Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 15, issue 4
Biogeosciences, 15, 1185-1201, 2018
https://doi.org/10.5194/bg-15-1185-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Biogeosciences, 15, 1185-1201, 2018
https://doi.org/10.5194/bg-15-1185-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Feb 2018

Research article | 27 Feb 2018

Smaller global and regional carbon emissions from gross land use change when considering sub-grid secondary land cohorts in a global dynamic vegetation model

Chao Yue, Philippe Ciais, and Wei Li Chao Yue et al.
  • Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France

Abstract. Several modelling studies reported elevated carbon emissions from historical land use change (ELUC) by including bidirectional transitions on the sub-grid scale (termed gross land use change), dominated by shifting cultivation and other land turnover processes. However, most dynamic global vegetation models (DGVMs) that have implemented gross land use change either do not account for sub-grid secondary lands, or often have only one single secondary land tile over a model grid cell and thus cannot account for various rotation lengths in shifting cultivation and associated secondary forest age dynamics. Therefore, it remains uncertain how realistic the past ELUC estimations are and how estimated ELUC will differ between the two modelling approaches with and without multiple sub-grid secondary land cohorts – in particular secondary forest cohorts. Here we investigated historical ELUC over 1501–2005 by including sub-grid forest age dynamics in a DGVM. We run two simulations, one with no secondary forests (Sageless) and the other with sub-grid secondary forests of six age classes whose demography is driven by historical land use change (Sage). Estimated global ELUC for 1501–2005 is 176Pg C in Sage compared to 197Pg C in Sageless. The lower ELUC values in Sage arise mainly from shifting cultivation in the tropics under an assumed constant rotation length of 15 years, being 27Pg C in Sage in contrast to 46Pg C in Sageless. Estimated cumulative ELUC values from wood harvest in the Sage simulation (31Pg C) are however slightly higher than Sageless (27Pg C) when the model is forced by reconstructed harvested areas because secondary forests targeted in Sage for harvest priority are insufficient to meet the prescribed harvest area, leading to wood harvest being dominated by old primary forests. An alternative approach to quantify wood harvest ELUC, i.e. always harvesting the close-to-mature forests in both Sageless and Sage, yields similar values of 33Pg C by both simulations. The lower ELUC from shifting cultivation in Sage simulations depends on the predefined forest clearing priority rules in the model and the assumed rotation length. A set of sensitivity model runs over Africa reveal that a longer rotation length over the historical period likely results in higher emissions. Our results highlight that although gross land use change as a former missing emission component is included by a growing number of DGVMs, its contribution to overall ELUC remains uncertain and tends to be overestimated when models ignore sub-grid secondary forests.

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Gross land use change such as shifting cultivation causes carbon emissions because carbon release in cleared forests is larger than absorption in regrowing ones. However, to appropriately account for this process, vegetation models have to represent sub-grid secondary forest dynamics. We found that gross land use emissions can be overestimated if sub-grid secondary forests are neglected in the model. Conversely, rotation lengths of shifting cultivation have a critical role.
Gross land use change such as shifting cultivation causes carbon emissions because carbon...
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