Logged and structurally degraded tropical forests are fast becoming one of the most prevalent land-use types throughout the tropics and are routinely assumed to be a net carbon sink because they experience rapid rates of tree regrowth. Yet this assumption is based on forest biomass inventories that record carbon stock recovery but fail to account for the simultaneous losses of carbon from soil and necromass. Here, we used forest plots and an eddy covariance tower to quantify and partition net ecosystem CO 2 exchange in Malaysian Borneo, a region that is a hot spot for deforestation and forest degradation. Our data represent the complete carbon budget for tropical forests measured throughout a logging event and subsequent recovery and found that they constitute a substantial and persistent net carbon source. Consistent with existing literature, our study showed a significantly greater woody biomass gain across moderately and heavily logged forests compared with unlogged forests, but this was counteracted by much larger carbon losses from soil organic matter and deadwood in logged forests. We estimate an average carbon source of 1.75 ± 0.94 Mg C ha -1 yr -1 within moderately logged plots and 5.23 ± 1.23 Mg C ha - 1 yr - 1 in unsustainably logged and severely degraded plots, with emissions continuing at these rates for at least one-decade post-logging. Our data directly contradict the default assumption that recovering logged and degraded tropical forests are net carbon sinks, implying the amount of carbon being sequestered across the world's tropical forests may be considerably lower than currently estimated.
|Proceedings of the National Academy of Sciences of the United States of America
|Early online date
|9 Jan 2023
|Published - 17 Jan 2023
This study was part of the SAFE Project, the Global Ecosystems Monitoring network (gem.tropicalforests.ox.ac.uk) and Imperial College's Grand Challenges in Ecosystems and the Environment Initiative. We acknowledge funding from the Sime Darby Foundation, the Biodiversity And Land-use Impacts on tropical ecosystem function (BALI) Project (NE/K016377/1) within the Natural Environment Research Council Human-Modified Tropical Forests Programme, the Malaysian Palm Oil Board (MPOB), and Centre for Tropical Forest Science (CTFS) in collaboration with HSBC Climate Partnership. The 52-ha Long-Term Ecological Research Project in Lambir is a collaborative project of the Forest Department of Sarawak, Malaysia, the Center for Tropical Forest Science of the Smithsonian Tropical Research Institute, USA (NSF awards DEB- 9107247 and DEB- 9629601), and Osaka City, Ehime & Kyoto Universities, Japan (Monbusho grants 06041094, 08NP0901 and 09NP0901). M.B.M. was supported by NERC studentship awarded through the Central England NERC Training Alliance (CENTA; grant referenceNE/S007350/1) and the University of Leicester, Y.M. was supported by the Jackson Foundation and European Research Council Advanced Investigator Grant, GEM-TRAIT (321131), Y.M., RME, and T.R. by NERC grant NE/P002218/1, and R.M.E. is supported by the NOMIS Foundation. TR also acknowledges support from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 865403). Maliau Basin and Danum Valley Management Committees, Royal Society South East Asia Rainforest Research Partnership (SEARRP), Sabah Foundation, Benta Wawasan, the State Secretary, Sabah Chief Minister’s Departments, Sabah Forestry Department, Sabah Biodiversity Council, and the Economic Planning Unit are acknowledged for their support and access to the sites in Sabah. Rostin Jantan, Rohid Kailoh, Suhaini Patik, Ampat Siliwong, Yehezekiel Jahuri, Robecca Siwaring, Jeffry Amin, Sarah Watson, Ryan Gray, Johnny Larenus, Unding Jami, Toby Marthews, Alexander Karolus, the Danum 50 ha plot team, Sylvester Tan, Xyxtus Tan, Nasir Muhi and Abilano Deres helped with the data collection. We thank Susan Page, Juan Carlos Berrio, Jörg Kaduk and Katie O’Brien for their constructive comments.
Data Availability Statement[field datasets; microclimate data] data have been deposited in [Zenodo] (https://doi.org/10.5281/zenodo.7307449, https://doi.org/10.5281/zenodo.7307447, and https://doi.org/10.5281/zenodo.3888375). Previously published data were used for this work [L. K. Kho et al. (59), T. Riutta et al. (16), and T. Riutta, et al. (20)].
- Tropical Climate