Restoring Soil Fertility, Productivity and Biodiversity through Participatory Agroforestry: Evidence from Madhupur Sal Forest, Bangladesh

Abstract

Species diversity and soil quality are deteriorating due to continuous disturbances in ecosystems caused by human interference. However, agroforestry is considered a good approach to minimizing environmental problems. Therefore, the objective of this study was to determine the impacts of participatory agroforestry on restoring soil fertility, farm productivity and biodiversity in the degraded Madhupur Sal forest of Bangladesh. The study purposefully selected 40 common agroforestry programs in Madhupur Sal forest for the collection of soil and plant data from 2020 to 2023. Agroforestry programs have improved soil organic matter, soil carbon, pH, and available N, P and K content to a substantial degree and protected soil degradation, enhancing yield. The soil improvement index represents the potentiality of agroforestry in restoring soil nutrients and carbon in the form of organic matter, which is an important indicator for carbon sequestration and mitigating the impacts of climate change. The resultant cost–benefit and land equivalent ratios were steadily higher, which corroborates the greater productivity and profitability of agroforestry compared to monoculture systems. In contrast, agroforestry restored 31 plant species, opening up opportunities for restoring plant species in the threatened forest ecosystem. Therefore, this study recommended selecting appropriate site-specific species for managing agroforestry and restoring ecosystems.

1. Introduction

The depletion of natural resources, particularly forests and land, has emerged as a significant global concern, affecting a substantial portion of the world’s impoverished population that relies on these resources for their daily sustenance [1,2,3]. The expansion of agricultural activities stands out as the primary driver behind the degradation and loss of biodiversity in forest ecosystems worldwide [4,5,6]. Addressing the biodiversity crisis and understanding the trade-offs between ecological and economic considerations necessitate the promotion of agroforestry production systems globally. Remarkably, over a billion hectares of agricultural land across the world exhibit more than 10% tree cover [7,8]. Consequently, agroforestry holds the potential to unlock numerous opportunities for restoring ecosystems and combating soil degradation in tropical and sub-tropical regions [9,10]. Various agroforestry practices have been developed, especially in developing countries like Bangladesh, proving effective in enhancing farm productivity and restoring biodiversity for small-scale farmers [2,11,12].

Agroforestry introduces plant and crop species to degraded forestland, serving as a habitat that conserves germplasm for endangered species, creates corridors for segmented or isolated species, and enhances biological diversity by contributing to ecosystem factors like erosion control, soil nutrient and microbial status, and water recharge. This, in turn, prevents soil degradation and the loss of surrounding habitat [9,11,13,14]. Demonstrating success in both tropical and temperate regions, agroforestry has proven to be an effective production system for increasing biodiversity and productivity [15,16,17,18]. Furthermore, implementing agroforestry in adjacent forest areas offers an opportunity for more efficient biodiversity conservation and ecosystem services, including soil health and productivity enhancement [9,16,19].

In Bangladesh, agroforestry practices in the degraded Sal forests commenced in 1989 under the supervision of the Bangladesh Forest Department [20,21]. The moist deciduous Sal forest, covering 0.12 million hectares, faces threats due to its proximity to a high-density poor population and is considered the most endangered forest ecosystem in the country [20,21,22]. Human interventions, such as illegal felling and encroachment, have been identified as the primary causes of Sal forest losses since Bangladesh’s independence in 1971 [20,23,24]. The deforestation and loss of Sal forest land in Bangladesh have been rapid, with approximately 35% of the forest cover disappearing by 1985, leaving only 10% of the original remaining Sal forests [13,23]. Additionally, more than 50,000 local people have illegally settled in the Madhupur Sal forest, engaging in commercial agriculture, including agroforestry practices [20,24,25].

The natural range of Sal forests in Bangladesh lies between longitudes 75° and 95° E and latitudes 20° to 32° N. These forests, comprising about 10% of Bangladesh’s total forest land, have undergone changes in biotic and abiotic features influenced by environmental and biotic factors over an extended period, as explained by succession theories. The Madhupur Sal forest, one of the significant patches of Bangladesh Sal forest, serves as a poignant example of deforestation and forest cover losses. In response, the Bangladesh Forest Department initiated people-oriented forest management through participatory agroforestry in 1989. This program, involving the plantation of fast-growing tree species alongside understory agricultural crops, has become a popular and productive initiative in Bangladesh, aiming to protect Sal forest biodiversity while providing income for rural communities [21,26,27].

Despite recognizing severe forest land degradation in the Madhupur area, researchers emphasize the crucial role of agroforestry in addressing forest land degradation and conserving biodiversity in Bangladesh. Agroforestry delivers essential services to local communities, including both material and non-material benefits, and plays a regulatory role in environmental services [9,28,29]. Nonetheless, gaining a profound insight into the broader impacts of agroforestry on biodiversity restoration, land degradation mitigation, and simultaneous productivity maintenance necessitates in-depth, research-driven knowledge. This research aims to provide a comprehensive overview of the productivity of agroforestry. As part of this study, we aim to assess various soil components in agroforestry and compare them with conventional agriculture and Sal forests. Our focus extends to scrutinizing the current agroforestry practices in Madhupur Sal forests, with a particular emphasis on evaluating the role of agroforestry in addressing soil degradation, boosting productivity, and conserving species in Bangladesh.

2. Materials and Methods

2.1. Location of the Study

In Bangladesh, moist deciduous plain land Sal forests are distributed over the relatively drier central and northwestern parts of the country [20,26] (Figure 1). This area is mainly made up of the districts of Tangail, Mymensingh, Gazipur and Dhaka; of the above, it is in the Madhupur Sal forest (under Tangail and Mymensingh districts), particularly where the majority of the Sal forests exist [24]. The Madhupur Sal forest is located in the northeastern part of the Tangail Forest Division, with a small portion running along the boundary with the Mymensingh Forest Division [24,30] (Figure 1). The green portion of Figure 1 represent the Madhupur Sal forest area.

The climate of the Madhupur Garh varies slightly from north to south, with the northern reaches being much cooler in winter. Average temperatures vary from 29.3 °C to 21.1 °C in summer, falling to 20 °C in winter, with extreme lows of 10 °C. Rainfall ranges from 1500 mm to 2100 mm annually, and the average is 2011.6 mm. The mean annual relative humidity (RH) and total evaporation are 84.8% and 1050 mm, respectively [31]. The Madhupur Sal tract belongs to Bio-ecological Zone No. 3 and the 28th AEZ (Agro Ecological Zone) of Bangladesh [31].

Although all areas of Madhupur Sal forest have been subjected to some degree of use, and the large wildlife species (e.g., tiger, leopard, elephant, sloth bear, spotted deer, etc.) have been extirpated, it is notable that an important plant variety still remains. The dominant species (more than 80%) of this Sal forest is the commercially profitable Sal (Shorea robusta) tree [32].

2.2. Description of the Participatory Agroforestry

The Madhupur Sal forest has faced a severe deforestation problem; accordingly, the BFD started plantation programs in the name of people-oriented forestry with fast-growing tree species dominated by Acacia spp. since 1989 [13,26,33]. The participants cultivated shade-tolerant crops in association with fast-growing trees; therefore, the program is called participatory agroforestry. A total of 3327 participants (one household equates to one participant) were involved in these programs (Local Forest Office Record 2022). In this program, each participant is allocated 1 ha of forestland for plantation lasting for one ten-year cycle (equal sharing of benefits with BFD); the program duration is a total of three cycles that last for a 30-year bilateral contract. The demand of local farmers and their spontaneous participation in participatory agroforestry programs were remarkable, as mentioned by the local forest offices.

2.3. Sampling, Data Collection and Analysis

The study collected secondary information on participatory agroforestry practices from the local forest offices. The agroforestry programs were implemented in the four ranges of Madhupur Sal forest; therefore, the study purposefully selected the 10 most common agroforestry practices from each range, and a total of 40 existing agroforestry practices and 40 agricultural plots were selected for data collection (

Table 1. Different agroforestry practice in four ranges.

Range 1	Range 2	Range 3	Range 4
Pineapple–Acacia-based agroforestry	Ginger–Acacia-based agroforestry	Turmeric–Acacia-based agroforestry	Papaya–Lemon-based agroforestry
Pineapple–Mahagony-based agroforestry	Pineapple–Sal-based agroforestry	Papaya–Eucalyptus-based agroforestry	Pineapple–Lemon-based agroforestry
Papaya–Sal-based agroforestry.	Papaya–Neem-based Agroforestry	Colocasia–Lichi-based agroforestry	Papaya–Beleric-based agroforestry
Pineapple–Teak-based agroforestry	Papaya–Mingiri-based agroforestry	Papaya–Kusum tree-based agroforestry	Colocasia–Lannea-based agroforestry
Papaya–Betel Nut-based agroforestry	Pineapple–Quickstick-based agroforestry	Papaya–Mahagony-based agroforestry	Colocasia–Sal-based agroforestry
Colocasia–Eucalyptus-based agroforestry	Pineapple–Guava-based agroforestry	Pineapple–Diospyros-based agroforestry	Pineapple–Betel Nut-based agroforestry
Colocasia–Guava-based agroforestry	Papaya–Jarul-based agroforestry	Pineapple–Jackfruit-based agroforestry	Colocasia–Gamar-based agroforestry
Colocasia–Lemon-based agroforestry	Pineapple–Jujube-based agroforestry	Papaya–Orange-based agroforestry	Pineapple–Eucalyptus-based agroforestry
Pineapple–Dillenia-based agroforestry	Papaya–Lichi-based agroforestry	Pineapple–Lombu tree-based agroforestry	Pineapple–Cotton tree-based agroforestry
Colocasia–Mahogany-based agroforestry	Papaya–Cotton tree-based agroforestry	Pineapple–Lichi-based agroforestry	Mixed Fruit tree-based agroforestry

). The agroforestry practices are shown below.

In these practices, recommended amounts of chemical fertilizers, in particular urea, TSP (Triple Super Phosphate) and MP (Muriate of Potash), were applied. The farmer utilized 4–5 tons per hectare of vermicompost and 10–12 tons per hectare of compost fertilizer. Additionally, Sal leaves were employed for field mulching, serving as an organic fertilizer source.

In the soil degradation analysis, soil samples were collected with the help of an augur from 0 to 30 cm depth of each segment of agroforestry land. Three soil samples were collected from each agroforestry plot and made into a composite sample for analysis of the soil pH, organic matter (OM), organic carbon (OC), total nitrogen (N), available phosphorous (P) and potassium (K) (

Table 2. Relevance of key soil parameters that have affected nutrient-supplying capacity of forest soils.

Soil Parameters	Relevance	References
Organic C or Organic Matter (OM)	Basic indicator of soil quality that indicates nutrient availability with specific scoring functions to be used for plant productivity, nutrient cycling, pesticide and water retention and soil structure, and forest nutrition	[34,35,36,37,38]
pH	Nutrient availability, pesticide absorption and mobility, acidification status, and buffering acid input	[34,36,38,39]
Total N	Forest nutrition, atmospheric N deposition, leaching potential, and availability of plants	[34,36,39,40]
Available P	Stock of macronutrients, weathering rates, and plant growth	[34,38,39]
Exchangeable K		[34,36,37]

).

The collected soil sample was air-dried, mixed well, and passed through a 2 mm sieve and stored in a bag for laboratory analysis. The soil sample analysis was carried out in the Humboldt and USDA (United States Department of Agriculture) soil laboratories situated in the Soil Science Department of BAU. The soil pH was determined using a glass electrode pH meter (WTW pH 522) at a soil–water ratio of 1:2.5, as described by Ghosh et al. (1983) [41], and electrical conductivity was determined using a conductivity meter method, as described by Jackson (1958) [42]. The percentage of soil organic carbon was analyzed using the Walkley–Black method modified by Anderson and Ingram (1994) [43]. Soil organic matter was calculated from the content of the organic carbon by a Van Bemmelen factor of 1.73. Assuming that organic matter contains approximately 58% C. Total N was determined using the semi-micro Kjeldahl method. Soil available P was measured using the Bray and Kurtz method outlined by Tandon (1995) [44], where phosphorus was extracted from soil using 0.5 M NaHCO₃ at a nearly constant pH of 8.5. Exchangeable K was extracted using 0.15% CaCl₂ and measured following the turbidimetric procedure improved by Hunter (1984) [45], where the turbidity was measured using a spectrophotometer at 420 nm.

Regarding the productivity of agroforestry, the study collected data from farmers’ fields and also used interview techniques. Each farmer was asked to provide their total tree–crop production and benefit–cost data throughout the year as agroforestry provided outputs at different times. So, the study calculated all crops and tree outputs on a yearly basis and converted them into hectares per BDT (local currency, USD 1 ≈ BDT 96). We undertook a thorough field survey for our research, using a well-organized questionnaire as our main tool. We focused on talking to farmers who practice agroforestry and conducted structured interviews to gather information systematically. The total number of participants was 100, including those 40 agroforestry plot owners and farmers who were selected randomly. Our questionnaire had a mix of close-ended and open-ended questions. Most were close-ended, giving participants predefined options for quick responses and easy quantitative analysis. We also included open-ended questions to receive participants’ suggestions and thoughts in their own words.

The improvement index seeks to evaluate the dynamic progression of key well-being indicators over time, transcending static snapshots and providing a nuanced measure of advancements in the quality of life, representing the percentage change in performance attributable to improvements [46].

The soil improvement index involves assessing various soil properties and conditions to gauge improvements over time or as a result of specific interventions.

Haake (2009) used the following formula to calculate the improvement in performance for a jump h compared with a baseline jump h_o [47],

$$ImprovementIndex=100\times \frac{h}{h_{\mathrm{o}}}$$

(1)

The study evaluated the impact of agroforestry on soil fertility changes; the soil nutrient status under agroforestry was compared with natural Sal forest stands as a baseline value, which is expressed as the improvement index in percentages.

The soil improvement index has been calculated by modifying Formula (1) as follows:

$$SoilImprovementIndex=100\times \frac{\mathrm{x}}{{\mathrm{x}}_{\mathrm{o}}}$$

(2)

$$\mathrm{x}=\left|{\mathrm{x}}_{\mathrm{o}}-\left.\mathrm{x}1\right|\right.$$

here x_o = soil property of the Sal forests; x₁ = soil property of the agroforestry field.

If the soil nutrient of Sal forests is 100, then, due to agroforestry, the soil nutrient value will be

Soil nutrient value = 100 ± Soil Improvement Index

(3)

The benefit–cost ratio (BCR) of each agroforestry practice was determined using the following formula:

$$\mathrm{B}\mathrm{C}\mathrm{R}=\frac{{\sum }_{i=0}^n{B}_t(1+i)t}{{\sum }_{i=0}^n{C}_t(1+i)t}$$

(4)

where B_t = total income in ith year, C_t = total cost in ith year, t = number of years, and i = interest (discount) rate (assuming 11% interest rate) [48]. This study also evaluated the land equivalent ratio (LER) of agroforestry practice as it refers to a valuable productivity indicator to evaluate yields from cultivated trees and crops together in comparison to yields from monocultures over the same period. So, LER is the sum of the fractions of the intercropped yields divided by the mono-crop yields,

LER = C_i/C_s + T_i/T_s

(5)

here C_i and T_i = crop and tree yield in agroforestry or intercropping conditions; C_s and T_s = crop and tree yield under monocropping conditions [49]. Monocrop and tree data were also collected from farmers filed at the Madhupur Sal forest area.

Finally, the study determined the plant species richness (number of different species in a specific area) toward the biodiversity data of agroforestry practices through the use of the count quadrat method. In this method, plant species data were collected from 40 agroforestry plots, and a quadrat size of 20 m × 20 m was set in each plot for recording trees (having a dbh of 10 cm) data. However, to measure the shrub and climber data, a 10 m × 10 m sub-quadrat was used, and a 5 m × 5 m sub-sub-quadrat was used to record herbs/crop and seedlings data, respectively. The study also calculated the Shannon–Wiener Index, which denotes the total number of species in a habitat (richness) and their abundance, using the following equation:

$$\mathrm{H}=-\sum [\left(pi\right)\times \mathrm{l}\mathrm{n} \left(pi\right)]$$

(6)

where H = Shannon diversity index, pi = n/N proportion of individuals of ith species in a whole community, n = individual of a given species and N = total number of individuals in a community [50].

In addition, the study also collected plant species richness data from agriculture and Sal forests through similar quadrat techniques, and a total of 40 quadrat plots were used for each agriculture and Sal forest land. In this process, the plant species comparisons among agroforestry, agriculture and forestry were determined.

The data collection was carried out at different time intervals from 2020 to 2023 with a six-member research team consisting of researchers, MSc and PhD students of Bangladesh Agricultural University. The obtained data were validated and scrutinized before being entered into a computer; afterward, the computed data were analyzed using the SPSS statistical program.

3. Results

3.1. Soil Fertility

The chemical analysis of the soil samples indicated significant variations between Madhupur agroforestry soil, conventional agricultural soil and Sal forest soil. The findings consistently showcased that Madhupur agroforestry soil performed favorably across various parameters compared to conventional agricultural soil. Significant discrepancies were observed, particularly concerning soil organic matter (

Table 3. Paired t-test of different soil components on different practices.

Paired Differences
	Std. Deviation	Mean	95% Confidence Interval of the Difference		t	p-Value (2-Tailed)
			Lower	Upper
AF OM–Sal OM	0.06083	2 ± 0.02720	−0.18553	−0.03447	−4.044	0.016 *
AF OM–AG OM	0.15209	1.5 ± 0.0681	0.64516	1.02284	12.262	0.000 **
Ag OC–AF OC	0.06465	1.16 ± 0.025	−0.62628	−0.46572	−18.884	0.000 **
Sal OC–AF OC	0.05675	0.87 ± 0.0281	−0.03246	0.10846	1.497	0.209
AF pH–Sal pH	0.08019	5.77 ± 0.035	−0.46357	−0.26443	−10.150	0.001 **
AF pH–AG pH	0.02387	5.42 ± 0.010	0.33236	0.39164	33.904	0.000 **
AF N–Sal N	0.14832	0.11 ± 0.06633	−0.20417	0.16417	−0.302	0.778
AF N–AG N	0.01581	0.09 ± 0.0070	0.01037	0.04963	4.243	0.013 *
AF P–Sal P	0.10164	11.53 ± 0.04	1.49980	1.75220	35.773	0.000 **
AF P–AG P	0.15875	9.89 ± 0.070	4.69289	5.08711	68.880	0.000 **
AF K–Sal K	0.03782	0.3 ± 0.01691	−0.08095	0.01295	−2.010	0.115
AFK–AGK	0.08456	0.24 ± 0.037	−0.01499	0.19499	2.380	0.076

AF = Agroforestry; Sal = Sal forest; AG = Agriculture; OM = Organic matter; OC = Organic carbon. (*) = Significant; (**) = Not significant.

). The disparity in soil organic matter content was notably high between Sal forest soil (1.95%) and agroforestry soil (2.05%). The Sal forest soil exhibited a substantially higher concentration of soil organic carbon, measured at 2.05%, indicating its superior carbon content in comparison to the agricultural soil, which registered a lower soil organic carbon percentage at 1.05%. There is a highly significant difference between agricultural soil and agroforestry soil on the basis of organic matter, but if we consider Sal forests and agroforestry, then there is no significant difference. In assessing the soil properties, it was evident that all soil samples exhibited an acidic nature. However, the agricultural soil displayed the lowest pH value at 5.25, whereas the agroforestry soil showed a moderate pH level (5.60), followed by Sal forest soil (5.94).

The analysis revealed that the total nitrogen content was substantially improved in agroforestry soil, measuring at 0.11%, closely resembling the content found in Sal forest soil at 0.12%. In contrast, agricultural soil demonstrated limited total nitrogen content, recording only 0.07%. Furthermore, agroforestry soil exhibited significantly higher phosphorous availability, recording 12.35 ppm, while agricultural soil displayed the lowest available phosphorous content at 7.44 ppm. Notably, no significant disparities were observed in terms of exchangeable potassium among the agroforestry, agricultural, and Sal forest soil samples. These findings underscore the positive impact of agroforestry practices on soil quality, particularly in enhancing nutrient levels such as total nitrogen and available phosphorous. Further studies are essential to explore the mechanisms behind these improvements, providing valuable insights for sustainable soil management practices. These outcomes suggest the potential benefits and advantages of Sal forest soil, emphasizing its enriched organic matter and higher soil organic carbon levels. Further comprehensive analysis and investigation are warranted to explore and understand the underlying factors contributing to these variations, providing insights for informed agricultural practices and soil management strategies.

Soil Improvement Index

The results showed that most of the degraded soil parameters were restored near standard/baseline values (red color line in Figure 2), while P content was higher in agroforestry production systems. The figure clearly showed the improvement trend of soil fertility in the Sal forest area through the intervention of agroforestry programs.

3.2. Productivity

Agroforestry programs have been evidenced to be income-generating and livelihood-enhancing production systems in Bangladesh. The fast-growing Acacia tree-based agroforestry in the Madhupur Sal forest showed a diversified output from a limited piece of land, and thus, small-scale farmers earned cash income at different times of the year. On average, an agroforestry program receives USD 5416.67 in crop income and USD 606.25 in tree income per year. Agricultural labor costs affect agroforestry production costs, and the farmers also mentioned that fertilizer and transport costs greatly increased after the Russia–Ukraine war in 2022.

The net income of the agroforestry programs showed that farmers received more than USD 4171.98 per year, which was profitable in comparison to agriculture (USD 738) or forestry (USD 412) production in the Madhupur Sal forest area mentioned by all farmers. Accordingly, the benefit–cost ratio (BCR) of 2.57 and land equivalent ratio (LER) of 2.742 also showed that agroforestry production was highly profitable for local farmers. The agroforestry farmers also informed that due to the higher productivity of participatory agroforestry, the competition for involvement in participatory agroforestry programs in government-owned forestland was extremely high here in Bangladesh. The benefit–cost ratio and land equivalent ratio of the agroforestry program clearly demonstrate that the program was highly productive and economically profitable for local farmers (

Table 4. Production costs and benefits of agroforestry practices at Madhupur Sal forest.

	Items	Income/Costs In Taka (USD 1)	Net Benefit In Taka (USD) (Total Income − Total Cost)	BCR	LER
Agroforestry	Land cultivation costs	14,590 (151.98)	400,510 (4171.98)	2.57	2.742
	Tree saplings and seedlings costs	21,700 (226.04)
	Crop seedlings/rhizomes//sucker buying costs	27,800 (289.58)
	Agriculture labor costs	68,100 (709.38)
	Organic manures and fertilizer costs	22,000 (229.17)
	Crop protection costs	10,500 (109.38)
	Intercultural operation costs	16,300 (169.79)
	Crop harvesting costs	23,000 (239.58)
	Tree felling/harvesting costs	18,200 (189.58)
	Transport costs	2500 (26.04)
	Total cost	224,690 (2340.52)
	Crop income	520,000 (5416.67)
	Timber income	47,000 (489.58)
	Firewood/fuel wood income	2400 (25.00)
	Tree thinning income	7600 (79.17)
	Fodder income	1200 (12.50)
	Forest Department Income/Govt Revenue	47,000 (489.58)
	Total income	625,200 (6512.50)
Agriculture	Land cultivation costs	19,540(205.68)	70,060 (737.66)	0.35	0.58
	Crop seedlings/rhizomes//sucker buying costs	35,000(368.42)
	Agriculture labor costs	69,700(733.68)
	Organic manures and fertilizer costs	28,000(294.73)
	Crop protection costs	15,200(160)
	Crop harvesting costs	27,900(293.68)
	Transport costs	2500(26.31)
	Total cost	197,840 (2082.52)
	Crop income	266,760 (2808.18)
	Fodder income	1140 (12)
	Total income	267,900 (2820.18)

¹ USD ≈ BDT 96 (Bangladeshi Taka, local currency).

).

3.3. Biodiversity

The commercial agriculture production system focuses on supplying only food production services that grow large areas of monocultures for economic output (45, Udawatta). Agroforestry could harbor greater species richness and diversity compared to agricultural cropping systems. The study found 31 plant species in the agroforestry field in the Madhupur Sal forest area. Although fast-growing tree species are emphasized in agroforestry production, a number of local tree and shrub species were observed in the study area (

Table 5. Plant species found in different land use practices in Madhupur Sal forest.

	Agroforestry	Sal Forest		Agriculture
Tree	Acacia auriculiformis	Adina cordifolia	Holarrhena pubescence
	Aegle marmelos	Aegle marmelos	Lagerstroemia parviflora
	Areca catechu	Albizia chinensis	Lagerstroemia perviflera
	Artocarpus heterophyllus	Albizia procera	Lagerstroemia speciosa
	Azadirachta indica	Alstonia scholaris	Lannea coromandelica
	Bombax ceiba	Anthocephalus cadamba	Litsea monopetala
	Citrus limon	Aphanamixix polystachaya	Milisua roxburghiana
	Dillenia pentagyna	Artocarpus chaplasha	Milisuna velutina
	Diospyros malabarica	Azardirachita indica	Moringa oleifera
	Eucalyptus camaldunensis	Barringtonia acutangular	Neolamarckia cadamba
	Gliricidia sepium	Bombax ceiba	Phyllanthus emblica
	Gmelina arborea	Butea monosperma	Schelichera oleosa
	Khaya anthotheca	Careya arborea	Semecarpus anacardium
	Lagerstroemia speciosa	Cassia fistula	Shorea robusta
	Lannea coromandelica	Dilenia indica	Stereospermum colais
	Litchi chinensis	Dillenia pentagyna	Streblus asper
	Mangifera indica	Diospyros blancoi	Syzygium cumini
	Melia azedarach	Erythrina variegate	Tamarindus indica
	Schleichera oleosa	Ficus bengalensis	Terminalia arjuna
	Senna siamea	Ficus hispida	Terminalia bellirica
	Shorea robusta	Ficus infectoria	Terminalia chebula
	Swietenia macrophylla	Gmelina arborea	Xylia kerrii
	Swietenia mahagoni	Haldina cordifolia	Citrus aurantifolia
	Tectona grandis	Randia dumetorum	Zizyphus rugosa
	Terminalia bellirica
Herb	Ananas comosus	Achyranthes aspera	Ageratum conyzoiodes	Ananas comosus
	Carica papaya	Agremone Mexicana	Paederia foetida	Brassica juncea
	Colocasia esculenta	Aloe vera	Phyllanthus reticulatus	Curcuma longa
	Curcuma longa	Alpinia malaccensis	Pligonum hydropiper	Musa spp.
	Zingiber officinale	Alpinia nigra	Rauvolfia sarpentina	Oryza sativa
		Blumea laccera	Cessus quadrangulais	Vigna mungo
		Chenopodium	Diosocorea pentaphylla	Vigna radiata
		Colocasia esculenta	Ipomoea paniculate	Zingiber officinale
		Commelina bengalensis	Lagenaria siceria
		Curcuma zeoderica	Mikania cordata
		Cynodn dactylon	Vitis quadrangularis
		Detura innoxia	Ocimum bssilicum
		Musa spp.
shrub	Psidium guajava	Abroma augusta	Mallotus philippensis
	Ziziphus mauritiana	Antidesma acidum	Microcos paniculate
		Ardisia colorata	Psidium guajava
		Averrhoa carambola	Ricinus communis
		Bursera serrata	Zanthoxylum rhetsa
		Clerodendrum inerme	Ziziphus mauritiana
		Ficus carica	Calotropics gigantea
		Adhatoda vasica	Clerodendrum squamatum
		Alocasia indica	Glycosmis pentaphyla
		Manihot esculenta	Acacia pennata
		Mimosa pudica	Calamus viminals
		Steudnera virosa	Cissus adnate
			Smilax zeylanica
Fern		Dryopteris filix
Climber		Scindapsus aureus	Cuscuta reflexa
			Dioscorea hispida
		Passiflora foetida	Entada phaseoloides

). In the biodiversity of Sal forests, there are a number of plant species (102 species), of which there are 56 tree species.

The diversity index of Madhupur agroforestry was recorded to range from 0.365 to 0.077. The highest diversity index was observed in Acacia auriculiformis (0.365), followed by Ananas comosus (Pineapple) Curcuma longa (Turmeric) and Zingiber officinale (Ginger) (Figure 3). The results clearly showed that the Acacia tree is the most dominant tree species all over the agroforestry production area of Madhupur Sal forest in Bangladesh.

4. Discussion

Agroforestry, integrating trees into agricultural landscapes, has emerged as a pivotal strategy to address environmental conservation and enhance productivity [9]. Globally, 40% of terrestrial land is designated for cropland and pasture, with over 43% of cropland having only 10% tree cover [9,15]. This highlights the potential of agroforestry to reconcile environmental conservation with agricultural productivity. The conversion of forests into commercial agriculture has adversely impacted soil quality, making agroforestry a crucial practice for soil conservation. Studies conducted in deforested areas like the Madhupur Sal forest in Bangladesh revealed positive impacts on soil parameters [51]. Agroforestry improved soil organic matter, carbon content, pH, and nutrient availability, countering the decline associated with natural forest conversion [2].

Organic matter, which is essential for soil fertility and tree crop productivity, has declined with the conversion of natural forests into agriculture [2]. In Bangladesh, agroforestry in the Madhupur Sal forest exhibited a notable increase in soil carbon due to nutrient-rich litterfall and root exudates [2]. The continuous input of leaf litter and root exudates also positively influenced nitrogen and potassium content, aligning agroforestry soil values with those of natural forest soil. Despite higher phosphorous levels attributed to fertilizer application, the soil improvement index, based on various parameters, demonstrated the restorative capacity of agroforestry soil compared to degraded agricultural soil [2].

Agroforestry’s role extends beyond environmental conservation to food security for small-scale farmers. The practice contributes to higher yields of both annual crops and woody perennials [52,53]. Economic analyses of Madhupur agroforestry programs showed a positive benefit–cost ratio (BCR) over a 10-year cycle, with an average BCR of 2.57 [14,25,54]. Farmers earned USD 4171.98 annually, while the government and forest department generated an average revenue income of USD 48,900.8 from participatory agroforestry programs. LER, evaluating relative yields of tree and crop species in agroforestry versus monoculture, consistently favored agroforestry, with an average LER of 1.742 [55,56,57,58]. This suggests that agroforestry in Madhupur was not only highly productive but also economically viable for farmers in Bangladesh.

Biodiversity conservation is a significant aspect of agroforestry, as it inherently supports higher biodiversity compared to monocrop systems [9,17,59,60,61,62]. The presence of woody plants plays a crucial role in increasing biodiversity, mirroring forest conditions [63,64]. In the degraded Sal forests of Bangladesh, agroforestry restored 31 plant species, transforming an area once dominated by monoculture and synthetic fertilizers [65]. These agroforestry programs also served as wildlife corridors, supporting various species like monkeys, deer and birds [3,66]. However, the introduction of foreign tree species, such as Eucalyptus, in the Madhupur agroforestry programs posed challenges by negatively affecting soil water and nutrient status [9,67]. Despite these challenges, proper planning and management, incorporating soil and climate-appropriate species, could mitigate adverse environmental impacts and further enhance biodiversity.

Agroforestry presents a multifaceted solution to global challenges, offering a harmonious integration of environmental conservation, food security, and economic viability. The study conducted in the Madhupur Sal forest of Bangladesh demonstrates the positive impact of agroforestry on soil quality, economic outcomes for farmers and biodiversity conservation. The findings emphasize the need for careful species selection and management to optimize agroforestry benefits while minimizing potential negative consequences. Agroforestry stands as a promising land use practice, contributing to sustainable and resilient agricultural systems on a global scale.

5. Conclusions

The invention of commercial agriculture has had a severe impact on the degradation of natural Sal forests in Bangladesh, as well as deteriorated forest soil and biodiversity to a great extent. Thus, the adoption of agroforestry for the cultivation of trees and crops together would be a good strategy to restore soil and biodiversity in the environment. Therefore, this study determined the impacts of agroforestry on soil restoration, productivity and biodiversity in the Sal forested areas of Bangladesh. The introduction of agroforestry in the form of a participatory approach with the patronization of the Bangladesh Forest Department has no doubt restored the degraded soil of Sal forests in comparison to agricultural soil. The increase in soil organic matter through the continuous addition of leaf litter from agroforestry species has notably enhanced land productivity. The cost–benefit analysis showed that agroforestry is a profitable production system and also 2.74 times more productive in terms of the land equivalent ratio. Local farmers’ spontaneous participation in agroforestry in the Madhupur area revealed that those programs were highly productive and economically profitable for farmers. Another valuable outcome of agroforestry programs was the biodiversity conservation potential aimed at increasing the species in the degraded Sal forests, and the selection of appropriate site–climate-specific plant species could enhance the biodiversity restoration process through participatory agroforestry. Finally, the study contends that the long-term impacts of agroforestry on soil restoration, farm productivity, and biodiversity restoration should be evaluated and that the selection of appropriate site-specific plant species for proper management strategies of agroforestry programs in degraded forestland should be recommended.