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...
| Main Authors: | , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-07-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | http://www.hydrol-earth-syst-sci.net/20/2691/2016/hess-20-2691-2016.pdf |
| 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. |
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| ISSN: | 1027-5606 1607-7938 |