PARENT SESSION
Symposium #26: Gasping for CO₂: Ecological Effects of Past Variations in Atmospheric CO2.

Organized by: JW Williams and S Cowling
Thursday, August 8. 9:00 AM to 11:30 AM. Turquoise Ballroom, TCC.

What controls the historical and current abundances of C4 plants.

Ehleringer, James^*,1, Cerling, Thure¹, ¹ University of Utah, Salt Lake City, UT

ABSTRACT- Approximately 80% of all C4 taxa are monocots (grasses and sedges), with the remaining 20% being dicots spread among a large number of families. There is significant interest in identifying the factors controling the evolutionary expansion and distributions of C4 taxa today and over the past 6-8 million years. In todays natural ecosystems, C4 taxa are found primarily in only five habitat types: grassland ecosystems, savanna ecosystems, monsoonal desert ecosystems, disturbed landscapes, and halophytic (saline) ecosystems. By far the broadest distributional abundances of C4 taxa occur in grassland and savanna ecosystems; in these ecosystems the C4 taxa are almost exclusively grasses. C4 taxa are far less common in desert ecosystems. Within monsoonal desert ecosystems and disturbed landscapes, both C4 monocots and dicots can be found. With the exception of the monsoonal desert and halophytic ecosystems, none of these are classified as arid ecosystems. Instead C4 taxa are found primarily in semi-arid and humid ecosystems. This contrasts with the notion that C4 photosynthesis is an adaptation to drought or that C4 photosynthesis is common in arid regions. In this presentation, we review the data on CO₂ and temperature as factors influencing the paleo- and current-distributions of C4 monocots. We evaluate the seasonality of precipitation as a compounding factor influencing interpretations of the climatic factors influencing the abundances of C4 plants under both current and paleo conditions.

KEY WORDS: carbon dioxide, photosynthesis, stable isotope