Effects of the woody component (trees, shrubs, palms, and bamboos) of an agroforestry system on soil and crops.
Abstract
Agroforestry Systems (AFS), or the association of trees with crops (or animals), is a strategy for land management and use that allows production within the sustainable development: (a) environmentally (production environmentally harmonic); (b) technically (integrating existing resources on the farm); (c) economically (increase in production), and (d) socially (equality of duties and opportunities, quality of life of the family group). As an intentional integration of trees or shrubs with crop and animal production, this practice makes environmental, economic, and social benefits to farmers. Given that there is a set of definitions, rather than a single definition of Agroforestry (AF) and AFS, it is justified to explore the historical evolution and the minimum coincidences of criteria to define them and apply them in the recovery of degraded areas. Knowing how to classify AFS allows us to indicate which type or group of AFS is suitable for a particular area with its characteristics. The greatest benefit that AFS can bring to degraded or sloping areas lies in their ability to combine soil conservation with productive functions. In other words, AF is arborizing agriculture and animal production to obtain more benefits including climate change adaptation and mitigation by ecosystem services.
Keywords
- agroforestry
- agroforestry systems
- silvopastoral systems
- land use
- tree biomass
- climate change mitigation
- carbon sequestration
- live fences
- shade trees
1. Introduction
Agroforestry systems (AFS) date back to the Mayan civilization, from 600 to 300 BC, with an apogee estimated to have lasted until 300 or 900 AC. this culture developed in the region of humid forests, but it is claimed that its agrarian system would have developed in the highlands of Guatemala until reaching the Yucatan jungle, where they practiced a pre-Hispanic style of agriculture adapted to forest management, which may well be called agroforestry [1, 2, 3]. The Mayan were poly-farmers; so that, they can be considered a culture with knowledge of land use and forest management; they used to practice a shifting cultivation system, which implies rotation of land use with periods of farming and resting the soil, and sometimes the selective logging leaving some useful trees. They farmed in small fields or clearings in the forest, and from the neighboring forest they took medicine, food, and building materials. This whole system of management of the natural forest and itinerant agriculture was based on the knowledge of the phenological cycle of certain trees. They also practiced horticulture and fruit growing in a multi-story system [4, 5].
It is estimated that in Latin America (LA) the AFS reaches an area between 200 and 357 million ha, including 14–26 million ha in Central America (CA), the most prominent are the commercial Silvopastoral Systems and the AFS of perennial crops under shade including coffee and cocoa plantations [6]. Although, these figures may have changed today given that the SPS has increased due to climate change mitigation actions and the AFS with coffee and cocoa may have decreased. An updated LA inventory of agroforestry areas would be valuable to land planners, resource managers, and decision-makers. This limits the amount of data that can be useful for multi-scale efforts.
A conceptual controversy may arise about whether agroforestry is a forestry activity or an agricultural one. Agroforestry as a concept should not be confused with other related terms, such as forest farming, which covers all the effects of forests and trees on the environment and agriculture, particularly the related socio-economic aspects. So not any kind of random combination of forest, fruit trees, ornamental trees, or service trees with crops or pastures is defined as an agroforestry system. It is also required that their combination be intentional, carried out systematically, and to produce various types of products; the system is the result of an important interaction, both ecological and economical between various types of crops; and that the system maintains or, as far as possible, improves the productive capacity of the land. There are three essential conditions to define an AFS: (1) at least two plant species interacting biologically; (2) at least one of the plant species is a tree or woody perennial; and (3) at least one of the plant species is managed for crop production (annual or perennial) or forage [2, 3, 7].
2. Historical development
One of the first documents on agroforestry in CA (CA) was possibly that of Cook in 1901 [8] who recognized several beneficial effects of shade trees, particularly legumes, on coffee plantations. Later, Holdridge in 1951 [9] described the use of
According to Holdridge [11], there are three major basic land uses agricultural, grazing, and forestry, and while other human activities occupy land (such as for industrial purposes, urban developments, and transportation infrastructure), they do not directly use the soil resources in the sense of the three major uses. Agroforestry activity arises when one of the main uses, agriculture or grazing overlaps with forestry. The mixture of species with different requirements also allows an enhancement of the interception of radiation by vertical stratification of the components and better use of horizontal space [12].
Combe in 1979 [13] identified three main fields of hypotheses related to AFS within the framework of economics, ecology, and forestry.
It is the set of land use and management techniques that involves the combination of trees with crops (annual and/or perennial), with animals, or with both at the same time, in a plot, either simultaneously or successively, to obtain advantages of the combination.
These combinations can be simultaneous or staggered in time and space, and their objective is to optimize the production of the system and ensure sustained performance [7, 14].
With the creation of the International Centre for Research in Agroforestry-ICRAF (Currently World Agroforestry Centre) in Nairobi, Kenya in 1977/78, a space for discussion and analysis of agroforestry issues was established. Within this framework of internal debates, the initial ideas were refined, and a definition was agreed upon in which the criterion of “deliberate association” and that of “significant ecological and/or economic interactions between its components” was highlighted [12, 15]. In the decade of the 80s, there was agreement that agroforestry is a modality of integrated land use that seeks greater production, especially under conditions of marginal land or low level of inputs in the same area, and some cases of AFS in CA are exemplified, such as the coffee plantations or shaded cocoa plantations of
Initially, most studies in agroforestry were descriptive from a biophysical point of view, in addition, it was accepted that agroforestry was a new name for a set of old practices; but much attention was paid to socio-economic aspects [18, 19], which have been widely discussed by other authors [20], and include a great diversity of products such as wood, foliage, fruits, resins, fuel and fodder; and numerous environmental services (climatic, hydrological, soil, ecological) and human (ethical and esthetic). Most definitions highlight the interactions among plant or animal components and their local environment and the spatial and temporal patterns of productive activities. Furthermore, open the possibility of considering and planning the social relations of production, and the interactions between communities and the outside world. Most of these aspects have been contemplated by Montagnini et al. [21, 22] in their comprehensive books on agroforestry.
3. Interactions among components in AFS
The functioning and adaptability of AFS depend on a dynamic relationship between plant species (a woody component with annual or perennials crops) and their abiotic environment (soil and water), as well as physical and chemical interactions in the environment (rainfall, temperature). These interactions and processes are of great importance for the long-term sustainability of the system. While the interactions are complex and interrelated simultaneously, they can be simplified from the point of view of the biological relationship between the two basic populations of an AFS, the woody component, and a crop; they may benefit or damage each other; or in other cases, the relationship may be neutral, all this depending on species and density of the tree component, the type of shade it produces according to the type of, type of canopy, tree crown, its branching habit, all of which have a fundamental role in AFS. After all, and since an AFS is an agroecosystem, which according to Hart [23], is an ecosystem that includes an agricultural or livestock productive component (crop populations, domestic animals, or both), an AFS can be syncretical defined as an agroecosystem with a woody perennial or tree component (Figure 1).
The effects of the woody component (trees, shrubs, palms, and bamboos) of an agroforestry system on soil and crops of an agroforestry system on soil and crops are very important because AF can increase farm productivity in several ways; first, the total output per unit area of tree/crop/livestock combination is greater than any single component alone; second, crops and livestock protected from the damaging effects of wind are more productive; and third, new products make the financial operations of a small agricultural enterprise more diverse. These effects are shown both on the soil and in crops and are outlined in Table 1.
Effects of the woody component | |
---|---|
On soil | In crops |
The leaf litter is the source of organic matter | Shade avoids excessive exposure to radiation |
Nutrient supply | Intercept and mitigate wind |
Improves soil structure | Attenuate the impact of rains |
Controls erosion | Shade reduces air temperature |
Favor water infiltration | Increase relative humidity |
Limit runoff | Reduces weed dispersion |
Reduces soil temperature | The positive effect of shadow |
4. Canopy effects in agroforestry systems
The canopy is a set of crowns and branches of the trees; it is like a filter that intercepts the photosynthetically active radiation (PAR) or light that reaches the associated crops under the canopy and modifies it in quantity and quality. This interception projects a shadow, with physical effects (light/shadow, absorption efficiency, spectral modification of the transmitted light), and physiological actions are also triggered, such as photocontrol of germination, elongation of internodes, leaf expansion, and the development of the photosynthetic structure in the associated crops (Figure 2).
The canopy is characterized by having a structure and a floristic composition that can be managed, thus regulating the amount of shade depending on the crop’s needs and the farmer’s objectives. To measure the density of the agroforestry canopy, the Leaf Area Index (LAI) can be used, which represents the sum of all the existing leaf areas in a soil area. The LAI is an indicator of the canopy’s ability to intercept solar radiation and predict the type of shade it produces dense, medium, or light shadow. The type of shade that the canopy produces can also be expressed in the percentage of coverage of the cups, in expressions such as 50% shade; although it is not necessarily an accurate indicator because the shadow is a dynamic process that moves on the floor of the AFS as the sun makes its apparent movement on the horizon. The position, shape, and accumulation of tree shadows, in different places and at different dates and times of an agroforestry plot, can be calculated with
5. Classification and characterization of agroforestry systems
Classifying agroforestry systems including their environmental and site variants took a long time, without reaching a consensus or a global classification. In this context, ICRAF conducted a global inventory of AFS between 1982 and 1987, the results of which resulted in a classification scheme that is generally accepted today [16, 17, 18]. This inventory was designed to collect, synthesize, and disseminate information on existing AFS in developing countries. As a result of it, Nair in 1993 concluded that “
Classification of traditional AFS | Type of AF system | Example of agroforestry systems |
---|---|---|
Sequential | Shifting agriculture | The traditional agriculture of cutting and burning trees practiced since ancient times. |
Taungya Systems | A temporary combination of a forest plantation during its initial phase, with the production of annual crops until the shade of the canopy allows it. | |
Simultaneous | Trees with annual crops | |
Trees with perennial crops | Growing coffee or cocoa under shade trees such as | |
Agroforests | Management of secondary forests, in association with one or more tree species of economic utility. Systems Quesungual o Kuxum-Rum. | |
Silvopastoral systems | Association of trees with pastures and livestock. Grazing in forest and fruit plantations. | |
Mixed home gardens. | Characterized by their complexity, are multi-specific, combine various forms of life and maintain production throughout the year. | |
Linear systems or in alignment | Live fences | Fences with live poles to which the wire is fixed and periodically pruned. |
Live hedges | A row of tree species was established at very close distances. | |
Windbreak curtains | Multiple rows of tree species are planted perpendicular to the direction of prevailing winds. |
6. Sequential agroforestry systems
Sequential AFS occurs at a site where there is a chronological succession between a period with annual crops and another with a forest component; that is, annual crops and regeneration of the natural forest or tree plantations follow each other over time. This category includes modalities of migratory agriculture with fallow management and taungya systems, where annual crops are made interspersed between rows of trees in the stage of establishment of a forest plantation until the foliage of the trees is developed (Figure 3).
6.1 Shifting agriculture
The migratory agriculture also called itinerant, nomadic, or “
6.2 Taungya system
The
7. Simultaneous agroforestry systems
Simultaneous AFSs occur at a site where there is a simultaneous and continuous combination of an agricultural component with a forestry component, whether timber, fruit, multi-use, or service trees. These AFSs include all kinds of tree associations with annual or perennial crops, mixed home gardens, agroforests, and silvopastoral systems.
7.1 Trees with annual crops
7.1.1 Alley cropping
Alley cultivation is a simultaneous AFS of trees with annual crops, consisting of rows of trees, usually 4–6 m apart between rows x 2 m between trees, interspersed with annual crops between rows of planting (Figure 6). Trees are pruned before planting and branches are left in alleys to incorporate organic matter into the soil and in turn suppress weeds. In this AFS, conveniently, the trees are of nitrogen-fixing species (Fabaceae such as
Reminding that the main mechanisms of gain of N in the soil are: (a) N contributed with the rainfall; (b) N from non-symbiotic fixation; (c) N from symbiotic fixation; (d) N provided by organic fertilizers; and e) N from the mineralization process from fresh remains (vegetable and animal), in this case, the fallen leaves and branches from the trees. Therefore, this is a production system that adapts well to low fertility soils in degraded areas and to dry and semi-arid areas, since it favors the restoration of fertility and physical conditions of the soil. In addition, producers can obtain from trees other products such as poles, firewood, fodder, green manure, and atmospheric nitrogen fixation. The latter has current importance because the action of reducing nitrogen fertilization is a way to reduce nitrous oxide emissions into the atmosphere, so it is considered a way to mitigate climate change. In areas, with steep slopes, the rows of trees can be established in contour lines as a living barrier for water conservation and to deter erosion. In addition, they are a way of conserving the soil that does not require physical conservation structures.
7.1.2 Crops under the cover of forest curtains
This category includes any form of short-cycle agricultural monocultures or polycultures such as corn, beans, onions, celery, lettuce, tomatoes, coriander, and other horticultural species, in association with windbreaker curtain-like trees in windy areas, multiramified live fences, or rows of trees in contour lines in hillside areas. All these alternatives that integrate crops with the planting of trees are a form of conservation and restoration of degraded areas, which contribute to conserve biodiversity and water resources. Crops undercover, forest.
7.1.3 Trees with perennial crops
The simultaneous association of trees and perennial crops is a common practice in CA. The most prominent examples are coffee and cocoa crops under shade. The beneficial effects of shade trees, particularly
7.1.4 Coffee and cocoa plantations under shade
These systems simultaneously combine trees with perennial crops, such as
Coffee (
Cocoa (
7.1.5 Agroforests
The concept of agroforests, despite being traditional systems, has been incorporated more recently into the definitions and classifications of FAS. Agroforests are areas with a predominance of trees and shrubs or communities that resemble forests, where there are plots or clearings with agricultural practices along with structures typical of natural forests due to their floristic composition and their multiple stratifications. An example is the so-called
7.1.6 Silvopastoral systems
Silvopastoral Systems (SPS) are agroecosystems in which a tree component is deliberately associated with an herbaceous one (natural or improved pastures) and a livestock production component (domestic animals) in the same site so that there are biological interactions between both to maximize the land use. In other words, they temporarily and spatially combine the maintenance of pastures (natural or cultivated ones) with livestock production activities, along with tree species. To this the silvopastoral practices can be added, in which the woody component does not need to be in the same site as the animal component because forage can be transported; such as the case of forage banks or living fences, which are pruned, and the forage produced by the pruning is supplied to confined animals [39, 40]. The limits, inputs, outputs, components, and interactions are shown in Figure 9. Silvopastoral systems found most frequently in CA are: (a) trees in pasture lands, including grazing in secondary forests and fallows; (b) grazing in forest and fruit plantations; (c) living fences; (d) perimeter shelterwood; and (e) fodder banks or crop and utilization of forage trees and shrubs. Tree species identified in pasture lands in CA are diverse and are according to the characteristics of vegetation, climate, and altitude of each region. In most cases, the trees are from natural regeneration and have been allowed to grow in densities that do not affect pasture growth, in a range from 10 to 70 trees per hectare but can reach up to 100 trees; with a basal area (BA) ranging from 1 to 7 m2/ha, although some authors mention that is possible to have up to 200 trees/ha [42]. Among the most frequent tree species found in animal production systems is
8. Linear agroforestry systems
Linear or alignment systems are tree plantations in rows of one, two, or more rows, such as live fences, hedges, living barriers, tree, and shrub lines (timber, fruit, multi-use), and windbreak curtains, usually associated with an agricultural crop or grasslands. They are useful especially on small farms because they offer many opportunities for the production of goods and services of interest to the farmer and are one of the most commonly promoted agroforestry technologies in forest and agroforestry extension and development programs in CA [43, 44].
8.1 Live fences
A living fence is made up of live poles in a row of trees or shrubs that delimit a property, or you can divide or parcel it out internally. Depending on the species used, it can produce firewood, wood, fodder, flowers for honey, fruits, and poles among others. They are very common in the countries of CA and one of their most important functions is the delimitation of farms, or paddocks (Figure 10). The most commonly used tree species as living poles are
8.2 Windbreaker curtains
A windbreaker is a linear tree plantation, which forms a barrier, to mitigate the negative effects of winds and regulate microclimate conditions, which consists of spinning multiple lines of trees, established perpendicular to the direction of the prevailing winds. They are included in the AFS when they are associated with an agricultural or livestock production system. The trees are planted in several parallel rows, and the protection depends on the height of the curtain and the compactness of the tree crown to stop the wind. It is generally accepted that a windbreaker curtain provides services and benefits to agricultural establishments. In addition, a well-managed curtain also produces timber and fuel wood. Several aspects must be considered for the design of a curtain, among them are (a) Orientation; (b) Distance between trees and between rows of trees; (c) Density; and (d) Height of trees. The height of the curtain trees is the most important factor to consider in your design, as it determines the area it protects. The maximum wind mitigation distance of a curtain varies between 15 and 20 times the height of the trees. That is, if a curtain is 10 m high, it will protect up to a distance of 150–200 m. For instance, in León, Nicaragua, curtains of three strata and five lines of
9. Attributes and characteristics sought in agroforestry systems
There are desirable attributes and characteristics of AFS in different aspects: (a) As for the selection of the woody species, keep in mind that it is easy to establish and grow quickly; (b) Regarding the architecture, phenology, and compatibility of the woody species with the associated crops, it is desirable that they make little competition for water and nutrients; that they have an open and narrow crown with small leaves; that they have a strong root system and as far as possible deep; no allelopathic effects; with branches and stems that are not brittle and that do not host pests or diseases. (c) Regarding the management and physiology of the woody component, it is desirable that: they tolerate full exposure to the sun; have self-pruning of branches; tolerate frequent pruning; regrow easily; be easy to handle (without thorns or stinging latex); and that if possible, fix nitrogen. (d) As for ecological functions, they are desirable attributes that have functional biodiversity and promote biological control; provide habitats for avifauna and other non-harmful animals; encourage soil conservation and fertility and maintain foliage in the dry season. The main three groups of attributes and characteristics (Table 3) that farmers expect from an AFS are (a) productivity; (b) sustainability; and (c) adaptability [47, 48, 49].
Attributes | Desirable characteristics of agroforestry systems |
---|---|
Productivity | The system produces goods, merchandise, and services required by producers |
Sustainability | Maintains or increases productivity over time: producing while preserving and conserving producing |
Adoptability | It is accepted, even under socioeconomic and biophysical constraints prevailing locally |
10. Current trends in agroforestry
At present agroforestry has become a significant issue in scientific research because the human face new challenges to ensure food security and climate change mitigation. The research interest in the field has boosted, and about 139 countries have been involved with the research in the field of AF and connected topics. These publications cover 66 subject categories and a great diverse research theme. The most used keywords in AF research have been changed from “Intercropping”, “Alley cropping”, and “Multipurpose trees” to “Carbon sequestration”, “Ecosystem service”, and “Climate change” [50, 51]. Other topics like Small-island agroforestry in climate change and sustainable development goals have been developed [52]. Also, extensive analysis and proposals to face the challenges of the new millennium by first-line researchers, covering topics of Biodiversity Conservation, and Food Sovereignty, Climate Change have been brought together in a work of vast value for researchers and students [53].
11. Conclusions
Agroforestry systems are a viable option to reduce land degradation and generate income for rural families. However, due to the cost structure and the return period, technical and financial assistance (payment for environmental services) should be considered for the adoption and empowerment of these systems to be successful in the long term. CAn farmers are familiar with a set of traditional AFS, including shaded coffee, shaded cocoa, silvopastoral systems (SSP), and row trees.
The different modalities of the AFS allow the diversification of family farming, the sale of surplus production, and the efficient use of the natural resources of the farm (water, land, biodiversity, energy); factors that are linked to the degree of development of the peasant economy and that would allow more comprehensive productive, food, and nutritional schemes. Due to the similarities in their structure, energy flows, and nutrient cycles with natural forest ecosystems, AFS is considered to be an alternative for ecologically sustainable use for climatic zones where natural vegetation is a forest.
Agroforestry systems, whether traditional or innovative, allow the development of strategies for the maintenance of productivity based on the regulation of nutrient recirculation through the choice of species, planting densities, and the management of canopy shade on crops through pruning. All this makes it possible to maximize income and minimize the loss of nutrients from the soil.
Although the advantages of the tree component (trees, shrubs, palms, and bamboos) are always highlighted, there can also be negative effects on crops and soil when planting density and shade are excessive and when the choice of species is not the most appropriate.
There are ancestral agroforestry modalities (
Agroforestry is part of the concept of the Nationally Appropriate Mitigation Action (NAMA) mechanism, which is based on a combination of public and market incentives for the implementation of greenhouse gas (GHG) mitigation measures. An example is the NAMA for the coffee sector of CR, which constitutes a broad platform for coordination and participation of the sector together with governmental, non-governmental, and international cooperation entities, covering an area of more than 90,000 hectares and 50,000 producers, for the improvement of competitiveness (cost savings and diversification of the coffee agroforestry system), and seeks at the same time the differentiation of the sector maintaining its access to markets and contributing to a low emission economy.
In a brief summary of the above, Guatemala has experiences and achievements in community forest management, with more than 20% of forests managed communally or municipally (380,000 hectares managed sustainably by community concessions in Petén); in Panama, 54% of forests and carbon are in indigenous territories and indigenous peoples organized under the National Coordinator of Indigenous Peoples of Panama (COONAPIP); Nicaragua has interesting approaches in the Autonomous Regions (21 titled territories with more than 3.6 million hectares, which are more than 62% of the forests in North and South Atlantic Autonomous Regions (RAAN and RAAS); in Honduras, more than 400,000 ha are in the hands of communities since the Forestry Law of 2007, there is titling of seven territories and 760,000 ha in the Mosquitía; while in CR, indigenous peoples, who constitute 2% of the population with 12% of the forests in the country, have Indigenous Development Associations (ADI) and from these rights the Payment of Environmental Services (PES) was established in indigenous territories with institutions consolidated by the National Forest Financing Fund (FONAFIFO).
Finally, Agroforestry is a possible alternative to receiving payment for the environmental services (PES) they produce. In the case of CR and Guatemala, there exist formal PES programs that incentive agroforestry; promote the incorporation of trees in agroecosystems; as an alternative for the recovery of forest cover, income generation, and also as a means for the reduction of greenhouse gas emissions. Honduras and Panama provide environmental services in their legislation, and Dominican Republic is in the process of formally implementing PES. In the case of mixed crops involving timber trees, it will undertake to increase and/or reorder the number of trees and reduce the impact of the crop on soils and waters and that its activity coincides with the capacity of land use; in addition, they could constitute an opportunity to strengthen the processes of conservation, sustainable use and poverty reduction in the CA region.
Acknowledgments
I dedicate this review to the memory of Dr. Gerardo Budowski (1925–2014) with whom I was introduced to the concepts of agroforestry; also, to my colleagues in my initial experience (1981–1986) at The Tropical Agricultural Research and Higher Education Center (CATIE), where I took the first steps into this multi-disciplinary field that combines agriculture with forestry and livestock activities.
Conflict of interest
The author declares that the literature research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest, and without external funding.
Comments
The apparent limitation of the study would be that the author cited mainly old literature, but the author felt that the pioneering work of the researchers who laid the foundations of the AF both in the New and Old World could not fail to be recognized. However, it is recognized the mutual importance of both the pioneering and current researchers of these sustainable cultivation technologies.
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