Pollen records indicate the episodic expansion of grasslands in alpine tropical regions of Hawaii, Africa, and Central and South America throughout the Quaternary. Although C4 plants make up less than 5% of the biomass of modern alpine tropical grasslands, stable-carbon isotope data suggest that C4 plants were an important component of past grasslands. The isotope data reveal, in particular, the widespread presence of abundant C4 grasses in tropical ecosystems during the last glacial period. The increased abundance of C4 plants during the last glaciation relative to the present time points to a common mechanism responsible for a global expansion of C4 grasses. In this study, carbon isotope data of soil profiles and lake sediments from the Colombian Andes are employed to evaluate the possible mechanisms responsible for the glacial expansion of C4 plants in the tropical Andes.

Isotopic analysis of lacustrine organic matter reveals the abundant presence of C4 grasses in the Bogota Basin of the Colombian Andes during the last glaciation. Confirming this conclusion, soil horizons developed in the vicinity of the lake during the same time interval exhibit organic matter with high stable-carbon isotope values indicative of a C4-dominated ecosystem. In contrast, isotope data of soil horizons developed during the same time interval, but located above 3,000 m in altitude show no evidence of C4 grasses. In agreement with this finding, outcomes from the Biome-3 model indicate that above this altitude, low temperatures (<3oC) favored the development of C3-dominated grasslands over C4-dominated ecosystems during glacial intervals. Model outputs suggest that the expansion of C4 grasses in the northern Andes could result from lower concentrations of atmospheric carbon dioxide prevailing during glacial intervals. Model outputs also suggest that a 50% decrease in rainfall could also produce a C4-dominated grassland ecosystem in the tropical Andes.

Our combined isotopic and modeling results indicate the sensitivity of alpine tropical regions to climate change. In particular, these results suggest that lower concentrations of carbon dioxide and/or lower precipitation rates can promote the expansion of C4 plants in tropical regions even under cooler conditions characteristic of glacial intervals.