Forests and savannas are the ecosystems which cover the largest areas in the tropics (Fig. 1.3). Notwithstanding the wide public concern about destruction and decline of tropical forests, it appears that it is deeply ingrained in the nature of man to be frightened by the unknown in the darkness of dense and repellent forests. There were incidents of natural catastrophes in which falling trees and landslides with forests threatened man, as a conequence of which a call arose to remove the forests altogether. The contrast in our emotional reactions towards forest and savanna, respectively, is vividly expressed by Alexander von Humboldt (1982) in his Journey to South America:
"If one has spent many months in the dense forests along the Orinoco, if one got used there to seeing the stars only near the zenith like looking upwards from the bottom of a well as soon as one leaves the bed of the river, then wandering over the steppes2 has something pleasant and attractive in it. The new pictures which one perceives give deep impression; like the Llanero one enjoys the feeling of being able to look around so well. However, this comfort does not last long. If, wandering for eight to ten days, one gets used to the games of mirages and the brilliant green of Mauritia bushes, which appear mile after mile, one feels the desire for more variable impressions, one longs for the sight of the huge trees of the tropics ..."3
2 Note that A. von Humboldt did not yet distinguish between "steppes" and "savannas", which we now consider as the grasslands of temperate and tropical zones respectively (Vareschi 1980; Walter and Breckle 1984).
3 "Hat man mehrere Monate in den dichten Wäldern am Orinoko zugebracht, hat man sich dort daran gewöhnt, daß man, sobald man vom Strome abgeht, die Sterne nur in der Nähe des Zenit und wie aus einem Brunnen heraus sehen kann, so hat eine Wanderung über die Steppen etwas Angenehmes, Anziehendes. Die neuen Bilder, die man aufnimmt, machen großen Eindruck, wie dem Llanero ist einem ganz wohl, 'daß man so gut um sich sehen kann'. Aber dieses Behagen ist nicht von langer Dauer. Ist man nach acht- oder zehntägigem Marsch gewöhnt an das Spiel der Luftspiegelung und an das glänzende Grün der Mauritiabüsche, die von Meile zu Meile zum Vorschein kommen, so fühlt man in sich das Bedürfnis mannigfaltigerer Eindrücke, man sehnt sich nach dem Anblick der gewaltigen Bäume der Tropen ...".
Commercially it is important to remember that most tropical countries have to sustain increasingly large populations, savannas serving agriculture and forests providing resources, which are, however, renewable only to a limited extent. Although original CO2 sequestration and hence gross primary productivity of tropical forests is high, due to high rates of respiration and the rapid degradation of litter in the tropical forests, net CO2 uptake and net productivity is much reduced. It may even be lower than in a beech forest of the temperate zone (Fig. 1.4).
Large-scale deforestation proves to be irreversible (Medina 1991). Previous land use determines successions and recovery. In the Central Amazon it was found that sites which had been clear cut without subsequent use were dominated 6-10 years later by the pioneer genus Cecropia and had a high diversity, while sites used for pasture before abandonment were dominated by the pioneer genus Vismia and had a lower diversity. Seed source, effects on soil and mineral depletion lead to a more rapid return of primary forest species if deforestation is not followed by the use as pastures before abandonment (Mesquita et al. 2001). Therefore, it is possible to reconcile utilization and preservation of tropical ecosystems if human activities are directed in the right way, as vividly summarized by Whitmore (1990). For example, there are two types of shifting agriculture (slash-and-burn agriculture; Fig. 1.5). One of them is destructive and unsustainable. It is an invasive system, where fields are used until they are exhausted even for regrowth of secondary forest. The other is a cyclic system, where clearings are used for cultivation for 1-2 years and then left to recover so that they can be reused in due course during the cycle, without the need for further clearing of forest. This is a sustainable mode of shifting agriculture. Figure 1.6 shows the recovery of above-ground biomass of a wet tropical forest after a slash and burn activity. Clearly, full recovery to a level comparable to mature original forest may take 100-200 years if the magnitude of disturbance has not been too
large. However, if short-term cultivation after slash-and-burn action is followed by fallow periods of 20 years or longer, recovery mechanisms of forest ecosystems remain intact, and long-term cyclic utilization under low population pressure remains possible.
Scientifically, sustainable schemes of silviculture have been developed for tropical areas and should be extended and enforced, including appropriate methods of logging and timber removal (Whitmore 1990). Afforestation and timber plantations are established on degraded sites and may reduce the pressure on good natural forest. Trees exotic to the native tropical environments are frequently used for such aforestations. In Ethiopia the introduction of Eucalyptus historically has been praised for having saved the country economically (Zewde 1992). In Brazil the total area covered by plantations of Eucalyptus has increased 8-fold during 30 years in the last century (Fig. 1.7). Ecologists have underlined the detrimental effects of exotic tree plantations on the indigenous environment. The tremendous difference in ecological quality between a more or less undisturbed woodland and an exotic timber plantation may become immediately clear at a glance (Fig. 1.8). However, there are also great advantages where degradation of ecosystems leaves no other choice. Some of the disadvantages and advantages are compared in Table 1.1: (i) potential harmful effects on soil properties are juxtaposed by experience with propagation of exotic tree species and silviculture, (ii) displacement of local native vegetation and fauna must be compared with improved productivity, (iii) susceptibility to epidemic diseases and pests can be counterbalanced by various nurse effects of exotic tree plantations.
An appropriate forestry management can back up the advantages and an exotic Eucalyptus plantation does not need to look as sterile as that of Fig. 1.8B (see Fig. 1.9C,D). This is supported by the case story of a Eucalyptus saligna plantation
Fig. 1.7 Area covered by plantations of Eucalyptus in Brazil from 1965 to 1995 (DaSilva et al. 1995)
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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.