Stable isotope and modelling evidence for CO₂ as a driver of glacial–interglacial vegetation shifts in southern Africa

• Main Authors: F. J. Bragg, I. C. Prentice, S. P. Harrison, G. Eglinton, P. N. Foster, F. Rommerskirchen, J. Rullkötter
• Format: Article
• Language: English
• Published: Copernicus Publications 2013-03-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/10/2001/2013/bg-10-2001-2013.pdf

Atmospheric CO₂ concentration is hypothesized to influence vegetation distribution via tree–grass competition, with higher CO₂ concentrations favouring trees. The stable carbon isotope (&delta;¹³C) signature of vegetation is influenced by the relative importance of C₄ plants (including most tropical grasses) and C₃ plants (including nearly all trees), and the degree of stomatal closure – a response to aridity – in C₃ plants. Compound-specific &delta;¹³C 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 C₃ relative to C₄ plant dominance after the Last Glacial Maximum. Using a process-based biogeography model that explicitly simulates ¹³C discrimination, it is shown that precipitation and temperature changes cannot explain the observed shift in &delta;¹³C values. The physiological effect of increasing CO₂ concentration is decisive, altering the C₃/C₄ balance and bringing the simulated and observed &delta;¹³C values into line. <br><br> It is concluded that CO₂ 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;¹³Cvalues 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, CO₂ effects must also be taken into account, especially when reconstructing changes in climate between glacial and interglacial states. Second, rising CO₂ 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 CO₂ concentration.