U

1 10 102 103 Average plant height (cm)

ies and mats can be much different and highly protected from the outside (Reisigl and Keller 1987). The acaulescent rosettes (Fig. 12.11) closely adopt the soil temperature. In the Andes, Squeo et al. (1991) found that all ground-level plants (cushions and acaulescent rosettes) were freezing tolerant. Day-night temperature changes are somewhat buffered by the heat-storage capacity of the soil and stress may be less extreme than from air temperature alone (Fig. 12.12A). Moreover, for taller plants, a specific problem of the day-night freezing climate is not only the freezing during the night but also the process of thawing at the beginning of the day. If ice formed in leaves thaws more rapidly than ice in the stems, transpiration sets in, while water transport in the shoot is still blocked. Thus, thawing every morning after regular nocturnal freezing leads to cavitation in the conductive elements of the xylem and the serious problems of embolism (Ball et al. 2006).

This is avoided in acaulescent rosettes, where the whole plant body thaws at the same time. An example is given in Fig. 12.13 showing leaf temperatures and corresponding maximum photosynthetic electron transport rates at light saturation (ETRmax) in Haplocarpha rueppellii (Fig. 12.11A) during a clear day at 4,100 m a.s.l. in the Simien Mountains in Ethiopia. As irradiance (PPFD in Fig. 12.13) increased after sunrise, the rosettes thawed and leaf temperatures increased to the average value of 20 0C when a PPFD of 400 jumol m-2 s-1 was reached. Above 400 |u mol m-2s-1 leaf temperature was independent of irradiance which clearly indicates dissipation of heat via the soil. Maximum rates of photosynthetic electron transport obtained from measurements of chlorophyll fluorescence (Box 4.6) at light saturation were linearly related to leaf temperature between ~ 5 and ~ 20 0 C indicating limitation of photosynthesis by temperature dependence of electron transport in the thylakoids and/or biochemical reactions of CO2-assimilation.

Some acaulescent rosette plants follow the CAM mode of photosynthesis, e.g. Echeveria columbiana (Crassulaceae) in the Andes (Fig. 12.11B) (Medina and Delgado 1976). These plants thus must be able to maintain the intensive metabolism during the night as it is required for dark CO2-fixation. They may possibly achieve this in a state of supercooling or non-ideal equilibrium freezing (see Sect. 12.4).

Fig. 12.11 Small acaules-cent rosettes of Haplocarpha rueppellii, Simien Mountains, Ethiopia, 4,100 m a.s.l. (A) and Echeveria columbiana, Aguila Pass, Venezuela, 3,600 m a.s.l. (B) and a cushion of Helichry-sum newii, Shira Plateau, Mt. Kilimanjaro, Tanzania (C). (C Courtesy E. Beck)

Fig. 12.11 Small acaules-cent rosettes of Haplocarpha rueppellii, Simien Mountains, Ethiopia, 4,100 m a.s.l. (A) and Echeveria columbiana, Aguila Pass, Venezuela, 3,600 m a.s.l. (B) and a cushion of Helichry-sum newii, Shira Plateau, Mt. Kilimanjaro, Tanzania (C). (C Courtesy E. Beck)

Fig. 12.12A, B Air temperature and soil temperature (A) and subepidermal temperatures (B) of the hairy and the hairless variety of Tephrocactus floccosus growing at the same site. (Keeley and Keeley 1989)
Fig. 12.13 Relationships between leaf temperature and ambient irradiance (PPFD; dotted t and maximum photosynthetic electron transport rate at light saturation (ETRmax; solid line) in acaulescent rosettes of Haplocarpha rueppellii (see Fig. 12.11A). (Unpubl. results of the author)

In the sclerophyllous shrubs transpiration is reduced and water economy is sustained by leaf-xeromorphy with reduced leaf surfaces, folded leaves and other morphological/anatomical adaptations.

Rundel et al. (1994b) invoke two different reasons for the diversity of life forms found in páramo ecosystems, namely:

• the complexity of environmental stresses, i.e. the diurnal freezing cycles and water and nutrient deficiencies, and

• the occurrence of microhabitat mosaics.

However, additional research is required to determine the correlations between life forms, habitat conditions, and physiological characteristics.

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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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