Agroforestry System as the Best Vegetation Management to Face Forest Degradation in Indonesia
2021 年 10 巻 p. 14-23
詳細
2021 年 10 巻 p. 14-23
Indonesia, as a country with a tropical climate, has a forest area of 94.1 million ha, and in 2019 deforestation reached 3,500,637.7 ha due to large-scale illegal logging for various activities. In addition, the area of primary forest has decreased over the last 15 years and is positively correlated with land degradation, carbon sequestration, and crop production. The low carbon sequestration capacity triggers global warming which has an impact on increasing the average temperature in Indonesia. Therefore, the restoration of forest functions to support ecosystem stability is the first step in forest management planning. The integration of trees and plants, namely agroforestry, is an option in the management of vegetation that is beneficial to the ecosystem. Tree-based farming systems have the capacity to absorb carbon dioxide from the atmosphere above ground, such as trunks, branches, and leaves, and below ground, namely the root system. Agroforestry has three main functions, namely regulating rainwater (soil and water conservation), sequestering carbon (reducing the impact of global warming), and supporting the microclimate (crop production). These three functions are supported by vegetation. Agroforestry systems provide options to reduce the effects of global warming, increase crop yields, and support ecosystem stability. Thus, a well-managed and sustainable agroforestry system is the best vegetation management that can solve the problem of deforestation.
Indonesia has a tropical climate and has a forest area of ninth after Australia and Argentina (Fig. 1). Indonesia’s forests experienced a relatively rapid decline in the area from 2014 to 2019. In 2019, deforestation reached 3,500,637 ha [1] (Fig. 2). This reflects that the use of forests for various purposes (industry, plantations, agriculture) is experiencing a relatively fast rate and often without considering the function of the forest as a provider of ecosystem services. In addition, several studies have shown that deforestation causes an increase in global warming [2, 3, 4].
The decrease in forest area has a very significant positive correlation with carbon sequestration capacity [5]. Carbon dioxide emissions in Indonesia increased by 4.7% in 2017 [6]. In 2020, CO2 emissions per capita for Indonesia was 2.09 tons of CO2 per capita (Fig. 3). CO2 emissions triggered an increase in the average temperature of Indonesia’s mainland by 0.7 °C from 1981 to 2020 [7], and increased evaporation from the soil surface [8]. CO2 and temperature are the main factors that affect plant growth, development, and function. Changes in these two factors will affect the productivity, structure, and composition of the ecosystem [9, 10]. An increase in temperature can cause damage to soil nutrients, decrease microbial activity in the soil, increase the rate of degradation of organic matter in the soil, which ultimately causes soil erosion [11, 12].
Appropriate management efforts are needed to deal with forest deforestation, namely through well-organized vegetation management. It provides several functions, namely water, and soil conservation, mitigating the impact of global warming and crop production. Vegetation is critical in compensating for the negative effects of deforestation. Therefore, concrete steps are based on the role of trees in rainwater movement (soil and water conservation function), carbon sinks (global warming impact mitigation function), and microclimate support for successful crop production (plant production function).
Figure 1: Indonesian forest area 2014–2019 (Indonesian Central Statistics Agency, 2020)
Figure 2: Indonesia’s net deforestation rate 2013–2019 (Indonesian Central Statistics Agency, 2020)
Figure 3: Indonesia’s CO2 emissions 2001–2017 (BPS, 2018)
The integration of trees and crops is an option in vegetation management that benefits the ecosystem. Efforts can be made to reduce the impact of deforestation by increasing carbon sequestration. The potential for increased carbon sequestration can occur in agricultural land and forests through tree-based agricultural cultivation technology [13, 14]. The agroforestry system provides an option to reduce the impact of deforestation because it is considered capable of increasing carbon sequestration in the soil. This article thoroughly reviews the role of agroforestry supported by trees as a form of managing the impacts of deforestation.
Deforestation causes land degradation, and the approach to solving this problem is an activity that considers the combination of environmental services and human well-being. Agroforestry is an important option to reverse the impacts of deforestation and associated ecosystem functions [15, 16]. In particular, agroforestry plays an important ecological role, including land conservation (maintaining soil and water resources), carbon sinks, and supporting crop production.
The availability of suboptimal land in Indonesia is very high and reaches 16,025,000 hectares. The soil in suboptimal land has low quality, which is caused by several limiting factors such as sloping topography, low nutrients, organic matter, low soil moisture content, too low or too high pH, and even accumulation of metal elements that are toxic to plants [17, 18, 19]. Suboptimal land characteristics are improved through the application of tree-based agriculture (Agroforestry). This is supported by shade trees in agroforestry systems that can mediate soil functions on suboptimal land, namely improving soil texture, increasing soil nutrient content, soil moisture levels [15, 20, 21, 22]. One example of suboptimal land with high availability is peat land [23, 24]. Agroforestry systems can be applied to Indonesian peatlands, namely shallow (0–2 m) peat agroforestry systems with a combination of one year old trees and agricultural crops that contribute 1.1 tons ha-1 of carbon stock. Deep peat agroforestry systems (3–4m) with a combination of woody trees, non-timber trees and plantation crops contribute to a carbon stock of 3 tons ha-1. Meanwhile, monoculture crops produce an average carbon stock of 0.8 tons ha-1 [25, 26, 27, 28, 29]. In addition, the application of agroforestry systems in dry land can increase the soil water content, overcome limited resources in dry areas while minimizing environmental damage [30, 31].
2.1 The role of agroforestry as a rainwater regulator (Soil and water conservation function)Conversion of forest to agriculture disrupts the ecological function of the forest in absorbing rainwater and preventing erosion. Forest conversion can impair the performance of hydrological systems associated with infiltration. Various efforts to restore the function of the forest are carried out by empowering sub-optimal lands as land for agricultural and forest crop production [32]. An agroforestry system supported by high tree diversity can maintain high infiltration rates and can have a positive impact on hydrological function through (1) green canopy cover at tree and understorey level, (2) soil surface roughness, (3) litter in the soil surface, and (4) water absorption by trees and vegetation [31, 32]. Tree canopy cover > 80% qualifies as an “infiltration friendly” land use, due to higher rainfall but relatively low erosion rates [35]. In agroforestry systems with a higher number of perennial crops have lower erosion rates than annual crops [36].
One of the critical lands, namely the Muria mountain area, is caused by land use that does not pay attention to its carrying capacity, causing loss or reduced land function. Based on research that has been carried out by Budiastuti et al. [37] that the application of Albizia chinensis and coffee agroforestry is ideally applied to critical land in the Muria mountain region. This is because coffee needs shade and reduces erosion due to rain; Albizia chinensis tree serves as a shade as well as contributing to the soil as a source of nutrients. When it rains, this combined agroforestry system is able to control runoff and soil erosion. Trees in agroforestry systems play a role in soil improvement, protection (from wind and frost), and providing shade and mulch [38].
Agroforestry functions as a provider of ecosystem services, namely reducing soil erosion rates by up to 50% compared to monoculture crops [31, 37, 38]. This is supported by higher infiltration rates, lower runoff, higher proportion of soil macroaggregates. In addition, SOC increased by 21%, N storage increased by 13%, available N by 46% and available P by 11% while soil pH increased by 2% in agroforestry compared to monoculture crops [39, 40]. This shows that agroforestry is a form of sustainable land management [43].
Agroforestry has a function as land conservation so that it can be used to improve suboptimal land quality [44]. Several studies show that agroforestry systems can be used to increase soil fertility because it is supported by an increase in the diversity of fungi and soil microorganisms. The diversity of fungi and soil microorganisms plays a role in improving the physical and chemical properties of the soil [11].
The most extensive suboptimal land in Indonesia is a peatland. Indonesia’s peatlands contain 77% of the total tropical peat carbon stores. Poor peatland management can lead to degraded land, forest, and land fires. Land-use conversion of about 10 million ha of peatland results in annual emissions of 132–159 Mt C year-1 from peat oxidation and will increase greenhouse gas emissions [45]. Agroforestry is one alternative that can be proposed for peat restoration activities. Research result [46] that peatlands and marginal lands can be used for the cultivation of lowland rice, corn, and soybeans with an agroforestry system. This system causes the diversity of land to increase so that the ecological conditions and land structure become better. Thus, agroforestry is beneficial in reducing water and wind erosion, increasing soil nutrition.
Agroforestry plays a role in soil and water conservation; this is supported by an increase in biodiversity. Such systems can contribute to biodiversity conservation and minimize homogenization [47]. Agroforestry is considered a distinctive “land sharing” strategy as it supports biodiversity conservation and agricultural production [48]. Vegetation diversity in agroforestry systems increases beetle species [49], bird [50] and natural enemies [51]. These variations may affect the potential for biodiversity conservation of different taxa.
Soil microbial community in agroforestry system is higher than monoculture system. Mycorrhizal diversity in agroforestry systems is very high because tree shade affects the efficiency of mycorrhizal bioinoculants, phosphate solvents and rhizobacteria [52]. The agroforestry system increases the abundance of heterotrophic bacteria and fungi [53]. Agroforestry can also maintain microbial diversity important for soil health and productivity [54].
2.2 Role of agroforestry as carbon sink (Global warming impact mitigation function)Agroforestry systems are considered the most effective in carbon sequestration [55]. Tree-based farming systems have the capacity to absorb carbon dioxide from the atmosphere above the ground, such as stems, branches, and leaves, and below ground, namely the root system, [54, 55] and can reduce greenhouse gas emissions [58]. Soil carbon absorption in the agroforestry system was higher at 11.29 t C ha-1 year-1 than the monoculture system at 4.38 t C ha-1 year-1. Agroforestry systems with various trees and vegetation have the highest carbon stocks of 108.9 Mg ha−1 [5]. Carbon stocks are related to species density and diversity [59, 60].
Based on Hairiah et al. [61] that land-use systems affect the average aboveground carbon biomass. Average biomass carbon in forest systems (natural) 291.5 Mg ha−1, plantations 84.0 Mg ha−1, complex agroforestry 66.7 Mg ha−1, simple agroforestry 30.0 Mg ha−1, oil palm plantations 29.0 Mg ha−1, trees monoculture 50.9 Mg ha−1 and without trees 0.9 Mg ha−1. Treeless land-use systems show the lowest carbon biomass. This is because the function of trees is the highest carbon absorber. SOC absorption rates tend to be higher in agroforestry systems with broad-leaf tree species than narrow-leaf tree species [60, 61].
Conversion of a cocoa monoculture to a form of cocoa agroforestry can restore up to half of the secondary forest C stock in the landscape. This system increases carbon storage above ground and below ground. Trees in agroforestry as microclimate modifiers and support soil C storage. An increase of 1 g kg−1 (0.1%) of soil organic carbon increases the available water capacity of the soil by 6% (vol/vol). Increased capacity of available water, relevant for the survival of the dry season during the week [62, 63].
2.3 The role of agroforestry as a microclimate support (plant production function)Agroforestry plays a role in maintaining microclimate conditions such as light, wind stability, temperature and humidity. This is caused by trees as climate support [35, 64]. High temperatures above 30 ℃ cause the photosynthesis mechanism, leaves, and flowers to be damaged, affecting plant physiology and yields [67]. Tree-soil-plant interactions in agroforestry systems have a function in agricultural production. For example, a coffee plantation with the use of shade trees to support less than optimal environmental conditions [66, 67]. The role of agroforestry in coffee plantations can improve the microclimate, increase soil nutrients and diversify income and increase food security [70]. The agroforestry system increases cocoa by 31% and this increase is due to a decrease in the environmental temperature in the system [71].
Based on the research carried out, the higher nitrogen fertilization under sengon (Albizia chinensis) stands can increase the yield of rice seeds (Table 1). These results were supported by the high nitrogen, phosphate, potassium, and organic C content in the soil under the high sengon (Albizia chinensis) stands. The availability of soil nutrients under tree stands can encourage plant growth and yields [72]. However, the yield of rice in the agroforestry system is lower than in the monoculture system. The highest yield in the agroforestry system was only 2.7 tons ha−1 compared to the monoculture system at 7 tons ha−1 with 150 kg ha−1 of nitrogen fertilization. This is because the light under the tree stands is lower, so the photosynthesis process is not optimal. Based on the yield of peanuts under three years of age, Albizia chinensis seeds were higher than those of 4 and 5 years old (Table 2). The results correlated with the Albizia chinensis canopy density index and the intensity of light received by peanuts.
| Nitrogen Dose (kg ha-1) | Seed yield (ton ha-1) |
| 0 | 2.1 a |
| 50 | 2.2 a |
| 100 | 2.3 a |
| 150 | 2.7 b |
Note: Values followed by the same letter show no significant difference at 5% level of Duncan Multiple Range Test
| Sengon age (years) | Seed yield (ton ha-1) |
| 3 | 0.55 |
| 4 | 0.48 |
| 5 | 0.39 |
Agroforestry is a sustainable land use approach that can combine three main roles in supporting agricultural activities (rainwater management: soil and water conservation function, carbon sink, microclimate support). Trees as a supporter of the agroforestry system and become part of forest management. Trees in agroforestry systems are assumed to provide ecosystem benefits as forests [73] because it plays a role in providing ecosystem services and functions, and is able to create a microclimate that reduces temperature and heat stress, maintains soil moisture and produces nitrogen [74].
Tree root systems stabilize hillsides and riverbanks, reducing the risk of landslides [75]. Agroforestry will control the reactivation of landslides if the arrangement of trees and plants is based on morphological units formed from previous landslides [76]. The role of trees also supports the function of agroforestry to maintain biodiversity, namely increasing the abundance and diversity of microorganisms and pollinator [77]. Trees support ecosystem biodiversity [78] and is a major component in the water, nutrient and carbon cycle [79].
Indonesia’s deforestation continues to increase. This leads to increased land degradation, decreased carbon sequestration and crop production. Agroforestry has three main functions, namely regulating rainwater (soil and water conservation), absorbing carbon (mitigating the impact of global warming), and supporting microclimate (plant production). These three functions are supported by vegetation. Agroforestry systems provide options to reduce the effects of global warming, increase crop yields, and support ecosystem stability. Thus, a well-managed and sustainable agroforestry system is the best vegetation management that can overcome the problem of deforestation.
