Separation of Different Types of Tropical Forests

In the tropics seasonality is determined by rainfall. Seasonality of rainfall is absent over only a very narrow zone 1° north and south of the equator (Fig. 3.1). Seasonality of rainfall is the most important factor determining the definition of different types of tropical forest. Hence, a detailed separation of types of tropical forests is obtained when they are related to Klimadiagramm graphs (after Walter 1973, see Sect. 2.2.1, Box 2.1), which depict the duration and severity of dry and wet seasons. Humid and arid climates, respectively, are defined by the difference between rainfall and evaporation. Positive values indicate a humid climate, where precipitation is larger than evaporation, and negative values mark arid climates with evaporation larger than precipitation. As we have demonstrated above (Sect. 2.2.1), the Klimadiagramm technique based on temperature and precipitation is highly successful in demonstrating the degree of aridity and humidity of individual stations as well as large areas. In contrast to data on temperature and precipitation, which usually are readily available from many weather stations, measurements of free evaporation are scarce. Other factors, especially wind, may significantly affect evaporation. Thus, where evaporation data is available, it can add much to the information provided by Klimadiagramm graphs. As seen in the examples of seven stations in Venezuela (Fig. 3.2) the results of Klimadiagramm graphs regarding the occurrence of humid seasons is roughly confirmed by the rainfall minus evaporation (R — E) index. However, the wet seasons generally appear somewhat shorter with the R — E index, and on the basis of this criterion small apparent humid seasons of the Klimadiagramm graphs disappear entirely (Fig. 3.2 diagrams 1 and 2). These seven stations carry different types of forest and represent a latitudinal transect from the wet continental areas close to the equator to the drier coastal regions in the north of Venezuela (Fig. 3.3). A very strong effect of latitude on the expression of tropical forest types is highly evident, which agrees with the predictions of Fig. 3.1.

Using India and Venezuela as examples comparison of forest types is made with a diagram of precipitation versus extension of dry periods (Fig. 3.4). The corresponding Klimadiagramm graphs for Venezuela are shown in Fig. 3.5. The major

Fig. 3.1 Dry and wet seasons in the subtropical and tropical zone with zenithal precipitation. Wet seasons hatched, dry seasons dotted. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

types of forests distinguished in relation to the degree of seasonality which increases with increasing numbers of dry months per year and decreasing annual rainfall are:

• evergreen rainforest,

• seasonal rainforest,

• semi-evergreen moist and dry monsoon or trade-wind forest,

• drought-deciduous forest,

• thorn scrub and cactus forest.

Fig. 3.2 Diagrams for seven stations of a transect across Venezuela as shown in Fig. 3.3. The diagrams indicate the extensions of wet seasons according to the Klimadiagramm concept (thinner plus thicker vertical bars, i.e. where the rainfall curve is above the temperature curve; see Box 2.1), and the dry seasons according to the Klimadiagramm concept are dotted as usual. The thick line gives free evaporation. The R — E index is positive where the rainfall curve of the Klimadiagramm is above the evaporation curve, and this indicates the extensions of the wet seasons according to the R — E index (thicker vertical bars). The R — E index is negative where the rainfall curve of the Klimadiagramm is below the evaporation curve and this indicates the extensions of the dry seasons according to the R — E index (left white in the diagrams). Annual precipitation P; annual free evaporation E. (After Medina 1983, with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25, NL-1055 KV Amsterdam, The Netherlands)

Fig. 3.3 Transect across Venezuela with rainfall minus evaporation (R — E index) at different stations with various forest types. Numbers in parentheses indicate dry months per year after the R — E index. See also Klimadiagramms in Fig. 3.2. (After Medina 1983, with kind permission from Elsevier Science-NL, Sara Burger-hartstraat 25, NL-1055 KV Amsterdam, The Netherlands)

Fig. 3.3 Transect across Venezuela with rainfall minus evaporation (R — E index) at different stations with various forest types. Numbers in parentheses indicate dry months per year after the R — E index. See also Klimadiagramms in Fig. 3.2. (After Medina 1983, with kind permission from Elsevier Science-NL, Sara Burger-hartstraat 25, NL-1055 KV Amsterdam, The Netherlands)

Gradation of rainfall is also inherent in Beard's distinction (Beard 1946, 1955) of forests according to altitude (Fig. 3.6):

• low-land rainforest,

• lower montane rainforest,

• upper montane rainforest,

Some of the forest types will be discussed separately below.

Fig. 3.4A,B Forest types in India (A) and in Venezuela (B) related to annual amount of precipitation and the duration of drought periods (number of dry months per year). (After Walter and Breckle 1984 and Vareschi 1980, with kind permission of R. Ulmer)

3.2 Physiognomy of Different Types of Tropical Forests 3.2.1 Tropical Rain Forests

Tropical evergreen rainforests are said to contribute about 35% of global net primary production (Löscher et al. 2003). However, there is a problem of deciding what we define as "tropical rainforest". This is best illustrated by eight maps of Venezuela presented by Vareschi (1980), where he depicts the distribution of rainforest in this tropical country according to the views of different authors (Fig. 3.7). In the two

Fig. 3.5A-F Klimadiagramm graphs for the different forest types distinguished in the diagram of Fig. 3.4B

Fig. 3.6A-D Types of tropical forest at different altitudes. A Semi-evergreen lowland rainforest (East Venezuela). B Montane rainforest (northern range Trinidad). C Upper montane rainforest (cloud or fog forest; Rancho Grande, northern coastal range Venezuela). D Elfin forest (Serro Santa Ana, Paraguana Peninsula Venezuela)

Fig. 3.6A-D Types of tropical forest at different altitudes. A Semi-evergreen lowland rainforest (East Venezuela). B Montane rainforest (northern range Trinidad). C Upper montane rainforest (cloud or fog forest; Rancho Grande, northern coastal range Venezuela). D Elfin forest (Serro Santa Ana, Paraguana Peninsula Venezuela)

extreme cases either almost 2/3 of the whole country is covered by rainforest (upper left in Fig. 3.7) or there is no rainforest at all (lower right in Fig. 3.7); and there are gradations in between these extremes. In Brazil much of the Amazon basin is covered by rainforest. The Brazilian Atlantic forest also is a typical tropical rain forest. It belongs to the 25 biodiversity hot spots of the world. Of an original area of

Fig. 3.7 Distribution of "rainforest" (black) in the tropical country Venezuela according to different authors. (Vareschi 1980, with kind permission of R. Ulmer)

1,227,600 km2 only 91,930 km2 (7.5%) are remaining to date of which 33,084 km2 (35.9%) are protected, and there are 20,000 plant species, 8,000 of which are endemic (Myers et al. 2000).

The purist's definition of tropical rainforest requires that there should be no seasonality of rainfall whatsoever. Hence, the lack of any rainforest in Venezuela accords with the purist's definition (Fig. 3.7, lower right). If one begins to broaden the definition, of course, it becomes a matter of taste how far this term may be extended. How long should rainless periods be and in how many successive years should they occur in order to retain the term "rainforest"? Accordingly, we then derive the gradations seen in the maps of Venezuela, shown in Fig. 3.7.

In addition to seasonality there are other features which characterize tropical rainforests. One of the most conspicuous is the extraordinary diversity of tree species. In contrast to the temperate and boreal zones, where forests can be named after dominant tree species, e.g. spruce, fir, pine, beech, oak, birch etc., there is no dominance of anyone particular tree species in tropical rainforests. One may encounter up to 300 tree species per ha, which represent about 1 /3 of all plant species present. The most frequent tree species rarely represent more than 15% of all species of trees present (Jacobs 1988; Whitmore 1990). This phenomenon also makes it difficult to define the minimal quadrat of sampling plots in plant sociology. The minimal quadrat is given by the size of a plot in a system being studied above which the total number of species observed does not increase (see Sect. 3.3.1 and Fig. 3.15). Such minimal quadrats in tropical rainforests may be quite large and may never be attained.

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Renewable Energy 101

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