Continental-scale distributions of vegetation stable carbon isotope ratios

Abstract

The stable carbon isotope composition (δ13C) of terrestrial vegetation is important for a variety of applications in fields ranging from biogeochemistry to zoology to paleoclimatology. To a large degree, spatial patterns in plant δ13C are imparted by variations in the photosynthetic pathway (C3/C4) composition of vegetation in topical and subtropical regions. Thus, the fractional coverage of each vegetation type must be known in order to predict the spatial distribution of plant δ13C values. Our approach for predicting the C 3/C4 composition relies on the strong ecological sorting of C4 plants along temperature gradients, as well as the near-universal restriction of C4 photosynthesis to herbaceous growth forms. We build upon a previous approach to predict C3/C4 vegetation fractions using finer spatial resolution (500 m) MODIS datasets of vegetation growth form (i.e., percent cover of herbaceous, woody, and bare) and crop type coverage, along with precipitation and temperature climatologies. By combining these products, we predict the C3/C4 vegetation fraction at continental-to-global scales. We present a distribution of C 3 and C4 vegetation in Africa. The area of land in sub-Saharan Africa covered by C4 vegetation is 6.3 million square kilometre, or approximately 31% of the land surface. The δ13C of vegetation in Africa is estimated from the C3/C4 composition, assuming constant values of-27‰ and-12‰ for C 3 and C4 biomass. Strong precipitation gradients in Africa drive the C3/C4 spatial gradients, producing correspondingly strong gradients in vegetation δ13C. These isotopic gradients can be used to infer such information as the migratory connectivity of birds. © 2010 Springer Science+Business Media B.V.