Trees as part of nature-based water management

  • Trees link local to regional and global water cycles through their modification of infiltration, water use, hydraulic redistribution of soil water and their roles in rainfall recycling
  • Nature-based water management is complemented by technical interventions for water retention, redistribution, flow regulation and recycling, but it generally is more resilient and adaptive than concrete and steel structures
  • Understanding forest (and tree) water relations can be characterized by three paradigms: ‘paradise lost’, ‘blue-green water competition’ and ‘full hydrological cycle’
  • Agroforestry can contribute to enhancing nine specified ‘ecosystem services’ that relate to water, with priorities depending on context and ten prototypes for coinvestment
  • Four types of ‘boundary work’ are recognized at the governance level, to link local solutions to global and (sub)national problems

Water has been explicitly (or sometimes implicitly in its climate relationships) discussed in nearly all preceding chapters. Water links the plot, landscape and governance scales of the three agroforestry concepts (Chapter 1), it is a key determinant of tree growth and adaptations (Chapter 2), relevant traits can be a target of tree domestication (Chapter 3); water is an important component of soils (Chapter 4) and treesoil-crop interactions (Chapter 5). The pantropical analysis of agroforestry (Chapter 6) found climate (and specifically the ratio of rainfall and potential evapotranspiration) to be a major determinant of tree cover on agricultural lands. All the landscape examples dealt with water, through restoration and modification of microclimate (Chapters 7, 8 and 12), through contested land use rights and watershed functions (Chapters 9, 10 and 11). One of the key features of small islands (Chapter 13) is a shortage of freshwater storage, while excess and deficits of water are at the basis of many disasters (Chapter 14). In this chapter we will discuss how the shift in agroforestry concepts (from field/farm-level AF1, to landscape level AF2 and governance level AF3, as detailed in Chapter 1) has interacted with research and contributed to an increased understanding of the way all water-related aspects are interlinked, urgent in the current sustainable development discussion, and open to a wide range of tree and agroforestry- based interventions (with several examples of how such interventions have backfired where understanding was incomplete). Hydrological, ecological, social, economic and policy aspects of trees as part of various land uses in relation to water, are tightly linked (a Gordian knot?). Yet, the relationship between tree cover and human water security is strongly contested1 (Fig. 17.1), with ‘pumps’ versus ‘sponges’ as key features of forests2 and atmospheric recycling as arena of debate3.

Policy discussions on forest, trees, water and rights to land have changed over time, but with only a limited role for science-based understanding. In the colonial period presumed hydrological functions that can only be provided by ‘forest’ became a major rationale for the state’s claims on any land not yet converted, for example in Indonesia6. Ecohydrological discussion in the 1930’s focussed on unique functions of forests as sponge (retention) versus an appreciation of multiple land uses that secure infiltration (dependent on terrain, geology and surface conditions) and allow soils to act as sponge7. The debate tried to reconcile practical experience with mechanistic understanding of the water balance, with important implications for the types of forests to be conserved and/or restored. The debate was left unfinished at the end of the colonial period and replaced by other priorities. Space for agroforestry and partial tree cover, and for the agroforesters whose livelihoods depends on ‘state forest land’ had to be created by tackling both the scientific understanding of hydrology, and the power relations between national and local stakeholders of well-functioning landscapes (compare Chapter 9). Elsewhere, colonial policies to enforce soil conservation became part of the struggle for independence in East Africa, and it took long before the negative stigma of top-down prescribed solutions could be replaced by bottom-up initiatives, adjusted to local context. Currently, three forest-water paradigms coexist1 (Figure 17.3). They have been labelled ‘Paradise lost’ (line A in figure 17.1), ‘Blue-green water trade-off’ (line B in Figure 17.1) and ‘Full hydrological cycle’ (Area C in Figure 17.1). The latter includes the concept of an intermediate tree cover optimum at landscape scale, but also ‘rainbow water’ (atmospheric moisture) as part of the wider feedback system, and attributes hydrological impacts to at least five aspects of land cover (Leaf Area Index, surface litter layers, rooting depth, soil structure and specific effects on downwind rainfall). Agroforestry, seen as land use with intermediate tree cover or as a continuum between agriculture and forestry is closely associated with the latter paradigm. This aligns with a recent UN Water report8 on ‘Naturebased solutions’ that seeks a more coherent approach to the various aspects of water flows (availability, quality, avoiding disasters) and storage that matter to large numbers of people around the world (Box 17.1).

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