Managing forests for water [Chapter 3]
Forests are often managed for a wide range of purposes, such as wood production, recreation and biodiversity conservation. Healthy, well-managed forests also store and filter water as well as reduce surface runoff and flood risk. Regrowing forests, on the other hand, can reduce downstream water supplies. Forests that are unmanaged may become overstocked (i.e. have a very high density of trees per unit area). This, in turn, can increase susceptibility to insect outbreaks and the risk of wildfire from the accumulation of fuels (Shang et al., 2004), both of which can have significant impacts on the forest hydrologic cycle (Goeking and Tarboton, 2020). Additionally, some unmanaged and potentially overstocked forests use more water and therefore may produce less streamflow than managed forests (i.e. with less growing stock). Forest managers need to achieve a balance between optimizing water yield (Evaristo and McDonnell, 2019) and keeping sufficient canopy to minimize soil erosion, maintain albedo (i.e. the proportion of incident light or radiation reflected from a surface) and promote water quality. Competing trade-offs between water and non-water natural resource demands from forests is a major forest management challenge (Sun and Vose, 2016). The need for clean, abundant, consistent water supplies is likely to increase as the climate changes and the human population continues to increase (Sun and Vose, 2016). Currently, about 4 billion people are affected by water scarcity at least once in any given year (Mekonnen and Hoekstra, 2016); this number is projected to grow to 6 billion by 2050 (Boretti and Rosa, 2019). Therefore, forest management that is explicitly designed to increase high-quality water supply is needed urgently.