Co-assessment of biomass and soil organic carbon stocks in a future reservoir area located in Southeast Asia

Abstract

An assessment of the organic carbon stock present in living or dead vegetation and in the soil on the 450 km2 of the future Nam Theun 2 hydroelectric reservoir in Lao People's Democratic Republic was made. Nine land cover types were defined on the studied area: dense, medium, light, degraded, and riparian forests; agricultural soil; swamps; water; and others (roads, construction sites, and so on). Their geographical distribution was assessed by remote sensing using two 2008 SPOT 5 images. The area is mainly covered by dense and light forests (59%), while agricultural soil and swamps account for 11% and 2%, respectively. For each of these cover types, except water, organic carbon density was measured in the five pools defined by the Intergovernmental Panel on Climate Change: aboveground biomass, litter, deadwood, belowground biomass, and soil organic carbon. The area-weighted mean carbon densities for these pools were estimated at 45.4, 2.0, 2.2, 3.4, and 62.2 tC/ha, respectively, i.e., a total of about 115±15 tC/ha for a soil thickness of 30 cm, corresponding to a total flooded organic carbon stock of 5.1±0.7 MtC. This value is much lower than the carbon density for some South American reservoirs for example where total organic carbon stocks range from 251 to 326 tC/ha. It can be mainly explained by (1) the higher biomass density of South American tropical primary rainforest than of forests in this study and (2) the high proportion of areas with low carbon density, such as agricultural or slash-and-burn zones, in the studied area.

Fish population dynamic in the newly impounded Nam Theun 2 Reservoir (Lao PDR)

“…The main tributaries of the reservoir are the Nam Xot, the Nam Theun and the Nam On Rivers. The reservoir flooded almost 70% of dense to light forest, 23% of degraded forest, riparian forest and agricultural soils and the last 7% were composed of water system, swamps and villages (Descloux et al, 2011 …”

Evolution of the physico-chemical water quality in the Nam Theun 2 Reservoir and downstream rivers for the first 5 years after impoundment

“…In the Nam Ngum reservoir in Laos, standing trees almost completely eliminate wind-generated turbulence and consequently reduce the extent of mixing (Baxter, 1977). It may explain why the impact of annual destratification is lower in the Nam Malou area (RES3) initially covered by forest (Descloux et al, 2011). Contrary to the Petit Saut Reservoir in French Guiana , this stratification weakened then disappeared each year during the CD season due to a drop of air temperature (Descloux et al, this issue-a).…”

5 years of monitoring of zooplankton community dynamics in a newly impounded sub-tropical reservoir in Southeast Asia (Nam Theun 2, Lao PDR)

“…The NT2 Reservoir covers a surface area of 489 km² at its full supply level and potentially decreases to a minimum of 86 km² at the end of the dry season (Descloux et al, same issue). The reservoir flooded almost 70% of dense to light forest, 23% of degraded forest, riparian forest and agricultural soils and the last 7% were composed of water system, swamps and villages (Descloux et al, 2011). The reservoir is qualified as a dendritic and is relatively shallow with an average depth of 8 m (Chanudet et al, 2012).…”