Date Published: August 16, 2017
Publisher: Public Library of Science
Author(s): Michael Köhl, Prem R. Neupane, Neda Lotfiomran, Sylvain Delzon.
The world’s forests play a pivotal role in the mitigation of global climate change. By photosynthesis they remove CO2 from the atmosphere and store carbon in their biomass. While old trees are generally acknowledged for a long carbon residence time, there is no consensus on their contribution to carbon accumulation due to a lack of long-term individual tree data. Tree ring analyses, which use anatomical differences in the annual formation of wood for dating growth zones, are a retrospective approach that provides growth patterns of individual trees over their entire lifetime. We developed time series of diameter growth and related annual carbon accumulation for 61 trees of the species Cedrela odorata L. (Meliacea), Hymenaea courbaril L. (Fabacea) and Goupia glabra Aubl. (Goupiacea). The trees grew in unmanaged tropical wet-forests of Suriname and reached ages from 84 to 255 years. Most of the trees show positive trends of diameter growth and carbon accumulation over time. For some trees we observed fluctuating growth—periods of lower growth alternate with periods of increased growth. In the last quarter of their lifetime trees accumulate on average between 39 percent (C. odorata) and 50 percent (G. glabra) of their final carbon stock. This suggests that old-growth trees in tropical forests do not only contribute to carbon stocks by long carbon resistance times, but maintain high rates of carbon accumulation at later stages of their life time.
Close to 90 percent of the global terrestrial biomass carbon is tied up in forests, of which almost half is contributed by tropical and subtropical forests.  In the period from 2011 to 2015 the average annual net removals by forests reached -0.57 Gt C globally. By contrast, in the same period annual emissions from deforestation and forest degradation, which to a large extent took place in tropical and sub-tropical forests, reached 0.8 Gt C and 0.27 Gt C, respectively .
This study rests on a large dataset for three tree species of the Guiana Shield. We applied a novel approach that provides detailed insights into the carbon accumulation dynamics of individual trees over their entire lifetime. Annual diameter growth was obtained by tree ring analyses and converted into annual carbon accumulation by means of biomass functions.