Geological and Recent Distribution of C3, C4 and CAM Plants
Based on the physiological characteristics of C3 and C4 plants there is differentiation of these two photosynthetic types in the earth's vegetation. Because of their high-temperature optima for photosynthesis and the ability to use high light intensities, C4 plants, particularly perennials, are more prevalent than C3 plants in subtropical savannahs and grasslands with summer rains (Fig. 2.4.18A). In contrast, the main distribution area of C3 grasses (often annuals) is in the winter cold steppes (Fig. 2.4.18 B). As the dependence on temperature and light is affected by C02 concentration in the atmosphere, the balance in the distribution of the two physiological types is not fixed.
The distribution of CAM plants in comparison to C4 plants is botanically very interesting, but is of rather secondary importance regarding the C cycle of the earth. Succulents are mainly distributed in coastal deserts with cool nights (South Africa with Namibia, Chile, Baja California, Tenerife; Fig. 2.4.18 C). The PEP-carboxylase of these coastal succulents is adapted to low night temperatures. Independent of this, CAM plants can also be found in warm, humid, moist rain forests. These include epiphytic orchids and some tree species of the genus Clusia (Clusia-ceae; Liittge 1999). The PEP-carboxylase of these genera is adapted, like C4 plants, to high temperatures, showing that the distribution pattern of CAM succulents with their major distribution in cold coastal deserts does not depend on the temperature response of PEP-carboxylase but is probably caused by competition, i.e. succulents grow too slowly in order to compete in the tropics and in the tropical grasslands the humidity at night is too low, so that stomata open during the night would also lose too much water.
Ehleringer et al. (1997) showed, based on the C02, light and temperature dependence of photosynthesis during the night, that C3 plants are competitively superior to C4 plants at increasing C02 concentrations and as climates become warmer (Fig. 2.4.19 A, B). At the time of the biggest freeze in the glacial period, the atmospheric C02 concentration was at about 200 ppm; in this C02 climate C4 plants were superior to C3 plants even at temperatures of 10 °C during the growing season. It is assumed that with increasing C02 concentrations C4 plants will be out-competed and have to move into climates with higher temperatures. For current temperature and C02 distributions on earth, C3 plants dominate at latitudes greater than 45° north and south (Fig. 2.4.19 B).
This observation is strengthened by the N requirement, which is higher for C3 than for C4 plants (Fig. 2.4.19 D). Under drier conditions, the N concentration in leaves is high due to the activation of nitrification in the soil after rain. Also, growth of plants is limited by drought and N concentration thus increases. In addition,
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