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Gauci, Vincent. 2013 Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes. Nature, 493. 660-663. 10.1038/nature11818Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. 39Unlike boreal and temperate forests 5,6 , and higher latitude wetlands 7 , however, the loss of fluvial 40 organic carbon from tropical peats has yet to be fully quantified. 41To quantify the effect of peatland degradation on fluvial organic C loss, we monitored DOC and , and experienced similar annual rainfall (Table 1). 9TOC ( ; Fig. 1). This represents a 55% increase in 12TOC export from the disturbed sites (DPSF1 and 2) over IPSF. Of the annual TOC flux from each land-13 cover class, 94% was lost during the wet season (October-June), the result of higher measured 14 discharge rates (3.9 m 3 s -1 cf. 1.0 m 3 s -1 in the dry season). This was associated with high rainfall 15 rather than changes in C concentration, which remained relatively constant over the study period. 16As with seasonal flux variability, differences in discharge between land-cover classes determined TOC 17flux with higher discharge rates causing larger fluxes in DPSF1 and DPSF2 (1744 mm and 1724 mm, 18respectively) than in IPSF (907 mm). These higher discharge rates in disturbed land-cover classes 19were not counterbalanced by lower TOC concentrations, and occurred despite uniform rainfall across 20 sites (Table 1). This likely reflects a decline in evapotranspiration and increased runoff as a 21consequence of large scale biomass loss and drainage in both disturbed land-cover classes. DOC 22accounted for between 91-98 % of the TOC lost, with lower DOC:POC ratios for disturbed sites ( Table 23 1) suggesting that the drained and exposed peat is vulnerable to mechanical breakdown associated 24 with the increased runoff. 25Surface water DOC can derive from multiple sources, ranging from recent photosynthates to 26 decomposition or dissolution products from deep within the peat column. We used radiocarbon ( 3These data indicate that the increased DOC fluxes from disturbed peatlands are derived from 4 previously stable C stored within the peat column, and suggest that this loss of C from depth is 5 occurring throughout the seasonal hydrologic cycle. Application of an age attribution model (Fig. 2d) 6suggests that two-thirds of DOC in runoff from the DPSF2 site derives from peat carbon of 500-5000 7 years age. 35To quantify the impact peatland disturbance has had on regional long-term fluvial C loss, we applied 36 our TOC flux estimates to land areas of intact and deforested PSF prior to and after peatland 37 disturbance. We omitted industrial plantations from our calculations as, to our knowledge, there are 38 no quantitative data on fluvial C flux from this land cover class, although our DO 14 C data suggest that 39 these ecosystems may also ...

SummaryWetlands are the largest source of methane to the atmosphere, with tropical wetlands comprising the most significant global wetland source component. The stems of some wetlandadapted tree species are known to facilitate egress of methane from anoxic soil, but current ground-based flux chamber methods for determining methane inventories in forested wetlands neglect this emission pathway, and consequently, the contribution of tree-mediated emissions to total ecosystem methane flux remains unknown.In this study, we quantify in situ methane emissions from tree stems, peatland surfaces (ponded hollows and hummocks) and root-aerating pneumatophores in a tropical forested peatland in Southeast Asia.We show that tree stems emit substantially more methane than peat surfaces, accounting for 62-87% of total ecosystem methane flux. Tree stem flux strength was controlled by the stem diameter, wood specific density and the amount of methane dissolved in pore water.Our findings highlight the need to integrate this emission pathway in both field studies and models if wetland methane fluxes are to be characterized accurately in global methane budgets, and the discrepancies that exist between field-based flux inventories and top-down estimates of methane emissions from tropical areas are to be reconciled.

Abstract.Concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) were analysed from the source to the mouth of the River Sebangau in Central Kalimantan, Indonesia during the dry and wet seasons in 2008/2009 and an annual total organic carbon (TOC) flux estimated. DOC concentrations were higher and POC concentrations lower in the wet season compared to the dry season. As seen in other tropical blackwater rivers, DOC concentration is consistently around 10 times greater than POC concentration. We estimate the annual TOC flux discharged to the Java Sea to be 0.46 Tg year −1 comprising of 93% (0.43 Tg) DOC and 7% (0.03 Tg) POC. This equates to a fluvial TOC loss flux per unit area over the entire Sebangau catchment of 88 g C m −2 yr −1 . When extrapolating the River Sebangau DOC loss flux (83 g C m −2 yr −1 ) to the peat covered area of Indonesia (206 950 km 2 ), we estimate a DOC loss of 17.2 Tg C yr −1 or ∼10% of current estimates of the global annual riverine DOC discharge into the ocean.

Carbon sequestration and storage in peatlands rely on consistently high water tables. Anthropogenic pressures including drainage, burning, land conversion for agriculture, timber, and biofuel production, cause loss of peat-forming vegetation and exposure of previously anaerobic peat to aerobic decomposition. This can shift peatlands from net CO 2 sinks to large CO 2 sources, releasing carbon held for millennia. Peatlands also export significant quantities of carbon via fluvial pathways, mainly as dissolved organic carbon (DOC). We analyzed radiocarbon ( 14 C) levels of DOC in drainage water from multiple peatlands in Europe and Southeast Asia, to infer differences in the age of carbon lost from intact and drained systems. In most cases, drainage led to increased release of older carbon from the peat profile but with marked differences related to peat type. Very low DOC-14 C levels in runoff from drained tropical peatlands indicate loss of very old (centuries to millennia) stored peat carbon. High-latitude peatlands appear more resilient to drainage; 14 C measurements from UK blanket bogs suggest that exported DOC remains young (<50 years) despite drainage. Boreal and temperate fens and raised bogs in Finland and the Czech Republic showed intermediate sensitivity. We attribute observed differences to physical and climatic differences between peatlands, in particular, hydraulic conductivity and temperature, as well as the extent of disturbance associated with drainage, notably land use changes in the tropics. Data from the UK Peak District, an area where air pollution and intensive land management have triggered Sphagnum loss and peat erosion, suggest that additional anthropogenic pressures may trigger fluvial loss of much older (>500 year) carbon in high-latitude systems. Rewetting at least partially offsets drainage effects on DOC age.

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Quantifying the relationship between tree diameter and height is a key component of efforts to estimate biomass and carbon stocks in tropical forests. Although substantial site‐to‐site variation in height–diameter allometries has been documented, the time consuming nature of measuring all tree heights in an inventory plot means that most studies do not include height, or else use generic pan‐tropical or regional allometric equations to estimate height.Using a pan‐tropical dataset of 73 plots where at least 150 trees had in‐field ground‐based height measurements, we examined how the number of trees sampled affects the performance of locally derived height–diameter allometries, and evaluated the performance of different methods for sampling trees for height measurement.Using cross‐validation, we found that allometries constructed with just 20 locally measured values could often predict tree height with lower error than regional or climate‐based allometries (mean reduction in prediction error = 0.46 m). The predictive performance of locally derived allometries improved with sample size, but with diminishing returns in performance gains when more than 40 trees were sampled. Estimates of stand‐level biomass produced using local allometries to estimate tree height show no over‐ or under‐estimation bias when compared with biomass estimates using field measured heights. We evaluated five strategies to sample trees for height measurement, and found that sampling strategies that included measuring the heights of the ten largest diameter trees in a plot outperformed (in terms of resulting in local height–diameter models with low height prediction error) entirely random or diameter size‐class stratified approaches.Our results indicate that even limited sampling of heights can be used to refine height–diameter allometries. We recommend aiming for a conservative threshold of sampling 50 trees per location for height measurement, and including the ten trees with the largest diameter in this sample.

Net Primary Productivity (NPP) is one of the most important parameters in describing the functioning of any ecosystem and yet it arguably remains a poorly quantified and understood component of carbon cycling in tropical forests, especially outside of the Americas. We provide the first comprehensive analysis of NPP and its carbon allocation to woody, canopy and root growth components at contrasting lowland West African forests spanning a rainfall gradient. Using a standardized methodology to study evergreen (EF), semi-deciduous (SDF), dry forests (DF) and woody savanna (WS), we find that (i) climate is more closely related with above and belowground C stocks than with NPP (ii) total NPP is highest in the SDF site, then the EF followed by the DF and WS and that (iii) different forest types have distinct carbon allocation patterns whereby SDF allocate in excess of 50% to canopy production and the DF and WS sites allocate 40%-50% to woody production. Furthermore, we find that (iv) compared with canopy and root growth rates the woody growth rate of these forests is a poor proxy for their overall productivity and that (v) residence time is the primary driver in the productivity-allocation-turnover chain for the observed spatial differences in woody, leaf and root biomass across the rainfall gradient. Through a systematic assessment of forest productivity we demonstrate the importance of directly measuring the main components of above and belowground NPP and encourage the establishment of more permanent carbon intensive monitoring plots across the tropics.