Summer Every Day Winter Every Night

The cold tropics (Sect. 1.2) comprise the "regions within the tropics occurring between the upper limit of continuous, closed-canopy forest (often around 3,500-3,900m) and the upper limit of plant life (often around 4,600-4,900m)". In this way Rundel et al. (1994a) define "tropical alpine environments". They use "alpine" as a more general term in an attempt to avoid regional terms like páramo and jalca for the moist Andes and puna for the drier Andes in South America and Afro-alpine and moorland in Africa. However, "alpine" is also a regional term applying to environments outside the tropics. On the other hand, since the conditions and the physiognomy of vegetation are similar on tropical mountains in different continents, especially in Africa and South America, we might as well choose the term páramo. Increasingly, this is used as the general term to describe vegetation in the cold tropics extending from somewhat above 3,000 m to nearly 5,000 m above sea level (Fig. 12.1).

The high altitude tropical environments were succinctly described by Hed-berg's (Hedberg 1964a) aphorism "summer every day and winter every night". The most important feature of the tropical alpine zone is the "Frostwechselklima"1 (Troll 1943) with an extraordinarily high amplitude of day/night fluctuations of humidity and especially of temperature (Fig. 12.2). Clearly, general characteristics of tropics are strongly accentuated in these high altitudes, such that daily oscillations of temperature are much more pronounced than the seasonal ones. Thus, nocturnal frosts followed by high day-time temperatures represent the most conspicuous stress to which plants are exposed in these environments. Additional, often very important stressors, are limited water supply and mineral nutrition.

1 Day-night freezing climate.

Fig. 12.1A-C Profiles of high tropical mountains in South America (A Chimborazo and Cotopaxi of the Andes) and Africa (B Mount Kenya; C Mt. Kilimanjaro) with the altitudinal vegetation zones indicated, and in C also the approximate annual precipitation. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

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Fig. 12.1A-C Profiles of high tropical mountains in South America (A Chimborazo and Cotopaxi of the Andes) and Africa (B Mount Kenya; C Mt. Kilimanjaro) with the altitudinal vegetation zones indicated, and in C also the approximate annual precipitation. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

Fig. 12.2A,B Thermohygrograms obtained in September 1981 at the Shira Plateau on Mt. Kilimanjaro at about 3,950 m (A) and in March 1985 on Mt. Kenya at 4,200 m altitude (B). Upper part of each graph temperature ° C; lower part relative humidity %. (Courtesy E. Beck)

12.2 The Stress Factor Frost

With temperatures below 0 0C every night, frost is the permanently dominating environmental stress factor ("stressor") in the tropical alpine habitat. Some relations between cold stress and cold resistance are presented in Fig. 12.3 according to the stress concept (see Sect. 3.3.2; Box 3.1).

First of all we need to distinguish between low-temperature stress at temperatures above the freezing point (0 to +6 0C) and stress caused by sub-freezing temperatures. Low-temperature stress may lead to a loss of fluidity of membrane lipids or to an increase in membrane rigidity with many consequences for membrane permeability and intracellular compartmentation. It also slows down many metabolic reactions. It may cause injury, elastic and plastic strain, and it requires chilling resistance. It is largely presented by the upper panel in Fig. 12.3 but will not be discussed here further, since we are dealing with sub-freezing temperatures

Fig. 12.3 Cold stress (left part) and cold resistance (right part) with low temperatures above the freezing point (upper panel) and subfreezing temperatures (lower panels) as stress factors. The terminology presented in this scheme according to the stress concept (see Box 3.1) is used in the present chapter to discuss adaptations of páramo plants

Fig. 12.3 Cold stress (left part) and cold resistance (right part) with low temperatures above the freezing point (upper panel) and subfreezing temperatures (lower panels) as stress factors. The terminology presented in this scheme according to the stress concept (see Box 3.1) is used in the present chapter to discuss adaptations of páramo plants in the páramo habitat. Sub-freezing temperatures sooner or later may lead to ice formation, i.e. the crystallization of cellular water. For stress resistance, the location of ice crystal formation in the cells is critical. If it is on the outer face of the cell walls, i.e. apoplastic, ice formation is tolerable and thus frost resistance may be achieved through freezing tolerance. However, if ice crystals are formed in the cell interior, i.e. intracellularly, this always leads to cell death, and frost resistance can only be achieved by freezing avoidance. These two cases are represented by the lower two panels in Fig. 12.3. They constitute options with different advantages and disadvantages, and it is interesting to note that Afro-alpine species commonly tolerate extracellular freezing while Andean species apparently rely on the freezing avoidance mechanism, as will be shown below in Sects. 12.4.1 and 12.4.2 respectively.

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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.

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