Significant Carbon Loss from a Natural Tropical Peatland

Significant Carbon Loss from a Natural Tropical Peatland under Current Climate Chandra S. Deshmukh*, Dony Julius, Chris D. Evans, Nardi, Ari P. Susanto, Susan E. Page, Supiandi Sabiham, Fahmuddin Agus, Adibtya Asyhari, Sofyan Kurnianto, Yogi Suardiwerianto, Ankur R. Desai *corresponding author at chandra_deshmukh@aprilasia. com Asia Pacific Resources International Holdings Limited (APRIL), Indonesia

Motivation for Research • Groundwater level fluctuation defines carbon accumulation or loss from tropical peatlands; • Increase in frequency and intensity of drought and climate extremes like El Nino causes significant groundwater level drop, particularly in dry season; • Measurements of ecosystem-scale carbon exchange over tropical peatland ecosystems remains limited; • Given their significant carbon stocks, the fate of tropical peatlands and their role in global carbon cycle under current and future climate is unknown. Rain and evapotranspiration Control Groundwater level Groundwater flow Determines Govern Topography and transmissivity Creates Peat carbon accumulation/loss Reference: Cobb, et al. (2017). How temporal patterns in rainfall determine the geomorphology and carbon fluxes of tropical peatlands. Proceedings of the National Academy of Sciences of the United States of America, doi: 10. 1073/pnas. 1701090114 2

Research Objectives 1. Determine the magnitudes of net ecosystem carbon dioxide (CO 2) and methane (CH 4) exchanges over a natural tropical peatland while incorporating all existing source and removal pathways; 2. Understand the link between CO 2 and CH 4 exchanges and associated changes in the environmental variables. Importance: understanding present and future role of tropical peatland in global carbon cycle => Greenhouse Gas Inventories 3

Study Site: Natural Tropical Peatland in Sumatra, Indonesia • Situated in Kampar Peninsula, a heterogeneous landscape of ~700, 000 ha; • Ombrotrophic (acidic and nutrient-poor) peatland largely formed within the past 8000 years; • Groundwater level fluctuates following seasonal rainfall pattern, which can reach 70 cm blow the peat surface in the late dry season. Sumatra Study site Indonesia 4

Approach: Eddy Covariance Technique • Measures net balance of all vertical CO 2 / CH 4 source and removal pathways; • Provides high frequency measurements (every 30 min) => temporal variability; • Provides ecosystem-scale measurements (> 200 ha around the tower) => spatial variability CO 2 analyzer • CH 4 analyzer Vertical wind speed Measurements height: above canopy – (51 m above ground surface at top of the tower) 5

Cumulative Net Ecosystem CO 2 Exchange Cumulative net ecosystem CO 2 exchange (g CO 2 -C m-2) Cumulative net ecosystem CO 2 exchange • CO 2 source when groundwater level is below the peat surface; • CO 2 removal only when groundwater level is close to/above the peat surface (shaded area in the above graph); • On an annual basis, tropical peatland ecosystem is no longer a carbon sink under the current climate. 6

Cumulative Net Ecosystem CH 4 Exchange Cumulative net ecosystem CH 4 exchange (g CH 4 -C m-2) Cumulative net ecosystem CH 4 exchange • CH 4 source throughout the study period, with a sharp increase in emissions when groundwater level is close to/above the peat surface (shaded area in the above graph) Reference: Deshmukh, CS, Julius, D, Evans, CD, et al. Impact of forest plantation on methane emissions from tropical peatland. Glob Change Biol. 2020; 26: 2477– 2495. https: //doi. org/10. 1111/gcb. 15019 7

Net ecosystem CO 2 exchange (g CO 2 -C m-2 day-1) Groundwater Level Controls Net Ecosystem CO 2 Exchanges • Lower groundwater level enhances peat aeration and thereby oxidative peat decomposition. Therefore, ecosystem respiration dominates over gross primary production, which results in CO 2 emissions; • Higher groundwater level suppresses peat aeration and thereby oxidative peat decomposition. Therefore, gross primary production dominates over ecosystem respiration, which results in CO 2 sequestration from photosynthesis. 8

Net ecosystem CH 4 exchange (mg CH 4 m-2 day-1) Groundwater Level Controls Net Ecosystem CH 4 Exchanges • CH 4 emissions increase exponentially with higher groundwater levels; • High groundwater level supports vegetation-mediated transport during daytime => dissolved CH 4 can be taken up by the root system and emitted to the atmosphere. Reference: Deshmukh, CS, Julius, D, Evans, CD, et al. Impact of forest plantation on methane emissions from tropical peatland. Glob Change Biol. 2020; 26: 2477– 2495. https: //doi. org/10. 1111/gcb. 15019 9

Significant Carbon Loss from a Natural Tropical Peatland • Our results indicate that tropical peatland ecosystems are no longer a carbon sink under the current climate; • If we apply a 100 -year global warming potential of 34 for CH 4, this implies that CH 4 emissions contributed to ~36% of the total CO 2 eq emissions. 10

Impact of Climate Change on Tropical Peatlands? Our results, which are among the first eddy covariance exchange data reported for any tropical peatland, (1) should help to reduce the uncertainty in the estimation of CO 2 and CH 4 emissions from a globally important ecosystem under current climate; 2) should help to improve our understanding of the role of natural peatlands under future climate => modelling/extrapolation opportunity. 11