Interpreting C₃ and C₄ Vegetation Dynamics in the Pleistocene from a Physiological Perspective

Rowan F. Sage. Department of Botany, University of Toronto, 25 Willcocks Street. Toronto, ON M5S3B2 Canada

The atmospheric CO₂ content has been below 280 ppm for 96% of the past 400,000 years, and below 200 ppm for at least 20% of the past 400,000 years. Low CO₂ should favor C₄ over C₃ plants, even at cooler temperatures where C₃ species are currently favored. Consistently, the paleoecological record indicates that in tropical climates, there was an expansion of C₄-dominated biomes at the expense of C₃-dominated biomes in the late-Pleistocene. In temperate latitudes, theoretical models also predict expansion of C₄-dominated biomes, but this is not generally observed. For example, in southern North America, there should be a shift from C₄ to C₃ dominated biomass at 15 to 12 kya, when atmospheric CO₂ levels rose from 180 to 260 ppm; however, most sites from the Great Plains show C₃ dominance over this period. C₄ grasses expand across the Great Plains following the warming of the climate between 10 and 5 kya. These results indicate low CO₂ affects C₃ relative to C₄ biomass less then expected, and that temperature is the more important control. To further examine this issue, we have been studying mechanisms controlling C₃ and C₄ photosynthesis at a range of temperature, CO₂ and nutrient levels. Two reasons for the failure of C₄ plants to exploit low CO₂ in temperate biomes have been identified. First, CO₂ responsiveness requires an abundance of mineral nutrients. In the absence of soil fertility, photosynthetic carbon acquisition appears to be sufficient at 180 ppm to meet growth demands. For example, in White Lupines grown under phosphorous deficiency, increasing CO₂ from 180 to 380 ppm has little effect on photosynthesis and growth. Hence, where nutrient supply is limiting, ecological success could be more dependent on nutrient acquisition and use rather than CO₂ supply, and in this regard, C₄ versus C₃ photosynthetic differences may minor. Second, there may be inherent weaknesses within the C₄ photosynthetic apparatus that limits performance in cool climates, regardless of CO₂ level. If C₄ plants exploited temperate environments 20,000 years ago, they would need to have been competitive in the cooler climates of the time. To examine this issue, we have been studying photosynthetic temperature responses species in species of the genera Amaranthus, Bouteloua, and Muhlenbergia that occur at the cold extreme of the C₄ distribution range. These species have evolved tolerance of cold during the growing season, and therefore are excellent models to study the potential of C₄ species to exploit low CO₂ conditions during cooler periods. One alpine C₄ grass, Muhlenbergia richardsonis, can even tolerate freezing during the growing season. Gas exchange work with these species indicate they lack the ability to match the photosynthetic performance of C₃ species at low temperature, possibly due to a strong limitation imposed by low levels of Rubisco activity. C₄ species contain less than half as much Rubisco as C₃ species of similar life-form, and this reduced amount of Rubisco may impose a low ceiling on C₄ photosynthesis at cool temperature. Low expression of Rubisco may thus be an inherent limitation that restricts C₄ performance in the cold, regardless of CO₂ level, and warmer conditions may be required to remove this limitation. Consistently, M. richardsonis in the alpine zone is restricted to microsites where leaf temperatures during the day can rise to levels where C₄ photosynthesis is favored.