The role of tree-fall dynamics in long-term carbon storage of tropical peatlands

The forested peatlands of the Earth's tropical belt represent a major terrestrial carbon pool that may contain over 90 petagram C. However, the mechanisms that led to the build-up of this significant pool of carbon are poorly understood. Moreover, the rates of carbon uptake by peat accumulation in these tropical settings have rarely been quantified and natural variations in uptake on decadal to millennial scales are not well constrained. We studied carbon accumulation dynamics on these timescales of a peat swamp forest dominated by the dipterocarp Shorea albida - a unique forest type that, until recently, widely covered the lowlands of northwest Borneo. The impressive Shorea albida trees may reach heights of 70 m and are therefore strongly susceptible to windthrow and lightning. Such natural disturbances cause these trees to fall and uproot - excavating over 1 m deep holes into the peat that fill with water to become tip-up pools. The analysis of high-resolution aerial photographs and radiocarbon dated peat cores from our field site in Brunei together with the construction of a simulation model of peat accumulation allowed us to determine the role of tree-fall and tip-up pools in carbon storage. In a hectare of Shorea albida forest four tip-up pools form per decade. A pool completely fills with organic matter within 200 years according to our pollen record and a dated pool deposit stored 40 kg C m-2 of the total 110 kg C m-2 large local peat carbon pool. The carbon accumulation rates in these pools reach over 800 g C m-2 yr-1 - within the range of annual litterfall in dipterocarp forests. The simulation model indicates that up to 60% of the peat deposits under Shorea albida forests could be derived from infilled pools. Tip-up pools are therefore local hotspots for carbon storage in tropical forested peatlands.