PARENT SESSION
Monday, August 7, 5:00-6:30 pm
Poster Session 6 - Biogeochemistry and nutrient cycling
Exhibit Hall, Ballroom Level, Cook Convention Center

Atmospheric CO₂ controls carbon allocation and ecological success in African savanna trees.

Kgope, Barney^*,1, Midgley, Guy¹, Bond, William², Nagy, John³, Woodward, Ian⁴, ¹ South African National Biodiversity Institute, Claremont, South Africa² University of Cape Town, Rondebosch, South Africa³ Brookhaven National Laboratory, Upton, NY⁴ University of Sheffield, Sheffield, United Kingdom

ABSTRACT- Savanna ecosystem structure may be sensitive to atmospheric [CO₂]. Glacial periods saw contraction of tree cover and expansion of C₄ grasslands in Africa and elsewhere, commonly attributed to drier conditions. However, low glacial [CO₂] may have affected tree cover by reducing growth rates of saplings recovering from fire or herbivory – a problem of carbon allocation to woody shoots, and irrelevant to grasses. Simulations suggest that grass-fuelled fires eliminated African savanna trees due to low sapling re-growth rates at LGM [CO₂]. Here we report the first experimental evidence of savanna tree carbon allocation response to [CO₂] ranging from very low LGM levels to super-ambient levels. We show that changes in [CO₂] since LGM, and especially from pre-industrial to ambient conditions, have greatly enhanced growth, anti-herbivore defenses, and re-sprouting vigour of saplings. The C₄ grass showed no such responses. Increased [CO₂] may therefore have contributed significantly to expansion of trees into grasslands after the last glacial and particularly to increasing tree cover observed in many grassy ecosystems worldwide over the last century.

Key words: African Savanna, Carbohydrates, Carbon dioxide.