How temporal patterns in rainfall determine the PNAS PLUS geomorphology and carbon fluxes of tropical peatlands Alexander R. Cobba,1, Alison M. Hoytb, Laure Gandoisc, Jangarun Erid, Rene´ Dommaine,f, Kamariah Abu Salimg, Fuu Ming Kaia,2, Nur Salihah Haji Su’uth, and Charles F. Harveya,b aCenter for Environmental Sensing and Modeling, Singapore–MIT Alliance for Research and Technology, 138602 Singapore; bDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; cLaboratoire Ecologie´ Fonctionnelle et Environnement, Universite´ de Toulouse, CNRS, Institut National Polytechnique de Toulouse, Universite´ Paul Sabatier, F-31326 Castanet-Tolosan, France; dForestry Department, Ministry of Industry and Primary Resources, Jalan Menteri Besar, Bandar Seri Begawan BB3910, Brunei Darussalam; eDepartment of Anthropology, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560; fInstitute of Earth and Environmental Science, University of Potsdam, 14476 Potsdam, Germany; gBiology Programme, Universiti Brunei Darussalam, Bandar Seri Begawan BE1410, Brunei Darussalam; and hBrunei Darussalam Heart of Borneo Centre, Ministry of Industry and Primary Resources, Jalan Menteri Besar, Bandar Seri Begawan BB3910, Brunei Darussalam Edited by Donald E. Canfield, Institute of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Odense M., Denmark, and approved May 5, 2017 (received for review February 6, 2017) Tropical peatlands now emit hundreds of megatons of carbon by the fraction of time that peat is exposed by a low water table dioxide per year because of human disruption of the feedbacks (Fig. 1). that link peat accumulation and groundwater hydrology. How- The water table rises and falls in a peatland according to the ever, no quantitative theory has existed for how patterns of car- balance between rainfall, evapotranspiration, and groundwater bon storage and release accompanying growth and subsidence flow. Water flows downslope toward the edge of each peat dome, of tropical peatlands are affected by climate and disturbance. where it is bounded by rivers. This flow occurs at a rate lim- Using comprehensive data from a pristine peatland in Brunei ited by the hydraulic transmissivity of the peat—the efficiency Darussalam, we show how rainfall and groundwater flow deter- with which it conducts lateral flow—and follows the gradient mine a shape parameter (the Laplacian of the peat surface eleva- in the water table. The gradient in the water table is slightly tion) that specifies, under a given rainfall regime, the ultimate, steeper near dome boundaries where the flow of water is faster. stable morphology, and hence carbon storage, of a tropical peat- A steeper gradient near boundaries implies a domed shape in land within a network of rivers or canals. We find that peat- the water table, or groundwater mound, corresponding to the lands reach their ultimate shape first at the edges of peat domes domed shape of the peat surface. The doming of the peat surface where they are bounded by rivers, so that the rate of carbon is very subtle: Gradients are about 1 m/km (1). Nonetheless, it is uptake accompanying their growth is proportional to the area the dome’s gentle curvature that accounts for the carbon storage of the still-growing dome interior. We use this model to study within the drainage boundary. how tropical peatland carbon storage and fluxes are controlled by changes in climate, sea level, and drainage networks. We find that fluctuations in net precipitation on timescales from hours to Significance years can reduce long-term peat accumulation. Our mathematical and numerical models can be used to predict long-term effects of A dataset from one of the last protected tropical peat swamps changes in temporal rainfall patterns and drainage networks on in Southeast Asia reveals how fluctuations in rainfall on yearly tropical peatland geomorphology and carbon storage. and shorter timescales affect the growth and subsidence of tropical peatlands over thousands of years. The pattern of tropical peatlands j peatland geomorphology j peatland hydrology j rainfall and the permeability of the peat together determine peatland carbon storage j climate-carbon cycle feedbacks a particular curvature of the peat surface that defines the amount of naturally sequestered carbon stored in the peat- land over time. This principle can be used to calculate the ropical peatlands store gigatons of carbon in peat domes, long-term carbon dioxide emissions driven by changes in cli- Tgently mounded land forms kilometers across and 10 or more mate and tropical peatland drainage. The results suggest that ECOLOGY meters high (1). The carbon stored as peat in these domes has greater seasonality projected by climate models could lead to been sequestered by photosynthesis of peat swamp trees (2) carbon dioxide emissions, instead of sequestration, from oth- and preserved for thousands of years by waterlogging, which erwise undisturbed peat swamps. suppresses decomposition. Human disturbance of tropical peat- lands by fire and drainage for agriculture is now causing reemis- Author contributions: A.R.C. and C.F.H. designed research; A.R.C. and J.E. established the sion of that carbon at rates of hundreds of megatons per year field site; A.R.C., A.M.H., L.G., J.E., R.D., K.A.S., F.M.K., and N.S.H.S. performed research; (2–5): Emissions from Southeast Asian peatlands alone are A.R.C. contributed new analytic tools; A.R.C., A.M.H., and C.F.H. analyzed data; and A.R.C. equivalent to about 2% of global fossil fuel emissions or 20% of and C.F.H. wrote the paper. global land use and land cover change emissions (6, 7). Because The authors declare no conflict of interest. peat is mostly organic carbon, a description of the growth and This article is a PNAS Direct Submission. PLANETARY SCIENCES EARTH, ATMOSPHERIC, AND subsidence of tropical peatlands also quantifies fluxes of carbon Freely available online through the PNAS open access option. dioxide (1, 4, 8). Evidence from a range of studies establishes Data deposition: The data reported in this paper have been deposited in the Dryad Dig- that accumulation and loss of tropical peat are controlled by ital Repository (dx.doi.org/10.5061/dryad.18q5n). water table dynamics (4, 9). When the water table is low, aer- 1To whom correspondence should be addressed. Email: [email protected]. obic decomposition occurs, releasing carbon dioxide; when the 2Present address: Gas Metrology Laboratory, National Metrology Center, Agency for Sci- water table is high, aerobic decomposition is inhibited by lack of ence, Technology and Research, 118221 Singapore. oxygen, production of peat exceeds its decay, and peat accumu- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. lates. In this way, the rate of peat accumulation is determined 1073/pnas.1701090114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1701090114 PNAS j Published online June 12, 2017 j E5187–E5196 Downloaded by guest on October 2, 2021 tion under constant rainfall (17, 18). Although these subsequent works simulate the dynamics of peat production and decompo- sition in increasing detail, a strength of Ingram’s model was that it provided quantitative intuition for how peat dome morphol- ogy depends on peat hydrologic properties and average rain- fall. Could a principle like Ingram’s exist that describes peatland dynamics as well as statics and remains applicable with realistic drainage networks and rainfall regimes? We established a field site in one of the last pristine peat swamp forests in Southeast Asia and then used measurements Fig. 1. Ecosystem feedback leading to peat accumulation. Peat accumu- from this site to develop a mathematical model for the geomor- lation occurs because of waterlogging of plant remains and is therefore phic evolution of tropical peatlands that is simpler, yet more gen- determined by the proportion of time that peat is protected from aero- eral than Ingram’s model for high-latitude peatlands. Our model bic decomposition by a high water table. Over time, peat builds up into makes it possible to predict effects of changes in rainfall regime gently mounded land forms, or domes, bounded by rivers. The slopes in a and drainage networks on carbon storage and fluxes in tropical peat dome, although very small, govern groundwater flow toward bound- peatlands. The model predicted, perhaps surprisingly, that sur- ing rivers at rates limited by the transmissivity of the peat. face peat would be older near dome margins. We tested these predictions by radiocarbon dating core samples and comparing the age of each sample to the simulated age at its location and Once the peatland surface is sufficiently domed, water is shed depth. Finally, we explored the future of tropical peatlands under so rapidly that no more organic matter can be waterlogged within climate projections by simulating the geomorphic evolution of an the confines of the drainage network, and peat accumulation idealized peat dome under projected changes in rainfall patterns stops (10). This maximally domed shape sets a limit on how much and drainage. carbon a peat dome can sequester and preserve under a given rainfall regime (11). If the peat dome is flatter than its stable Methods shape for the current climate, it will sequester carbon and grow; Field Measurements. We established a field site in a pristine peat forest if it is more domed than its stable shape, it will release carbon in Brunei Darussalam (Borneo) to study a peat dome where current pro- and subside as peat decomposes. [In the tropical peat literature, cesses affecting peat accumulation are essentially similar to those during “subsidence” is used for a decline in the peat surface elevation, its long-term development (Fig. 2). At the site, we installed 5 piezometers regardless of mechanism (5).] The volume of this stable shape along a 2.5-km trail, 12 piezometers along a 180-m transect, and 3 through- times the average carbon density of the peat defines a capacity fall gauges.