**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 [1–3]. The Mayan were polyfarmers; 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 *Alnus acuminata* (alder) associated with grasslands in the highlands of Costa Rica (CR). This type of land use system was also described by Budowski in 1957 [10], who reported the success of *Cupressus lusitánica* as a windbreaker, in the highlands of dairy regions and *Cordia alliodora* as a shade tree in grasslands in humid lowlands, both in CR.

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.

*Economic hypothesis:* it is assumed that AFSs allow obtaining net income higher per unit area in the long term than the possible income with each isolated component.

#### *Agroforestry: An Approach for Sustainability and Climate Mitigation DOI: http://dx.doi.org/10.5772/intechopen.105406*

*Ecological hypothesis*: it is assumed that trees in an AFS contribute to the conservation of the environment and particularly of the soil, especially when the induced combination represents a simulation of the types of vegetation that would occur in natural successions. In addition to the effects on the soil, important impacts on the microclimate, the fauna, and other factors that affect the biological balance are assumed.

*Silvicultural hypothesis*: it is assumed that the trees in an AFS can and should be managed according to the principles of classical forestry, always considering the particular requirements of the associated crops. Adequate silvicultural treatment is an indispensable condition for achieving and optimizing the positive economic and ecological results exposed in the previous hypotheses. In CA, there was a historical process, which had its beginnings with the definition of Combe and Budowski [7], presented that year in the First Workshop of Agroforestry Systems held in Turrialba, CR that can be summarized as follows:

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 *Erythrina* and *Cordia* and in turn the concept of "agroforestry practices" is introduced as operational aspects of an AFS, for example, the pruning of the trees of the system [16, 17]. Nair's definition [18] summarizes the concept as follows: "*Agroforestry is a land-use system in which woody species are grown intentionally in combination with crops or cattle on the same land, either simultaneously or in a sequence. The objective is to increase the total productivity of plants and/or animals in a sustainable manner, especially under levels of low technical inputs and in marginal lands. It involves the social and ecological integration of trees and crops*". Simultaneously, in CR, was conceptualized a complementary definition includes requirements or conditions: *"Agroforestry is a form of land use for multiple crops in which some fundamental conditions are met: (1) There are at least two components that interact biologically; (2) At least one of the components is a perennial woody; and (3) At least two species are managed for "agricultural" purposes in the broad sense of the word"* [2].

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

#### **Figure 1.**

*Interaction among components of an agroforestry system, with limits, inputs, outputs, components, and interaction among components. Inputs are solar radiation, rainfall, fertilizers, and money invested in the system. Outputs are agricultural products, wood, and firewood from the tree component and ecosystem services. Management is what the farmer does with the components and inputs.*


#### **Table 1.**

*Effects of the woody component (trees, shrubs, palms, and bamboos) of an agroforestry system on soil and crops.*

a small agricultural enterprise more diverse. These effects are shown both on the soil and in crops and are outlined in **Table 1**.

## **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 *software* designed in CATIE called *ShadeMotion*, which requires supplying the number of trees, location, shape, size, and density of foliage of the trees; as well as, the size of the land, degree of slope, and geographical latitude where the plot is located [24–26]. In response to shade, most plants produce less dry matter, retain photosynthates in the shoot at the expense of root growth, develop longer internodes and petioles, and produce larger and thinner leaves. The net photosynthesis (NP) of the crop increases with the Leaf Area Index (LAI) but reaches a ceiling when LAI is around 3 and could be plotted like the adjunct one attached. LAI is defined as the relationship between the sum of green leaf areas of

#### **Figure 2.**

*Effect of the canopy on agroforestry systems.*

the canopy of shade trees per unit ground surface area (LAI = Σ leaf area/ground area, m2 /m<sup>2</sup> ; in broadleaf canopies).
