Drylands consist of hyper-arid, arid, semiarid, and dry subhumid categories of the aridity index classification, and occupy 60.95 million km
2 or 41% of the earth's land area. Out of the 2 billion inhabitants of the drylands, about 90% live in developing countries and are relatively more dependent on natural resources than other groups of populations. Between 6 million and 12 million km
2 of these areas are affected by desertification, reducing their capacity to sustain human livelihoods. Thus, tropical drylands are more exposed than other ecological regions to the threat of environmental degradation (
In this context, silvopasture, which is a major traditional agroforestry system (AFS) in the semiarid and dry subhumid regions of the world, merits special attention. In silvopastoral systems (SPS), various types of trees are grown in association with understory shrubs and forage species to support livestock operations (
In Brazil, drylands extend over 900,000 km
2, in the tropical biomes of Caatinga, Cerrado and Atlantic Forest. The Caatinga region that covers 10% of Brazil's land area and 1.4 % of total global area of drylands (
The semiarid Caatinga biome in the Northeast of Brazil that occupies about 845,000 km
2 is located between 3° to 17° S, and 35° to 45° W (
The soils of the Caatinga region are stony and shallow, with many outcrops of massive rocks. The main soil orders (US Soil Taxonomy) are Ultisols, Alfisols, and Oxisols (Argisol, Luvisol, and Latosol according to the Brazilian soil classification); in general, these soils are of average agricultural potential (
The main vegetation type of the biome is a deciduous woodland (
Livestock activities in the Caatinga biome are concentrated in a region that resembles a savannah, with an abundant herbaceous vegetation and arboreal shrub covering about 20%, and the tree density varying from 0 to 300 plants/ha. About 70% of the tree species of the Caatinga are fodder trees, some better forage as leafy materials, and others as litter in dry periods (
Livestock in the biome are mainly cattle, sheep, and goats. In the native forests without management, they derive more than 70% of their diet from the woody vegetation (
During the dry season, which is the season of nutritional stress, the Caatinga vegetation has little or no forage in the "bipedal foraging zone" (accessible to animals while standing up on two rear legs and leaning on to the tree) and leaf litter is the dominant component of the available forage. The quantity of leaf litter available during the dry season varies from 500 to 1,500 kg/ha, which constitutes 20-70% of the animal's annual diet (
In the Caatinga, more than 90% of the farms have herds of more than one animal species simultaneously (
Despite water limitations, more than 25 million people live in the Caatinga biome. Currently the main land-use systems of the biome are livestock and crop production and firewood collection from the native vegetation to support the industry and the local demand (
Historically, livestock-related activities were the primary occupation of the inhabitants. That situation has not changed, and continues to be very important for local farmers. In a drought year, the agricultural production in the state of Ceará, a major state of the Caatinga region, declines by up to 84%, whereas livestock activity drops by only 20% (
As mentioned, traditional management of the Caatinga biome includes site-specific silvopastoral activities of various intensities that are influenced by a multitude of ecological and sociocultural characteristics. Therefore, recommendations for management-improvement opportunities have to be of a broad nature ranging from improvement opportunities for native forests to SPS opportunities. Some of the suggestions and recommendations given below have been implemented to varying degrees in the region, while others are innovative opportunities based on experience from elsewhere.
The guidelines for sustainable management of the native Caatinga forest (
The carrying capacity of the arboreal shrub vegetation without management for supporting animal production is very low: an area of about 10 ha is necessary to support one life stock unit (LSU) of cattle and 2 ha/animal for sheep or goat. Overall, in the native vegetation without management, goats and sheep perform better than cattle (
Four main SPS management practices have been suggested for increasing forage productivity or animals' accessibility to forage of the native vegetation of the Caatinga region. These are: thinning, coppicing, thinning + coppicing, and enrichment (
Thinning
Thinning the tree stand opens up the overstory canopy and helps more sunlight to be transmitted to the understory and thus increase yield and availability of herbaceous understory plants for cattle and sheep (
One important consideration for this management is to decide which trees (of different species as well as individuals of the same species) to be removed
Some common norms to be observed are that 40% tree cover (around 400 medium size trees/ha) may be maintained, and thinning should be avoided in areas of slopes steeper that 25%. Thinning regimes should be designed such that the tree arrangements after thinning should result in one of the three configurations: (1) Savannah (trees dispersed on landscape); (2) tree clusters consisting of small groups of trees together as a cluster or group and the clusters scattered on the landscape; and (3) tree alleys in which trees are planted close together in single or double rows with wider distances between tree rows (as in alley cropping type of agroforestry). The alley type of arrangement is recommended for slopes of 10-25%, and the savannah and tree clusters for slopes less than 10%, for facilitating soil conservation (
Coppicing
Coppicing refers to cutting the trees' branches or trunks at a low height, usually 20-30 cm above ground, to facilitate vigorous re-sprouting of new shots in an accessible stratum for the animals, enhancing the quality of the animals' diet (
Coppicing should not be done uniformly to all tree species; the rate at which the trees resprout varies considerably among species. Some resprout vigorously, while some do not resprout at all and they die off once they are cut. Species that are preferred by animals and can withstand coppicing include:
Enrichment planting
Enrichment or enrichment planting refers to the practice of introducing additional plants into an existing stand of plants, increasing the overall productivity from the land.
The tree component to be used for such enrichment planting could be either exotic species such as leucaena (
Reports on the survival of tree seedlings used for enrichment planting in the Caatinga are not available; this is an aspect that needs investigation. Furthermore, studies from two arid regions, northwestern India (
Fodder bank
Management of the native vegetation can and sometimes should be done with techniques and treatments that may be new yet relevant to the location. Introduction of a "fodder bank" is one such strategy. A fodder bank is an assemblage of tree and shrub species that are predominantly fodder species, but are multipurpose in nature, providing multiple products and services such as for forage, fruits, soil fertility improvement, and biodiversity habitats. They can be assembled as woodlots, live fences, wind breaks, soil conservation barriers, and for similar other purposes. Usually, the fodder is cut and carried to stall-feed the animals, but sometimes are allowed to graze on the fodder bank in a controlled manner for defined periods of time (
The Caatinga Research Institution of the Instituto Nacional do Semiárido, Campina Grande/PB, is studying native and exotic trees for fodder banks; the species include
Non-conventional feed resources
Another plant that is suitable for feed supplementation during the dry season or drought periods is cactus. In the Caatinga, goats grazing on native vegetation half year had their diet quality improved in the reproduction period with the cactus
Crop residues of common crops could also be used to feed animals, but with nutritional supplementation; such common crops include maniçoba (
Conversion of orchards
Another type of SPS being adopted in the biome is the introduction of animals in orchards and plantations to help in weed management and for the manure. This is being practiced in the irrigated fruit-orchards of mango (
Special systems for small and medium-sized farms
Considering all the possible management approa-ches described above, different systems have been recommended for the Caatinga region with the general objectives of improving nutrient cycling, forage availability and soil resilience. Two main systems are described below, but several other possibilities exist (
The Saf-Sobral system is one that focuses on food production. Food crops are grown in the rainy season in about 20% of the total land holding (which could be < 10 ha). After the crop, a fodder bank is established where the animals can graze in the dry season on both the crop residues and the leguminous fodder trees. This is a promising option for smaller farms with land holdings of area > 3 ha. Considering that most of the farms in the biome are < 10 ha in area, this system has a great potential of adoption and can play an important role for food security (
Another promising system for the region is the Caatinga-Buffel-Leucaena System, for farms that are larger than 20 ha, but preferably larger than 100 ha. This system has three areas: an open pasture, a fodder bank, and the native vegetation. The native forest is 1/3 to 2/3 of the total area; animals can graze over the area mainly during the rainy seasons (3-4 months), when the native vegetation offers good forage of herbaceous and tree vegetation. The area of the open pasture should be no more than 80% of the area and the fodder bank 10-20% of the pasture area (
Silvopastoral systems practiced in different parts of semiarid to arid regions of the world have been described in several publications (
In Chile, SPS management practices include fodder banks, grazing in croplands, family gardens; but due to the high aridity in many regions, only a few tree species are able to survive producing forage/food,
In East and Central Africa, SPS are
Overall, the nature of management of these systems is in accordance with the general land-use scenarios and socioeconomic conditions of the regions and countries concerned. Thus, in Africa and the Indian subcontinent, the SPS systems are more subsistence-oriented and labor-intensive than in the Mediterranean and southern regions of South America, where the system management is more capital-intensive and less labor-intensive. An overview of those systems and their characteristics is beyond the scope of this paper. Instead, some common features related to livestock production and SPS in the semiarid regions worldwide and the commonalities and contrasts between the Caatinga SPS and SPS in other parts of the world will be examined briefly in the following sections.
Calculations and estimations for the systems' resilience
Estimations and calculations based on computer models are becoming popular in designing management options for sustainable of livestock activities. Detailed surveys and inventories of local conditions and stocks of natural resources including vegetation characteristics, species composition, tree cover density, biomass productivity, litter production, forage consumption, and animal carrying capacity are some of the points need to be measured for the Caatinga. To the extent possible, farmers should be educated about these calculations and their experience taken into proper consideration.
Some computer models are available to estimate the productivity of arid environments.
Soil resilience
In the management of any native vegetation as well as anthropogenic grasslands for livestock activities in drylands, plant productivity/survival is a main concern, for which soil resilience plays a key role. Studies have shown SPS as the most efficient land-management system in the Caatinga to minimizing soil degradation processes, reducing water erosion and losses of nutrients and carbon (
In Jodhpur, Rajasthan state, India, an arid region, the tree litter from a SPS was reported to have increased soil fauna abundance and soil fertility (
Seasonality and grazing regimes
Grazing limits are very important in the semiarid regions. Overgrazing can cause soil compaction, exterminate seed bank, and curtail the plant's capacity for regrowth. Management measures should be put in place to overcome the effects of dry season and drought years on the availability of forage and the impact of the animals on surviving plants as well as soil erosion. Grazing regimes can play an important role in that. Management of areas without animals can avoid effects of overgrazing and will be economically important; for example, by supporting those that require extra nutrients and energy such as pregnant or lactating animals.
In a study in Northern Oman, areas not grazed by goats had substantially higher species diversity and herbaceous mass yield compared to grazed areas (
A study in Ethiopia compared three grazing regimes: CC, seasonal grazing (SG), and continuous grazing (CG). The CG had the lowest species diversity and richness during the rainy season, and affected negatively the soil bulk density, total nitrogen, and the herbaceous basal cover which was 6.8 times smaller than CC. For the occurrence of not desirable or intended plants, like non-forage species, CC was also the best management for the dry season, although in general SG regime improved soil phosphorus (
Animal welfare
The maintenance of source of animal feed can be considered the main focus, but the effect of strong light intensity without shades over the animals in hot regions may play a significant effect on their welfare and productivity. The potential of microclimate amelioration by the trees to cattle for example is described for the Cerrado biome (
Salinization
Soil salinization is a major concern in many arid regions worldwide, and the Caatinga region is no exception to this (
The best species for halophytic fodder crop preferably contain > 5% protein and < 10% ash. Many halophytes can fulfill the animals' protein requirement, but if the species has 10-15% ash, it may be used as supplements to regular feed (
Social aspects
Social considerations are also very important for livestock and SPS activities in arid regions, especially when considering the adoption of new management strategies. Studies on the cost of implementation of SPS, economic evaluation and farmers perceptions should also receive more attention, both in the Caatinga and worldwide. Issues such as animal selection, educational level of farmers, gender role, land concentration, and local economy play important roles in land management in arid regions too as everywhere else.
In the Caatinga, for example, most farms are less than 10 ha in area and most farmers do not know the concept of overgrazing. In an effort to intensity land use, sometimes they adopt poor management practices that compromise the system's resilience, threatening food security (
Although some animal species may be preferred for ecological aspects, farmers may have important concerns that are not limited to just livestock productivity. In a study about decision models in a dryland in Zimbabwe, goats were almost always preferred compared to cattle, the opposite occurred only when cattle were used for draft power. Although cattle productivity decreased due to this activity, the considerable increase in crop production was reported to have compensated for that, especially by wealthier farmers (
Gender roles also should be considered for any future projects, and it may play a key role in rural projects.
Economic activities in the region can also push farmers to adopting a specific activity. In the Caatinga, for example, the firewood activity generates about 90,000 direct jobs in the rural zone and have become attractive during the past few years. Although in general this practice causes deforestation, a different output is described when the firewood-gatherers received governmental support. In 2015, a total of 4,000 families in 468 sustainable forestry management plans were maintaining or increasing the biodiversity of plants, with positive impacts on fauna, and increasing their domestic income (
In terms of ecological and land-use characteristics, the Caatinga region is distinctly different from and less studied than the two other ecological regions between which it is "sandwiched" - the Cerrado to the left and the Atlantic Forest to the right - as described in Section The Caatinga Region of Brazil. Although some studies on the Caatinga region have been conducted as reviewed in the previous sections, the region is perhaps one of the least-studied and economically backward in Brazil. Little is recorded on its ecology and land-use history, resource utilization patterns, and development potential. Therefore, the relevance of the Caatinga experience to other regions has to be projected based more on intuition than on scientific background. Nevertheless, given that the world's drylands and the proportion of world population affected by water scarcity and severe drought conditions are projected to expand by 10% by the end of this century under a high greenhouse-gas-emission-and-global-warming scenario (
The lack of a rigorous body of scientific knowledge on the Caatinga is a serious deficiency, but also a major opportunity for future efforts. The seemingly vast biodiversity of the region is one such exciting opportunity. Biodiversity of ecosystems is proving to be one of the best defenses against extreme weather and rising temperatures (
A major area of unexplored potential of the Caatinga biome in terms of the ecosystem services is related to soil carbon storage. Some of the studies from the Caatinga region on this aspect have been mentioned in Section Developments in SPS Management Worldwide with Relevance to the Caatinga. Additionally, numerous studies have recently been reported on enhanced soil carbon sequestration under SPS compared to woodlands, both natural and planted.
The SPS management practices in the Caatinga are basically similar to those of many arid regions; however, the inherently high level of biodiversity of the Caatinga offers several possibilities for arrangement of various tree/shrub/and grass components according to the needs and browsing/grazing habits of animals. Research-based knowledge of the local biodiversity and the specific management for each component while grown in combination with other plant species, and the scope for development of varieties from both the Caatinga and for other regions are two important management-related research priorities. Coppicing or/and thinning in a woodland with the adequate number of animals may create an even more sustainable system that would increase the growth of the grass component and accessibility of the fodder bank, and additional input of animal manure to the system.
Environmental and ecological specificity of the Caatinga biome is one of its unique features, which restricts the comparison and extrapolation of the management used in the biome to other hot semiarid regions. However, the limited studies in the region focusing on animal - vegetation interactions (behavior and response of animals to native vegetation, and vice versa) have helped develop management strategies to improve the productivity and sustainability of the systems. The lessons learned from these experiences could be valuable for the design of productive and sustainable SPS in other semiarid and more humid regions too. The social aspects mentioned above, though not analyzed in detail, may present formidable challenges and barriers for SPS adoption. Nevertheless, given that the options for such ecologically challenged environments are limited, the experiences from Caatinga management deserve serious consideration in global development paradigms.
Livestock production and management activities in drylands the world over play an important role in the livelihood and food security of local communities as well as sustainability of these fragile ecosystems. The traditional SPS of the Brazil's Caatinga region have stood the test of time as an excellent example of the resilience of the ecosystem and the ingenuity of the local farmers in managing it sustainably. The role of native trees in supporting animal production has been remarkably illustrated by this experience. The lack of any significant volume of scientific input in understanding the potential of these native species and opportunities for improving their productivity has, however, hampered the development of these systems for reaping substantial benefits that are feasible.
Relatively simple management interventions such as thinning, coppicing, and enrichment planting either alone or in combination could offer immediate benefits in the short term. In the longer term, careful introduction of desirable exotic species that are adaptable to local conditions biologically as well as socioeconomically could be a promising opportunity. Vegetation management techniques that allow at least 40% of biomass to be "reserved" for dry season forage have been shown to be extremely desirable for protecting the soils against forces degradation. Fodder banks and use of non-conventional fodder sources such as cactus, and introduction of fodder species into existing orchards and plantations are other management opportunities that deserve serious considerations.
The experience gained from other SPS systems in tropical semiarid regions such as system resilience in response to management, adjusting the seasonality of grazing regimes according to forage availability and weather conditions, and importance of animal welfare, could be adapted for the improvement of the Caatinga. Likewise, the Caatinga experience of SPS could be valuable for other semiarid regions around the world. Biodiversity and climate-change mitigation are two unique aspects of ecosystem services provided by such traditional systems that are seldom recognized. For example, the power of biodiversity in the wild, with which the Caatinga is enormously endowed but the extent of which has not yet been studied, is now known to surpass what has been predicted by experiments and models. Understanding these systems and protecting them for long term benefits rather than "exploiting" them for short-term returns is a strategy that needs careful consideration and successful implementation. This applies to SPS not only in the Caatinga region, but in the entire drylands worldwide.
The paper draws heavily from the results of the work of researchers and institutes of the Caatinga region that have been working for decades on developing SPS in the region. The authors are grateful to all of them.