Stable isotope and modelling evidence for CO sub(2) as a driver of glacial-interglacial vegetation shifts in southern Africa

Atmospheric CO sub(2) concentration is hypothesized to influence vegetation distribution via tree-grass competition, with higher CO sub(2) concentrations favouring trees. The stable carbon isotope ( delta super(13)C) signature of vegetation is influenced by the relative importance of C sub(4) plants...

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Bibliographic Details
Published in:Biogeosciences 2013-03, Vol.10 (3), p.2001-2010
Main Authors: Bragg, F J, Prentice, I C, Harrison, S P, Eglinton, G, Foster, P N, Rommerskirchen, F, Rullkotter, J
Format: Article
Language:English
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Summary:Atmospheric CO sub(2) concentration is hypothesized to influence vegetation distribution via tree-grass competition, with higher CO sub(2) concentrations favouring trees. The stable carbon isotope ( delta super(13)C) signature of vegetation is influenced by the relative importance of C sub(4) plants (including most tropical grasses) and C sub(3) plants (including nearly all trees), and the degree of stomatal closure - a response to aridity - in C sub(3) plants. Compound-specific delta super(13)C analyses of leaf-wax biomarkers in sediment cores of an offshore South Atlantic transect are used here as a record of vegetation changes in subequatorial Africa. These data suggest a large increase in C sub(3) relative to C sub(4) plant dominance after the Last Glacial Maximum. Using a process-based biogeography model that explicitly simulates super(13)C discrimination, it is shown that precipitation and temperature changes cannot explain the observed shift in delta super(13)C values. The physiological effect of increasing CO sub(2) concentration is decisive, altering the C sub(3)/C sub(4) balance and bringing the simulated and observed delta super(13)C values into line. It is concluded that CO sub(2) concentration itself was a key agent of vegetation change in tropical southern Africa during the last glacial-interglacial transition. Two additional inferences follow. First, long-term variations in terrestrial delta super(13)Cvalues are not simply a proxy for regional rainfall, as has sometimes been assumed. Although precipitation and temperature changes have had major effects on vegetation in many regions of the world during the period between the Last Glacial Maximum and recent times, CO sub(2) effects must also be taken into account, especially when reconstructing changes in climate between glacial and interglacial states. Second, rising CO sub(2) concentration today is likely to be influencing tree-grass competition in a similar way, and thus contributing to the "woody thickening" observed in savannas worldwide. This second inference points to the importance of experiments to determine how vegetation composition in savannas is likely to be influenced by the continuing rise of CO sub(2) concentration.
ISSN:1726-4170
1726-4189
DOI:10.5194/bg-10-2001-2013