Del Campo, A. D., González-Sanchis, M., Ilstedt, U., Bargués-Tobella, A., & Ferraz, S. (2019). Dryland forests and agrosilvopastoral systems: water at the core. Unasylva 251: Forests: nature-based solutions for water, 251(1), 27.
Dryland systems occur on all continents and cover about 41 percent of the Earth’s land surface, with little variation in this figure in recent decades (Cherlet et al., 2018). Drylands differ in their moisture deficit and can be classified in four subtypes according to the United Nations Environment (UNEP) aridity index (AI)1 as dry subhumid (0.65– 0.5), semiarid (0.5–0.2), arid (0.2–0.05) or hyperarid (<0.05) (Figure 1).2 Forests and grasslands are the dominant biomes in the dry subhumid and semiarid subtypes, respectively (more than 60 percent of the subtype areas). On the other hand, the arid and hyperarid subtypes are mostly treeless (FAO, 2016) and thus beyond the scope of this article.
Based on their underlying definition (i.e. by AI), annual potential evapotranspiration (PET) in dry subhumid and semiarid lands is considerably higher than annual precipitation, with frequent meteorological droughts. These atmospheric drivers lead to low soil moisture and this, in turn, means slow tree growth and low productivity, resulting in a socio-ecological context of water scarcity. Marked rainfall seasonality, with torrential events followed by long dry periods, and the combination of high intra- and interannual variability, put such regions within the “difficult” hydrology framework, which hampers water security, sustainable development and poverty reduction (Grey and Sadoff, 2007). southern Africa, Australia, the Middle East and Central Asia (Cherlet et al., 2018). The intensification of precipitation and other climatic extremes under warmer conditions is likely to increase water scarcity and moisture deficits in drylands and beyond. Climatic constraints increase the role of soil processes and properties in the regulation and magnitude of water-related issues in drylands, especially those concerned with resource storage (e.g. soil depth, infiltrability, deep-water storage and erosion). Thus, land-use and management practices, especially nature-based solutions, are extremely important for the soil–water–productivity complex. This article uses case studies in dryland on three continents to show the importance of a water-centred approach to dryland management for increasing resilience and adaptation to climate change