PARENT SESSION
Oral Session #54: Fire Ecology -- Trees, forests, woodlands.
Presiding: E. Menges
Wednesday, August 7. 8:00 AM to 11:30 AM. Apache Meeting Room, TCC.

Atmospheric CO₂, Fire and tree/grass interactions .

Bond, William^*,1, Midgley, Guy², Woodward, Ian³, ¹ Botany Department, UCT, Cape Town, South AFrica² National Botanical Institute, Kirstenbosch, South AFrica³ Plant and Animal Sciences, U. of Sheffield, Sheffield, U.K.

ABSTRACT- Trees and grass coexist in a precarious mix in tropical and sub-tropical savannas. Coexistence is locally unstable and one of other growth form may increase to dominate a site. Changes in the relative proportions of trees and grasses have occurred over historical, pre-historical and geological time spans. Over the last century or two, tree and shrub cover has increased in many parts of the world causing negative impacts on pastoral farming and conservation of savanna biota. Mesic savannas are subject to very frequent fires which prevent release of juvenile trees into adults size classes where fire damage is reduced. The frequency of release events is determined by the frequency and severity of fires but also the growth rates of juvenile trees. Here we report a simulation study of the effects of CO₂ on stem growth rates, and the consequences for woody plant cover in savannas. Our results, for South African savannas, suggest that trees would have been absent during glacial periods because of low CO₂ effects alone. Trees would have been present, but at low biomass, at pre-industrial CO₂ levels, but the model predicts much higher tree densities under current CO₂. These results are broadly consistent with the historical records for vegetation in the region. They suggest that variation in CO₂ alone can have major effects on tree biomass in fire-prone systems independently of climate change.

KEY WORDS: savanna, CO2 effects, vegetation change