Carbon dioxide and the uneasy interactions of trees and savannah grasses

Abstract

Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO 2 ) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO 2 has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO 2 effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO 2 effects. Changes in arid savannahs seem less obviously linked to CO 2 effects and may be driven more by overgrazing. Large-scale shifts in the tree -grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth -atmosphere linkages and the global carbon cycle in ways that are still being discovered.

Climate, CO<sub>2</sub> and human population impacts on global wildfire emissions

“…However, we also find a negative impact of rising CO 2 on wildfire emissions for all tropical savannah ecosystems, which outweighs the positive impact through increasing fuel load and is caused by an increase in the dominance of woody biomes at the expense of grass vegetation. This phenomenon of shrub encroachment, or woody thickening, in tropical savannahs has been repeatedly observed in field studies (Wigley et al, 2010;Bond and Midgley, 2012) and frequently attributed to CO 2 enrichment of the atmosphere (Morgan et al, 2007;Buitenwerf et al, 2012). This link is less observed for arid savannahs (Bond and Midgley, 2012), consistent with the finding here that in the most arid regions, no decrease in the grass fraction is predicted.…”

Past, present, and future distribution of Afromontane rodents (Muridae: Otomys) reflect climate-change predicted biome changes

“…At least three hypotheses can be advanced. (Bond et al 2003, Kgope et al 2010) particularly in mesic savannas (Bond and Midgley 2012), the absence of fire, and land management factors such as increased grazing pressure (Roques et al 2001, Bond 2008, O'Connor et al 2014) with implications for ecosystem function including hydrology (e.g. Huxman et al 2005) and cascading impacts on biodiversity (Bond 2008).…”

Integration of terrestrial observational networks: opportunity for advancing Earth system dynamics modelling

“…Plant-soil interactions are changing across the globe, whether it be the encroachment of woody species into polar, alpine and temperate grassland areas (Archer et al, 1995;Jackson et al, 2002), the increase in atmospheric CO 2 concentrations that potentially alter the depth penetrations of roots (Bond and Midgley, 2012;Van Auken, 2000), or changing land cover (agriculture, forest plantations; (Van Minnen et al, 2009)). Subsurface changes to the root system architecture (root function, density and depth) alters the injection (spatial distribution) of organic carbon into the ground, controlling microbial 25 productivity and respiration, macropore location, distribution and evolution, controlling the transport of most water that moves through soil (Beven and Germann, 1982), and spatial distribution of organic acids and root respiration (generation of CO 2 ; (Jones, 1998)).…”