The water regulation service of dryland agroforestry ecosystems

Trees constitute a structural component of biodiversity of much significance in driving the water cycle at local, watershed and global scales. Through this they underpin the water regulation service of terrestrial ecosystems, agroforestry systems included. Non- tree plants, that jointly with trees constitute the vegetation covering our planet’s lands are similarly instrumental. However, quantitatively and sometimes qualitatively, the ef - fect of trees on the water cycle, and hence their contribution to the water regulation service is much stronger than that of non-tree plants (Figure 6.4.3.1). There are many differences between trees and non-tree plants. Traits differentiating trees from non-tree plants in water cycle functionality include their physical dimen - sions expressed in greater height and volume of foliage and canopy, and in the below- ground depth and volume of their root system. Furthermore, their perennial life cycle and high morphological resistance to physical pressure confer resilience to the water regulation service. In addition, while non-tree plants occur in all terrestrial ecosystems, tree abundance, density, species richness and diversity, decline towards the climatic extremes of the planet and trees are rare or absent in polar, alpine and the desert ar - eas. However, almost all cultivated lands occur in areas with climate favorable to trees and their diversity, density and spatial distribution is determined here by management rather than climate. This is often the case of agroforestry systems, where the land user who manages the tree species composition and abundance determines the degree to which trees underpin the water regulation service of the system. In this section we will address the water regulation services of trees, even though other plants, including crops, contribute but usually with much smaller effect

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