The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas

被引:301
作者
Bond, WJ [1 ]
Midgley, GF
Woodward, FI
机构
[1] Univ Cape Town, Dept Bot, ZA-7701 Rondebosch, South Africa
[2] Natl Bot Inst, Climate Change Res Grp, ZA-7735 Claremont, South Africa
[3] Conservat Int, Ctr Appl Biodivers Sci, Washington, DC 20036 USA
[4] Univ Sheffield, Dept Anim & Plant Sci, Sheffield S11 9PE, S Yorkshire, England
关键词
C-4; grass; fire; low CO2; palaeoecology; savanna;
D O I
10.1046/j.1365-2486.2003.00577.x
中图分类号
X176 [生物多样性保护];
学科分类号
090705 ;
摘要
The distribution and abundance of trees can be strongly affected by disturbance such as fire. In mixed tree/grass ecosystems, recurrent grass-fuelled fires can strongly suppress tree saplings and therefore control tree dominance. We propose that changes in atmospheric [CO2] could influence tree cover in such metastable ecosystems by altering their postburn recovery rates relative to flammable herbaceous growth forms such as grasses. Slow sapling recovery rates at low [CO2] would favour the spread of grasses and a reduction of tree cover. To test the possible importance of [CO2]/fire interactions, we first used a Dynamic Global Vegetation Model (DGVM) to simulate biomass in grassy ecosystems in South Africa with and without fire. The results indicate that fire has a major effect under higher rainfall conditions suggesting an important role for fire/[CO2] interactions. We then used a demographic model of the effects of fire on mesic savanna trees to test the importance of grass/tree differences in postburn recovery rates. We adjusted grass and tree growth in the model according to the DGVM output of net primary production at different [CO2] relative to current conditions. The simulations predicted elimination of trees at [CO2] typical of the last glacial period (180 ppm) because tree growth rate is too slow (15 years) to grow to a fire-proof size of ca. 3 m. Simulated grass growth would produce an adequate fuel load for a burn in only 2 years. Simulations of preindustrial [CO2] (270 ppm) predict occurrence of trees but at low densities. The greatest increase in trees occurs from preindustrial to current [CO2] (360 ppm). The simulations are consistent with palaeo-records which indicate that trees disappeared from sites that are currently savannas in South Africa in the last glacial. Savanna trees reappeared in the Holocene. There has also been a large increase in trees over the last 50-100 years. We suggest that slow tree recovery after fire, rather than differential photosynthetic efficiencies in C-3 and C-4 plants, might have been the significant factor in the Late Tertiary spread of flammable grasslands under low [CO2] because open, high light environments would have been a prerequisite for the spread of C-4 grasses. Our simulations suggest further that low [CO2] could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbance, with fire favouring the spread of grasses.
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页码:973 / 982
页数:10
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