Detecting tropical peatland degradation: Combining remote sensing and organic geochemistry

PLOS ONE RESEARCH ARTICLE Detecting tropical peatland degradation: Combining remote sensing and organic geochemistry Chloe Brown1, Doreen S. Boyd ID1*, Sofie Sjögersten2, Christopher H. Vane ID3 1 School of Geography, University of Nottingham, Nottingham, United Kingdom, 2 School of Biosciences, University of Nottingham, Nottingham, United Kingdom, 3 British Geological Survey, Centre for Environmental Geochemistry, Keyworth, United Kingdom a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Brown C, Boyd DS, Sjögersten S, Vane CH (2023) Detecting tropical peatland degradation: Combining remote sensing and organic geochemistry. PLoS ONE 18(3): e0280187. https:// doi.org/10.1371/journal.pone.0280187 Editor: Arun Jyoti Nath, Assam University, INDIA Received: February 4, 2022 Accepted: December 22, 2022 Published: March 29, 2023 Copyright: © 2023 Brown et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by the Natural Environment Research Council [NE/L002604/1], under the Envision DTP. Additional support from the University of Nottingham. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. * Abstract Tropical peatlands are important carbon stores that are vulnerable to drainage and conversion to agriculture. Protection and restoration of peatlands are increasingly recognised as key nature based solutions that can be implemented as part of climate change mitigation. Identification of peatland areas that are important for protection and restauration with regards to the state of their carbon stocks, are therefore vital for policy makers. In this paper we combined organic geochemical analysis by Rock-Eval (6) pyrolysis of peat collected from sites with different land management history and optical remote sensing products to assess if remotely sensed data could be used to predict peat conditions and carbon storage. The study used the North Selangor Peat Swamp forest, Malaysia, as the model system. Across the sampling sites the carbon stocks in the below ground peat was ca 12 times higher than the forest (median carbon stock held in ground vegetation 114.70 Mg ha-1 and peat soil 1401.51 Mg ha-1). Peat core sub-samples and litter collected from Fire Affected, Disturbed Forest, and Managed Recovery locations (i.e. disturbed sites) had different decomposition profiles than Central Forest sites. The Rock-Eval pyrolysis of the upper peat profiles showed that surface peat layers at Fire Affected, Disturbed Forest, and Managed Recovery locations had lower immature organic matter index (I-index) values (average I-index range in upper section 0.15 to -0.06) and higher refractory organic matter index (R -index) (average R-index range in upper section 0.51 to 0.65) compared to Central Forest sites indicating enhanced decomposition of the surface peat. In the top 50 cm section of the peat profile, carbon stocks were negatively related to the normalised burns ratio (NBR) (a satellite derived parameter) (Spearman’s rho = -0.664, S = 366, p-value = <0.05) while there was a positive relationship between the hydrogen index and the normalised burns ratio profile (Spearman’s rho = 0.7, S = 66, p-value = <0.05) suggesting that this remotely sensed product is able to detect degradation of peat in the upper peat profile. We conclude that the NBR can be used to identify degraded peatland areas and to support identification of areas for conversation and restoration. Competing interests: The authors have declared that no competing interests exist. PLOS ONE | https://doi.org/10.1371/journal.pone.0280187 March 29, 2023 1 / 19 PLOS ONE Detecting tropical peatland degradation 1. Introduction Tropical peat swamp forests are recognised for their global importance as carbon sequestering and storing ecosystems, and their role for climate change mitigation [1,2]. Under stable conditions, lowland tropical peatlands have the potential to form one of the most efficient carbon capture environments, with peat accumulation rates up to ten times faster in the tropics compared with other peatlands (temperate, subarctic, boreal) [3–6]. However, increased climate variability, amplified by degradation associated with land use change and resource management, has the potential to shift tropical peatlands from a net carbon sink to a source [7–10]. South-East Asia has experienced the most rapid and destructive large-scale peatland degradation. Since the 1980’s, widespread degradation through deforestation [11–13], drainage [7,8] and frequent intense fire events [13–15], have dramatically increased the pressure on peat swamp forests in South-East Asia. Ombrotrophic tropical peat swamp forests are rainwater-fed ecosystems with environmental and topographic conditions favouring poor drainage, oversaturation and substrate acidification [16]. The peat deposits are formed and maintained by the continuous input of organic matter from evergreen swamp forest species under waterlogged conditions [17,18]. The surplus of water on the peatland floor creates an intermittent lack of oxygen, thus restricting microbial decomposition creating optimum conditions for peat accumulation, in particular during waterlogged conditions [19]. The microtopography of tropical peatlands creates an irregular forest floor, with elevated surfaces, hummocks and hollows [20,21]. When the organic matter inputs in the peat soil undergo microbial mediated aerobic and anaerobic decomposition greenhouse gasses (carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)) can be released into the atmosphere [22–25]. Tropical peatland degradation accelerates these processes releasing high levels of greenhouse gas emissions into the atmosphere through vegetation removal, peat oxidation and combustion [7,10,24–26]. Remote sensing has become one of the primary data sources for large-scale tropical peat swamp forest assessment [27]. This provides a rapid and economical approach to supplement traditional direct field-based methods such as coring and probing [28,29]. Typically, field-based measurements (with laboratory analyses) are used to determine peat properties (for example—peat thickness, bulk density, carbon content), with remote sensing data used to derive information on the area extent, land use change, fire events and vegetation cover [30–32]. Remote sensing data can be collected via a number of platforms such as planes, UAV’s, kites and satellites; in this paper we focus on data captured by satellites. Spaceborne data offers a low cost (often freely available) data solution, with regular resampling intervals and large area (...truncated)