Photo credit: CGIAR
To tree or not to tree?
It seems like a safe bet to say that planting trees is a good thing: who could argue with that? Well, not so fast. A group of researchers, after 15 years’ careful work, have finally confirmed that growing trees in a plantation—in this case, a teak plantation—actually increases water runoff, erosion, and downstream silting when compared to naturally regrowing forest areas.
According to the most recent Global Forest Resources Assessment, published by the UN’s Food and Agriculture Organization in 2015, more and more countries (13 at last count) are moving from losing forests to increasing their forested area. It is not too early to want to understand the impact of these changes.
Scientists from the CGIAR Research Program on Integrated Systems for the Humid Tropics and the French Institut de Recherche pour le Développement worked with colleagues from Lao PDR, and Vietnam to compare different land-use situations in two upland agricultural areas exposed to the same monsoonal climate. In the Houay Pano catchment of northern Laos they looked at what happened when local people gave up rice-based shifting cultivation and started cultivating teak trees in plantations. In the Dong Cao catchment of northeastern Vietnam, they looked at land that had gradually reverted to natural forest after being used for annual crops and fodder, or mixed-trees plantations.
In northern Laos, converting farm land to teak plantations increased the seasonal water flow in neighbouring streams; meanwhile, at the Vietnamese research site, moving from annual crop and mixed-tree plantations to naturally re-growing forest led to decreased water flow. This may appear counter-intuitive. It all came down to how the land was managed.
In Vietnam, the natural ground cover allowed rain to penetrate into the soil, which allowed plants to develop deeper and thicker root systems as well as a denser tree canopy. So the rainwater was better absorbed by the soil and then transpired by the growing trees (i.e. evapotranspiration), resulting in less water leaching into the streams during both the wet and the dry seasons, and an overall reduction of erosion.
Read the full article: CGIAR