite rocks of the Sierra Maigualida (at 05° 30' N, 65° 15' W, 2,040 m a.s.l.; A and B, in B shortly after a rain storm) and sandstone rock of the Serrania Paru (at 04° 25' N, 64° 32' W, 1,200 m a.s.l.; C), Guayana Highlands Venezuela, with cyanobacteria
Table 11.1 Elemental analysis (elements of order-number 11-80) by X-ray fluorescence spectroscopy of samples from sandstone rocks of the Serriania Paru (04° 25' N, 65° 32' W, 1,200 m a.s.l.), inselbergs along the Orinoco and the riverbed of the Orinoco near Puerto Ayacucho. (See Budel et al. 1994)

Element %

Serrania Parua

Inselbergs Orinocob

Riverbed Orinoco0

Al

37.9 ± 9.5

16.8 ± 1.3

7.8 ± 0.1

Si

46.0 ± 10.0

55.1 ± 4.2

18.9 ± 3.1

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