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1 Lacombe, Guillaume; Ribolzi, O.; de Rouw, A.; Pierret, A.; Latsachak, K.; Silvera, N.; Pham Dinh, R.; Orange, D.; Janeau, J.-L.; Soulileuth, B.; Robain, H.; Taccoen, A.; Sengphaathith, P.; Mouche, E.; Sengtaheuanghoung, O.; Tran Duc, T.; Valentin, C. 2015. Afforestation by natural regeneration or by tree planting: examples of opposite hydrological impacts evidenced by long-term field monitoring in the humid tropics. Hydrology and Earth System Sciences Discussions, 12:12615-12648. [doi: https://doi.org/10.5194/hessd-12-12615-2015]

Afforestation ; Plantations ; Tectona grandis ; Hydrological factors ; Humid tropics ; Ecosystem services ; Land use ; Soil conservation ; Water conservation ; Catchment areas ; Rain ; Runoff ; Models / Lao People s Democratic Republic / Vietnam / Houay Pano Catchment / Dong Cao Catchment

(Location: IWMI HQ Call no: e-copy only Record No: H047340)

http://www.hydrol-earth-syst-sci-discuss.net/12/12615/2015/hessd-12-12615-2015.pdf
https://vlibrary.iwmi.org/pdf/H047340.pdf

(3.39 MB) (3.39 MB)

The humid tropics are exposed to an unprecedented modernization of agriculture involving rapid and highly-mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability controlling habitats, water resources and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydro-meteorological variables has been operating in several headwater catchments in tropical Southeast Asia since 2001. The GR2M water balance model repeatedly calibrated over successive 1 year periods, and used in simulation mode with specific rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses and trend detection tests allowed causality between land-use changes and changes in seasonal flows to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow, led to intricate flow patterns: pluri-annual flow cycles induced by the shifting system, on top of a gradual flow increase over years caused by the spread of the plantation. In Vietnam, the abandonment of continuously cropped areas mixed with patches of tree plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration or planting) led to opposite changes in flow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.

2 Lacombe, Guillaume; Ribolzi, O.; de Rouw, A.; Pierret, A.; Latsachak, K.; Silvera, N.; Dinh, R. P.; Orange, D.; Janeau, J.-L.; Soulileuth, B.; Robain, H.; Taccoen, A.; Sengphaathith, P.; Mouche, E.; Sengtaheuanghoung, O.; Tran Duc, T.; Valentin, C. 2016. Contradictory hydrological impacts of afforestation in the humid tropics evidenced by long-term field monitoring and simulation modelling. Hydrology and Earth System Sciences, 20:2691-2704.

Humid tropics ; Hydrological factors ; Afforestation ; Natural regeneration ; Plantations ; Monitoring ; Simulation models ; Land use ; Land cover change ; Ecosystem services ; Forest conservation ; Catchment areas ; Rainfall-runoff relationships ; Stream flow ; Cropping systems ; Water conservation ; Soil conservation ; Soil surface properties ; Tectona grandis / Lao People's Democratic Republic / Vietnam / Houay Pano Catchment / Dong Cao Catchment

(Location: IWMI HQ Call no: e-copy only Record No: H047644)

http://www.hydrol-earth-syst-sci.net/20/2691/2016/hess-20-2691-2016.pdf
https://vlibrary.iwmi.org/pdf/H047644.pdf

The humid tropics are exposed to an unprecedented modernisation of agriculture involving rapid and mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability which controls habitats, water resources, and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydrometeorological variables has been operating in several headwater catchments in tropical southeast Asia since 2000. The GR2M water balance model, repeatedly calibrated over successive 1-year periods and used in simulation mode with the same year of rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses, and trend detection tests allowed causality between land-use changes and changes in seasonal streamflow to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow led to intricate streamflow patterns: pluri-annual streamflow cycles induced by the shifting system, on top of a gradual streamflow increase over years caused by the spread of the plantations. In Vietnam, the abandonment of continuously cropped areas combined with patches of mix-trees plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration vs. planting) led to opposite changes in streamflow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.

3 Lacombe, Guillaume; Bolliger, A. M.; Harrisson, R. D.; Thu Ha, T. T. 2016. Integrated tree, crop and livestock technologies to conserve soil and water, and sustain smallholder farmers’ livelihoods in Southeast Asian uplands. In Hiwasaki, L.; Bolliger, L.; Lacombe, Guillaume [IWMI]; Raneri, J.; Schut, M.; Staal, S. (Eds). 2016. Integrated systems research for sustainable smallholder agriculture in the Central Mekong: achievements and challenges of implementing integrated systems research. Hanoi, Vietnam: World Agroforestry Centre (ICRAF). Southeast Asia Regional Program. pp.41-64.

Soil conservation ; Water conservation ; Water availability ; Rainwater ; Water harvesting ; Sustainable agriculture ; Monoculture ; Smallholders ; Farmers ; Farmland ; Living standards ; Highlands ; Land degradation ; Land use ; Land management ; Erosion ; Plantations ; Tectona grandis ; Rubber ; Coffee ; Environmental impact ; Natural resources management ; Local communities ; Catchment areas ; Case studies / Southeast Asia / Lao People's Democratic Republic / Southwest China / Vietnam / Cambodia / Yunnan Province

(Location: IWMI HQ Call no: e-copy only Record No: H047859)

https://www.worldagroforestry.org/region/sea/publications/download?dl=/BK00186-16.pdf&pubID=3760
https://vlibrary.iwmi.org/pdf/H047859.pdf

(3.82 MB)

After reviewing the main causes and effects of land degradation and erosion in the uplands of mainland Southeast Asia, this chapter presents several case studies of recent land-use changes governed by economic, political and institutional transitions, the expansion of teak and rubber tree plantations in northern Laos and southwest China, respectively, and of monocropping coffee in the Central Highlands of Viet Nam. We explain how these environmental disturbances are altering water and soil resources across different geographic scales, from the agricultural plot to the headwater catchment. Examples of coping strategies combining field trials and participatory approaches are illustrated with several case studies taken from research for development activities conducted in Cambodia, Laos, Viet Nam and the Yunnan Province of China. These activities were part of the CGIAR Research Program on Integrated Systems for the Humid Tropics (Humidtropics) in the Central Mekong Action Area. We propose solutions for sustainable agricultural intensification to diversify income, improve dietary diversity and improve natural resource management. The accomplishment of these objectives requires longterm involvement with ethnic minority communities that have been the particular focus in the target areas. The three-anda-half-year lifespan of Humidtropics in the Mekong region was a short period. It would require extension to maintain the carefully built and nurtured relationships with local implementation partners and local farming communities, and reach its full promise.