Hydrological Functions of Tropical Forests: Not Seeing the Soil for the trees?

background

This literature review provides a summary of hydrological functions in tropical forests and includes differences in the hydrology of disturbed areas, secondary forests, and mature forests.

Conclusions & Takeaways

Reforestation is usually too small to have an effect on climate (local temperature and rainfall), but in some situations, such as montane cloud forest, the forests rely significantly on fog drip, so reforestation could have an impact on total soil water (water from fog drip averages 5-20% of total water but sometimes much higher). Conventional wisdom asserts that forests act as a sponge by soaking up the surface water during rainy season and releasing it slowly during the dry season, but this has not been demonstrated (and it is hard to study because of confounding factors like geology, natural climatic variation, and lag time for the reforested site to have a measureable effect). Forest clearing will result in greater surface runoff but reduced infiltration capability. Net effects on the flow regime will depend on the depth of the soil and the size of the underground water reservoir. Clearing of forests will result in greater sediment loads (erosion), but properly managed grazing or agriculture can have low rates of erosion. Large scale erosion events (landslides) are more dependent on geology (because they are deep) and surface vegetation doesn't matter. Litterfall is an important determinant of reduction in erosion, and restored forests often lag in their accumulation of litter, so reforestation can take several years (or decades) to reduce erosion. Modifications like check dams and sand traps may be more effective in reducing runoff than can reforestation.