Contradictory hydrological impacts of afforestation in the humid tropics evidenced by long-term field monitoring and simulation modelling

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...

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Published in:Hydrology and earth system sciences 2016-07, Vol.20 (7), p.2691-2704
Main Authors: Lacombe, Guillaume, Ribolzi, Olivier, de Rouw, Anneke, Pierret, Alain, Latsachak, Keoudone, Silvera, Norbert, Pham Dinh, Rinh, Orange, Didier, Janeau, Jean-Louis, Soulileuth, Bounsamai, Robain, Henri, Taccoen, Adrien, Sengphaathith, Phouthamaly, Mouche, Emmanuel, Sengtaheuanghoung, Oloth, Tran Duc, Toan, Valentin, Christian
Format: Article
Language:English
Subjects:
Abandonment
Afforestation
Afforestation effects
Agriculture
Analysis
Biodiversity
Calibration
Catchment area
Catchments
Computer simulation
Corn
Correlation analysis
Crops
Cultivation
Detection
Earth Sciences
Ecosystem services
Environmental changes
Environmental impact
Environmental monitoring
Environmental risk
Flood control
Flood risk
Forest communities
Forest regeneration
Forests
Headwater catchments
Headwaters
Hydrology
Hydrometeorology
Inspection
Land use
Modelling
Modernization
Mosaics
Plantations
Planting
Precipitation variability
Rain
Rainfall
Rainfall variability
Regeneration
Regeneration (biological)
Rice
Sciences of the Universe
Seasons
Service restoration
Shifting cultivation
Simulation
Soil
Soil conservation
Soils
Stream discharge
Stream flow
Streamflow
Streamflow regime
Studies
Tree planting
Trees
Tropical climate
Tropical environments
Variability
Visual inspection
Water balance
Water conservation
Water resources
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Summary: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.
ISSN:1607-7938
1027-5606
1607-7938
DOI:10.5194/hess-20-2691-2016