Articles | Volume 20, issue 7
Research article
08 Jul 2016
Research article |  | 08 Jul 2016

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

Guillaume Lacombe, Olivier Ribolzi, Anneke de Rouw, Alain Pierret, Keoudone Latsachak, Norbert Silvera, Rinh Pham Dinh, Didier Orange, Jean-Louis Janeau, Bounsamai Soulileuth, Henri Robain, Adrien Taccoen, Phouthamaly Sengphaathith, Emmanuel Mouche, Oloth Sengtaheuanghoung, Toan Tran Duc, and Christian Valentin

Abstract. 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.

Short summary
Laos and Vietnam have switched from net forest loss to net forest expansion between 1990 and 2015. Based on long-term field measurements of land use, river flows, and weather data, we demonstrate that forest expansion can have extreme, yet opposite, impacts on water resources, depending on how the newly established tree-based cover is managed. The conversion of annual crops to teak plantations in Laos or to naturally regrowing forests in Vietnam led to increased and decreased flows, respectively.