Why is agroforestry the most sustainable farming technique in drylands?

Much less than might be expected. A large number of dryland fruit or multipurpose trees survive, grow and produce fruit and biomass under the climatic conditions prevalent at the border between arid and semi-arid climate. Survival and productivity of such trees is enhanced by planting them in terraces, dams or pits where runoff water accumulates, allowing agricultural production deep into the arid dryland zone. Many dryland trees create a leave litter layer improving soil fertility and productivity. Similarly, windbreak effects and mutual shading create a superior microclimate compared to bare areas promoting emergence of self-sustained systems of continuously increasing productivity with enhanced resilience to climate calamities. The woodland shown below (Fig. 1) was planted in 1992 with minor initial irrigation and has since grown and developed without further inputs, at mean annual precipitation of 220 mm.

The leaf and seed litter layers provided by trees also protect soil from erosion and drying out. As shown in the graph, woodland soil protected by leaf litter (see Fig 2) maintains double soil humidity throughout the year. This enhanced moisture and nutrient content facilitated planting of commercial agroforestry species (an olive seedling in fig. 1) applying only three single 10 liter irrigations in early summer permitting the tree to get established and grow without any further care.

Such wooded dryland ecosystems will continue developing and improving as deep rooted trees continue recovering more and more limiting nutrients from deep soil and deposit them as leaf litter on the topsoil, and increasing amounts of moisture are able to infiltrate and accumulate in deep soil. The woodland soil shown has the highest levels of nitrogen, phosphate, potassium, soil organic matter (SOM) and water infiltration rates among all ecosystems tested.

Consequently it is possible to establish edible dryland forests in degraded drylands that provide food and fodder as well as a wide range of other ecosystem services such as animal feed, erosion control, carbon sequestration, biodiversity enrichment, and more. The fascination in this approach is the unexpected fact that all this can be created from essentially nothing, without significant inputs of irrigation water and fertilizer, solely by stimulating natural self-amplifying recovery mechanisms.

Trees producing edible fruit, seeds, foliage or other valuable goods have been the secret for survival of humans in drylands in all five continents throughout the evolutionary history of mankind. Trees are the dominant ecosystem component in semi-arid and sub-humid woodland, and spread deep into arid and hyper-arid zones along dry riverbeds or in temporary flood plains. Some of the major constituents of semi-arid and arid Mediterranean woodland are carob, oak, Ziziphus spina christe, Pistacia etc having edible seeds or fruit. Domesticated almonds, grapes, figs, pommegranates and olives also thrive in this climate without irrigation. Around the world, species such as Moringa, Marula, Balanites, other variants of Ziziphus, Prosopis and many more fruit and nut trees are well known to local inhabitants as essential food sources often available during the dry season (see figure 3). Useful species of interest for different climate zones can be found at the database of the World Agroforestry Center.

This is an important and often discussed subject with many claiming that trees compete with annual grassland vegetation for resources, and thus reduce rangeland productivity. However, this is only true under certain conditions and not a general rule, and depends on climate, soil, and the tree species planted. Some trees have leaves containing allelopathic substances that suppress germination of annual plants, or form a thick layer of resilient leaf litter that that can inhibit seed germination. Unfortunately many such trees thrive well in drylands and have been widely planted in the Negev, e. g, Eucalyptus sp, Acacia saligna and Acacia salicina, whereby the latter two also are profoundly invasive though they are still being propagated as agroforestry species in some countries.

Other species however are reported to significantly stimulate annual plant growth nearby, among them Faidherbia albida, Albizia lebbeck, carob and Acacia victoriae especially in arid environments. Those tree species, in contrast to above toxic ones, also produce edible foliage and seeds contributing to rangeland productivity. Incorporation of such species has been modelled to increase fodder yield in arid shrubland several fold, and also providing at least one year of fodder reserve during drought (see this publication on modeling the optimal grazing regime and Fig. 5 to learn more).

Several species such as Faidherbia albida are also commonly incorporated into cropping systems in Africa (Fig. 6), where they are reported to increase yields of maize or sorghum by a factor of two to three (http://www.slideshare.net/agroforestry/evergreen-agriculture-beatingfamine ). Several countries have adopted a declared policy of incorporating at least 10% of this tree species into their farmlands. The expected result is enhancing soil quality, resilience to climate extremes, and increased farm income and food security.