Abundant C4 plants on the Tibetan Plateau during the Lateglacial and early Holocene

Plants using the C4 (Hatch-Slack) photosynthetic pathway are key for global food production and account for ca 25% of terrestrial primary productivity, mostly in relatively warm, dry regions. The discovery of modern naturally-occurring C4 plant species at elevations up to 4500 m in Tibet and 3000 m in Africa and South America, however, suggests that C4 plants are present in a wider range of environments than previously thought. Environmental conditions on the Tibetan Plateau, including high irradiance, rainfall focused in summer, and saline soils, can favor C4 plants by offsetting the deleterious effects of low growing season temperature. We present evidence based on leaf wax carbon isotope ratios from Lake Qinghai that C4 plants accounted for 50% of terrestrial primary productivity on the northeastern Tibetan Plateau throughout the Lateglacial and early Holocene. Despite cold conditions, C4 plants flourished due to a combination of factors, including maximum summer insolation, pCO2 ca 250 ppmv, and sufficient summer precipitation. The modern C3 plant-dominated ecosystem around Lake Qinghai was established ca 6 thousand years ago as pCO2 increased and summer temperature and precipitation decreased. C4 plants were also intermittently abundant during the Last Glacial period; we propose that C4 plants contributed a significant portion of local primary productivity by colonizing the exposed, saline Qinghai Lake bed during low stands. Our results contrast with state-of-the-art ecosystem models that simulate <0.5% C4 plant abundance on the Tibetan Plateau in modern and past environments. The past abundance of C4 plants on the Tibetan Plateau suggests a wider temperature range for C4 plants than can be inferred from modern distributions and model simulations, and provides paleoecological evidence to support recent findings that C4 plant evolution and distribution was determined by a combination of climatic and environmental factors (temperature, irradiance, precipitation amount and seasonality, and soil salinity). Moreover, this finding highlights the exceptional sensitivity of high-elevation ecosystems to environmental change, and provides critical benchmarks for ecosystem model validation.