Thorn Scrub and Cactus Forests

Examples comparing the physiognomy of a thorn scrub and a cactus forests are shown in Figs. 3.10 and 3.11. The term cactus forest was coined by Vareschi (1980). Cactus-forests represent an example of thornbush-succulent forests. This type of forest in Madagascar covers a small area in the south-west of the island (see Fig. 2.14) dominated by Didieraceae (11 species in 4 genera), Euphorbiaceae and other succulent and deciduous woody species (Fig. 3.12). It is also characteristic of the "Caatinga" formation in NE Brazil (Fig. 3.13A). In Venezuela there is the "Espinar" in the area around Carora, where Vareschi (1980) distinguishes a thornbush-forest, with Mimosaceae (Haematoxylon praecox) and Caesalpiniaceae (Cercidium prae-cox) which has only one species of cactus, and a cactus-forest with 10 or more different species of cacti (Figs. 3.11 and 3.13B,C). However, both types of forest mutually intermingle and form a mosaic-like pattern. The term cactus forest relies on the acceptance of large columnar cacti as tree-like plants, and depends on the size of the area occupied by them. Alternatively or additionally, one may require the most important physiognomical aspect of a forest to be the formation of a closed canopy. The area around Carora in Venezuela, for which Vareschi originally coined the term, is dominated by columnar cacti of Cereus lemairei, Ritterocereus griseus and Cephalocereus moritzianus. However, it appears, that woody Mimosaceae, Cap-

Venezuela Forest
Fig. 3.10A,B Types of dry tropical forest. A Drought deciduous forest (Falcon, Venezuela). B Cactus-forest (Carora, Venezuela)

paridaceae and Caesalpiniaceae like Cercidium praecox very much add to the impression of a closed canopy, which is certainly given when one is walking around in these forests. The floor of these forests is bare of vegetation or covered by a for-

Venezuela Forest

Fig. 3.11A,B Transects of a thornbush forest (A) and a cactus forest (B) in the area of Carora, Venezuela. In A the trees are Mimosaceae (1), the Apocynaceae Plumería alba (2), and the Cac-taceae Pereskia guamacho (3). The only cactus is Cereus jamacaru (4). In B Cactaceae dominate the higher and lower strata. (Vareschi 1980, with kind permission of R. Ulmer)

Fig. 3.11A,B Transects of a thornbush forest (A) and a cactus forest (B) in the area of Carora, Venezuela. In A the trees are Mimosaceae (1), the Apocynaceae Plumería alba (2), and the Cac-taceae Pereskia guamacho (3). The only cactus is Cereus jamacaru (4). In B Cactaceae dominate the higher and lower strata. (Vareschi 1980, with kind permission of R. Ulmer)

bidding muddle of thorny cacti, particularly Opuntia wentiana and O. caribea. This may be due to a large extent to overgrazing by goats. At places where the access of goats is prevented, grasses are entering these cactus-forests, which then obtain a physiognomy more comparable to the equivalent thornbush-savanna in Africa.

The climate at the sites of these thornbush-succulent forests is strongly seasonal and very dry during most of the year. The Klimadiagramm of Carora (Fig. 3.14) for example only shows a pronounced wet period from September to December and a very short rainy season in May to June. The open canopy of these deciduous forests allows much penetration of full sunlight, and the plants are subject to stress by:

• limited nutrient availability especially nitrogen (N).

The ecophysiological responses of plants in dry tropical forests will be discussed in Sect. 5.2).

Fig. 3.12 Thornbush-succulent forest in SW Madagascar with Didieraceae and Euphorbiaceae. (Photographs courtesy M. Kluge)
Fig. 3.13A-C Caatinga in the state of Ceara, Brazil (A), and thornbush-succulent forests near Carora, Venezuela (B, C)

Fig. 3.14 Klimadiagramm of the thornbush-succulent forest site at Carora. (Vareschi 1980, with kind permission of R.

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Renewable Energy 101

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