All plants use C3 photosynthesis, which has been supplemented in some species with Crassulacean acid metabolism (CAM) and C4 photosynthesis. These two pathways are adaptations to dry environments. CAM photosynthesis, which occurs in about 10% of plant species, fixes CO2 in the dark and the resulting organic acids are deacidified during the day. Plants with CAM close their stomata during the day, which is an effective response to drought conditions and is preferentially found in desert-dwelling succulents, for example, cactuses and agaves, or in arboreal plants, for example, bromeliads and orchids, that is, in dry microenvironments. CAM has low levels of primary production.
C4 photosynthesis occurs in only c. 5% of plant species, principally in monocotyledon sedges and grasses and in some xeric-tolerant dicotyledon families, including chenopods and euphorbs. This form of photosynthesis occurs most commonly in arid environments, especially in regions with warm-season rainfall. In environments with both C3 and C4 plants, C4 plants occur principally in microenvironments that are warm and dry. C3 plants, which constitute about 85% of all plant species, including bryophytes, ferns, gymnosperms, and most dicotyledons and aquatic plants, preferentially occur in cooler and moister microenvironments.
C3 photosynthesis is often limited by the amount of CO2 that is available, whereas C4 is usually saturated at relatively low CO2 concentrations. Thus, while C4 species has greater productive yields than C3 species at low CO2 concentrations, C3 species outcompetes C4 species at elevated CO2 concentrations. In Mexico, for example, there was an increase in C3 species about 9000 years ago, which correlated with an increase in CO2, in spite of a simultaneous increase in aridity, which based on water conservation presumably would have led to an increase in C4 species. C3 plants also have increased in abundance since the beginning of the industrial age, possibly in association with the increase in CO2.
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