PLEISTOCENE COLONIZATION OF TROPICAL HIGH-ALTITUDE GRASSLANDS BY C4 GRASSES

Pollen and fossil records suggest that grasslands occupied larger areas in tropical regions, including alpine regions, during the last glacial interval. In particular, records from several ecotones in tropical South America reveal a reduction of rainforest coverage during the last glacial maximum and possibly during the early Holocene. C3 plants were assumed to dominate glacial Andean grasslands because modern C4 plants are abundant in tropical grassland ecosystems experiencing high light and warm conditions and because these C4 grasses are rare in alpine environments experiencing cold temperatures. It has been postulated that the rare occurrence of C4 grasses in cold climates is related to an inherent weakness of the C4 photosynthetic pathway at low temperatures possibly associated with the reduced regeneration of the enzyme phosphoenolpyruvate. However, isotope data from sediments recovered from a lake in the Colombian Andes indicate the prevalence of C4 plants at this high-altitude site during glacial intervals. Data from other tropical alpine regions also show the significant presence of C4 grasses during the last glacial interval, but the mechanism causing this alpine expansion of C4 grasses is still unknown.

A comparison of the timing of the expansion of C4 grasses in the Andes and records of temperature, rainfall, and concentrations of carbon dioxide not only reveals significantly lower temperatures in the Andes during glacial intervals, but also indicates decreased concentrations of atmospheric carbon dioxide and likely reduced rainfall. The prevalence of New World C4 grasses in the sub-family Chloridoideae over C3 plants in the Andes during the last glacial interval suggests the adaptation of lowland C4 plants to cold conditions despite the suggested relatively recent evolution of the C4 syndrome in warm tropical settings. This adaptation to cold temperatures indicates that the modern scarcity of alpine C4 grasses in the Andes is not related to cold temperatures, but it is related to the result of other conditions that favor C3 plant growth.