Agriculture that includes trees is generally known as agroforestry (Weston and Philip, 2020). However, this is a simplification, and agroforestry is in fact far more complicated. According to geographic information system (GIS) statistics, agroforestry covers 43% of all agricultural land in the world, equivalent to billions of hectares (Zomer et al., 2014), Agroforestry practices are common in the region of South Asia, primarily in Afghanistan, Pakistan, India, Bangladesh, and Sri Lanka, where these procedures have played a significant role since much earlier periods (Cannell, 1988). Agroforestry approaches in the area have changed over time, and they are now associated with the implementation of a variety of creative and traditional schemes to meet the elementary requirements of small-scale agronomists for fodder, grain, fuelwood, medical goods, and monetary income. The multi-functional home gardens used conventionally in the Asian region, in specific, are vital for improving diet and nutritional well-being (Waldron et al., 2017), diversification, and social and environmental factors due to their incorporation of organic fertilizer from plants, conservation of soil resources, and promotion of territories for enhanced agrobiodiversity climates that can deal with environmental change (Das and Das, 2005; Kumar, 2006). Experiences with these conventional systems illustrate the relevance of agroforestry in the 21st century for addressing land management challenges, such as food shortages, deforestation, habitat destruction, biodiversity loss, and climate variability.
In Asia, traditional dominant land-use systems, such as house gardens, have been altered as a result of land-use augmentation (Kumar and Nair, 2004). As increasing population and food demands pushed towards monospecific production technologies (Capistrano, Marten and Yokokura, 1986), forest and agricultural policies began to promote fast-growing commercial species, negatively impacting agroforestry in Asia. By modifying biological patterns and interactions, industrial agriculture systems have created new socioeconomic and environmental issues (Matson et al., 1997; Lobao and Meyer, 2001).
Agroforestry policies have been adopted in several countries. For example, India's national agriculture policy encourages farmers to use agroforestry practices to boost their revenue by offering loans and technology.
Since the 1960s and 1970s, corporations and administrations in various countries with low or moderate income have invested in research on agroforestry projects (Akbar, Baig and Asif, 2000; Garrity et al., 2010), examining the interrelations among ecosystem-oriented services and agroforestry around the globe, particularly in Asia (Jose, 2009). (Chang, Karanth and Robbins, 2018) investigated bird species diversity in the agroforestry of the coffee sector in India and discussed the effect of riparian buffer solutions on biomass in China. Additionally, (Tiwari et al., 2012) determined maize ecological services from mountainside agroforestry in Nepal. However, all of these previous research findings have fixated merely on a single form of agroforestry. In addition, (Bohra et al., 2018) evaluated the socioeconomic implications of agroforestry, (Basu, 2014) supervised discussions to determine the effects of agroforestry on mitigating weather adjustment, and (Goswami, Verma and Kaushal, 2014) discussed the anticipated biomass and carbon confiscation in various agroforestry structures.
Overall, the breadth of this research is constrained, and there is a shortage of reliable testimony regarding its usefulness. This is largely due to the difficulty of interpreting the interrelationships among ecosystem services and agroforestry as well as the time it takes to synthesize regional studies. This is in turn the result of a variety of factors. First, ecological services are interconnected, and multidimensional market-offs vary by facility and spatial scale (Mupangwa et al., 2007). Second, the advantages are reported in outcomes (Hillbrand et al., 2017), while the disadvantages (Jose, Gillespie and Pallardy, 2004; Rivest et al., 2013) are not. Third, focus on various specialty agroforestry techniques has resulted in a lack of systematic research.
Research to synthesize ecology and agroforestry service awareness has been performed in certain areas, including Europe (Torralba et al., 2016), some high ranking-revenue nations (Brown et al., 2018), and some low–middle-revenue regions (Miller et al., 2017). However, no research has sequentially recognized and defined the nature, quantity, and attributes of agroforestry and ecological facility research in South Asia. Various previous studies have described meta-study inquiries to compensate for the limitations of systematic mapping (de Oliveira et al., 2016; Santos, Crouzeilles and Sansevero, 2019). By organizing and presenting the outcomes of previously undertaken experiments on agroforestry from across the region of South Asia, this research provides a complete and methodical outline regarding agroforestry and its effects on ecosystem services. The methodical plan attempts to collect known information and investigate the features and inclinations based on prevailing discoveries in the field, providing the first synthesis of information on South Asian agroforestry and the related ecological systems.
1.1. Hypothetical Background
1.1.1 Agroforestry
Agroforestry is commonly referred to as a "current phrase for an ancient exercise" because the term was not used in research until the mid-1970s, even though the practice had been in use for considerably longer (Cannell, 1988). Agroforestry is a compilation of territory-based structures, methods, and skills that incorporate wood perennials into yield and livestock production (Atangana et al., 2014). The essential conditions for agroforestry require that a system of land should have at least two plant or animal species, one of which must be a wood perennial. Furthermore, there should be ecological and economic connections among two or more manufacturing systems, such as tree–fish, tree–livestock, or tree–crop. Agroforestry systems have a longer cycle than advanced agriculture systems and are more complex physically, financially, environmentally, and functionally (Nair, 1993).
The various forms of agroforestry include silvopasture, Agrisilviculture, agrosilvopastoral, urban agroforestry, and forest farming as well as tree integration with fisheries and beekeepers. Agrosilvopastoral is a tree-incorporated approach to farming animals and crops, whereas silvopasture is a tree-incorporated approach to plants and cattle, and agrosilvopasture is a tree family-incorporated approach that combines cattle and yields. Forestry cultivation is a method of raising cattle or growing crops and non-timber forest products (NTFPs) in the woods (Atangana et al., 2014). Agroforestry practices are categorized by consumption and evaluation standards, such as the arrangement and composition of the structure's modules. They are also identified based on the chronological cycles used to introduce new genera in the approaches and the role of trees in the agroforestry activities, such as supplying shade, decreasing wind, and preserving soil. Because various agroforestry systems are better suited to specific environmental circumstances than others, ecological suitability and the degree or level of contribution to commercial management systems may also differ (Nair, 1993).
1.1.2. Service to the Ecosystem
According to (Ehrlich and Mooney, 1983) the term "ecosystem services" to draw attention to the human actions that cause land degradation and the subsequent decreased functionality and benefits provided by ecological systems. With a developing awareness of the significance of ecological structures and the inextricable link among ecology facilities and human impacts, researchers have looked at essential ecology services from a variety of perspectives (Daily, 1998). To analyze and represent an ecosystem, there are a variety of techniques to categorize ecological benefits (Boyd and Banzhaf, 2007). According to the Millennium Ecosystem Assessment’s theoretical structure, which was established by several specialists, ecology service areas comprise specialized functions influencing personnel provision, control, and social service jobs (MEA) (Kalaba, Quinn and Dougill, 2013). Supporting facilities as well as photosynthesis, nutrient uptake, and initial production are all dependent on indirect services (Kalaba, Quinn and Dougill, 2013). Forests and trees, when correctly managed, provide a broad array of essential services, including foodstuff, wood, unprocessed resources, and health supplies (Maass et al., 2005; Kalaba, Quinn and Dougill, 2013). Furthermore, pollination (Ricketts, 2004; Balvanera et al., 2005), biodiversity (Polasky et al., 2011; Maes et al., 2012), insect control (KLEIN et al., 2006; Letters et al., 2013), nutrient run-off mitigation (Logan, 1993), and soil nutrient improvement (Power, 2010) are some of the benefits offered by timber lands at the community and state stage. Studies at the domestic and worldwide levels describe how woodland environment facilities operate and assist in watershed preservation (Daily and Matson, 2008), regulating services, and resource conservation, as well as the difficulties in incorporating ecosystem services into decision-making and implementation (De Groot et al., 2010). Based on references discussed earlier (Assessment et al., 2005; De Groot et al., 2010; Masiero et al., 2019), ecosystem services are categorized using both geographical and operational range (see Table 2). This aids in characterizing the essential ecological services supplied by trees and agroforestry practices on a regional, domestic, and worldwide level. This classification is thus regarded as significant from the perspective of this systematic review.
When seeking to analyze the connection between nature and human development, ecosystem services operate as a transformational lens, illuminating the agricultural production and influencing factors that should be examined (Tscharntke et al., 2005; Swinton et al., 2006; Farrell et al., 2010). Because it balances manufacturing and ecological preservation at the countryside level, agroforestry has been regarded as an ecologically safe and low-cost territory-usage strategy for landscape regeneration (de Oliveira et al., 2016; Hillbrand et al., 2017). Due to the fact that incorporated systems of agroforestry are more effective at gathering agricultural reserves, such as water and solar emissions, they can yield more than monocultural systems under certain conditions (Cannell et al., 1996; Jose, 2009). Overall, agroforestry was revitalized due the mix of productivity and conservation and improvement it provides.
Each form of agroforestry has a different impact on ecosystem services. Below is a list of the specific implications of agroforestry practices on ecosystem services, organized by the type of ecological system (Table 2). Food (McAdam et al., 2009), fiber, natural resources (de Oliveira et al., 2016), freshwater (Jose, 2009), forest resources (Hillbrand et al., 2017), NTFPs, medicinal supplies, genetic resources, and decorative resources (de Oliveira et al., 2016) are all provided by well-managed agroforestry. Agroforestry further provides erosion management, climate change action, nutrient availability, carbon storage and sequestration, and insect control (Hillbrand et al., 2017).