A Tree-Planting Vehicle for Promoting the Sustainable Development of Desert Greening

Abstract

Preventing land desertification is one of the 17 sustainable development goals of the United Nations, which can effectively promote the sustainable development of desert greening. Currently, tree plantation is the most effective way to achieve this goal. However, the existing tree-plantation activities have some imperfections, including low efficiency, labor-intensiveness, challenging environments, and the low survival rate of saplings. Therefore, to contribute to the sustainable development of desert greening, this paper presents a practical desert tree-planting vehicle based on scientific and effective design and evaluation methods. First, based on the survey results, we used the objectives tree method to clarify the design objectives of the tree-planting vehicle. Second, the functional system boundaries of the tree planting vehicle were clarified using the function analysis method. Third, several alternatives were obtained using the finite structure and morphological analysis methods. Finally, an optimal solution was obtained using fuzzy comprehensive evaluation. This optimal design scheme has the characteristics of mechanical automatic planting, a closed cockpit, and large-capacity storage space, which can improve the construction efficiency and labor intensity, thereby contributing to the sustainable development of desert greening.

1. Introduction

In September 2015, the United Nations Sustainable Development Summit was held in New York, and the 193 member states of the United Nations officially adopted 17 sustainable development goals. The 15th sustainable development goal is to “Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, halt and reverse land degradation, and enhance biodiversity”. Since then, a growing number of countries and regions have paid attention to the issue of combating land desertification. Desertification refers to the land becoming desert in arid, semi-arid, or dry sub-damp areas [1]. Roberto [2] once introduced that the coverage area of global desertification is about 36 million square kilometers, which is about 24.1% of the earth’s land surface area. In this case, about one-sixth of the earth’s population is threatened with desertification. Further, the United Nations Environment Programme has stated that about 12 million hectares of the earth’s landmass are transformed into new deserts every year [3]. Currently, more than 30% of the dry land on the planet is suffering from desertification, and this impact is spreading to semi-arid land [4]. Precisely, desertification is particularly severe in Africa, Asia, and North America. In terms of the harm of desertification, it could threaten the living environment of human beings and could seriously affect the sustainable development of the ecological environment and economic society [5,6,7,8,9,10,11]. Specifically, the main hazards of desertification can be summarized as follows: (1) the available land resources are reduced, resulting in the reduction of agricultural and animal husbandry production; (2) moving dunes could bury roads or railways, thereby threatening public transport safety; and (3) it could aggravate natural disasters such as smog and sandstorms, thereby disrupting people’s living environment. Therefore, we should consider the causes of desertification and then use scientific and practical technologies and equipment to retard or avoid the desertification of the land.

The formation of land desertification can be attributed to two factors, namely natural factors and human factors [12,13]. Natural factors refer to an area being affected by the subtropical high belt year-round, causing the area to become a desert due to the scarcity of rainfall. Human factors refer to the rapid disappearance of green vegetation on the surface due to humans’ uncontrolled mining, logging, and grazing, resulting in land turning into deserts. In contrast to natural factors, Zhang et al. [12] believed that human factors are the primary and direct driving forces that cause the continuous increase in desertification areas. Recently, many scholars have continued to focus on controlling land desertification [14,15,16,17,18,19]. In these references, the primary governance methods are engineering construction and tree planting. Because tree plantation can prevent the continuous spread of desert areas and improve cohesion within the soil [4,20], tree planting can be considered a crucial measure to combat desertification. Therefore, it is imperative to improve global land desertification through tree plantation.

Scholars have provided a variety of plants suitable for planting in desert areas. Generally, Hedysarum scoparium, Caragana korshinskii, Artemisia desertorum, Oxytropis aciphylla, sea buckthorn, Haloxylon, Populus euphratica, bamboo willow, Pinus sylvestris, Hedysarum scoparium, Hedysarum mongolicum, Hippophae rhamnoides, and Salix psammophila are often used in tree plantation efforts worldwide [14,15,16,17,21]. Still, the climate, soil quality, moisture, and other factors in desert areas should be considered in actual selection. In addition, Tuo et al. [14] provided several critical factors for tree plantation: (1) the robustness and integrity of the plant root system must be maintained; (2) specific pesticides (i.e., trichlorfon and cottonseed cake) must be used to prevent disease and insects; and (3) plants must be provided with sufficient nutrition supply. Besides, saplings need to be cultivated for 2–3 years before being used for tree plantation [14]. Precisely, only large-sized seedlings with a circumference of trunk diameter ranging from 1 cm to 2.5 cm and a height of about 200 cm should be transplanted to desert areas. After planting, to ensure that the survival rate of saplings reaches 90.0%, it is necessary to provide fertilizer, nutrient solutions, water, and pesticides [14,22].

Currently, tree-plantation activities still adopt traditional manual planting (e.g., Ant Forest in Alipay) or modified planting equipment (i.e., semi-mechanized). The traditional tree-planting method has disadvantages such as slow speed, large labor consumption, and the low survival rate of saplings [23,24]. In contrast to traditional tree-planting methods, mechanized tree-planting methods could save workforces, improve planting efficiency, adapt to extreme environments, and provide more care for workers. However, although people realize the advantages of mechanized and automated tree planting, few scholars have provided information for a competitive tree-planting vehicle for desert areas [25]. Moreover, although some high-tech devices (i.e., drones) are used to detect the ecological environment of forests, few scholars have used high-tech devices to serve tree plantation [26]. In addition, although some conceptual designs of tree-planting vehicles have been published on design websites, these designs did not systematically consider the design objectives, functional systems, or distribution of functional units [25,27,28,29]. Thus, to improve the efficiency of tree plantation and prevent the spread of desertification, it is necessary to design a systematic and practical desert tree-planting vehicle based on scientific and effective research methods.

2. Theoretical Foundations and Methods

2.1. Objectives Tree Method

In design activities, clarifying the design objectives of the target product is a crucial first step. Design objectives can also be referred to as customer requirements, user needs, or product use. Generally, design objectives can be derived directly from the design brief (i.e., provided by customers). However, to obtain more accurate and comprehensive design objectives, it is necessary to further expand the original design objectives through in-depth user interviews or team discussions. In other words, the initially proposed design goals are abstract or fuzzy. Thus, designers must actively expand a series of secondary design objectives based on the original design goals to obtain more specific design goals. Cross [30] proposed a method for managing design objectives, namely the objectives tree method. In this paper, we use the objectives tree method to manage the design objectives of the desert tree-planting vehicle and then obtain a clear objective tree diagram. The specific operation steps of the objectives tree method are as follows [31]:

• List the design objectives through investigations, interviews, and discussions and expand the design objectives as much as possible;
• According to the primary and secondary relationship between the design objectives, all design objectives are grouped;
• Draw an objective tree diagram to present the relationship between design targets.

2.2. Function Analysis Method

After design objectives are set, the function analysis method can be used to clarify the functional system of the target product [30,31]. The function analysis method could regard the functional system of the target product as a “black box”: the left end of the “black box” is the necessary “input”, the right end is the desired “output”, and the interior consists of a series of secondary functions. Specifically, the internal secondary functions can be expressed as “verb + noun”. Furthermore, to ensure functional innovations, designers should actively broaden the functional system boundary of the target product. This paper reconstructs the functional system of the tree-planting vehicle using the function analysis method based on design objectives. Then this functional system could be transformed from a “black box” into a “white box”. The specific operation steps are as follows:

• Define the functional system of the target product based on “input” and “output” (i.e., black box);
• Decompose the overall function of the target product into a series of necessary secondary functions;
• In the “black box”, from left to right, use a block diagram to represent the relationship between secondary functions;
• Plan out several reasonable functional system boundaries;
• Search for appropriate elements to realize the association between secondary functions.

2.3. Finite Structure Method (FSM)

Because “form follows function” is the basic principle of product design, after clarifying the functional system of the target product, the shape design can be carried out. Then, considering that the products with the same primary function would produce a variety of shapes due to the different combinations of secondary functional units, based on the obtained functional system, various possible shapes of the target product could be obtained using the FSM [32]. Specifically, the FSM could represent the secondary functional units as two-dimensional geometric figures and then use these geometric figures to analyze the spatial layout of the target product. In this paper, we rationally analyze the shape of the desert tree-planting vehicle in two-dimensional space using the FSM. The specific operation steps of FSM are as follows:

• Determine the primary functions of the target product;
• Disassemble the primary function into a combination of several secondary functional units;
• Use geometric figures to represent secondary functional units and analyze various possible layouts in two-dimensional space;
• Reflect on the feasibility of various layouts according to the design objectives and then obtain several reasonable spatial layouts.

2.4. Morphological Analysis Method

Fritz Zwicky [33] proposed the morphological analysis method in 1948, also called the morphological synthesis method or morphological matrix method. This method can be used to analyze all possible combinations of shape elements. Theoretically, the combination scheme of hundreds of morphological elements can be obtained using the morphological analysis method [34]. Currently, the morphological analysis method is widely used in various fields, namely design engineering [34], manufacturing engineering [35], industrial engineering [36], and ergonomics [37]. In this paper, based on the obtained several spatial layouts, we further used the morphological analysis method to concretize the shape of the desert tree-planting vehicle. The operation steps of the morphological analysis method are as follows:

• List the shape elements of the target product;
• Present various types of each shape element;
• Establish a morphological chart;
• Obtain several alternatives through rational screening.

2.5. Fuzzy Comprehensive Evaluation

Because an object has multiple properties, various factors will affect it. Therefore, various factors should be considered comprehensively to evaluate this object through a process called comprehensive evaluation. In addition, if these factors are fuzzy in the evaluation process, this process can be called fuzzy comprehensive evaluation (FCE) [38,39,40]. Recently, FCE has been widely used to evaluate products such as public fitness equipment [41], balance bikes [34], and bicycles [42]. This paper uses FCE to evaluate several alternatives for tree-planting vehicles. Specifically, FCE includes five steps: establishing the factor set, establishing the weight set, establishing the evaluation set, single-factor fuzzy evaluation, and fuzzy comprehensive evaluation. The specific steps are as follows:

(1)

Establish a factor set:

The factor set is a general set composed of various factors that affect the judgment object. It can be expressed as:

$$U=\left\{u_1, u_2,\cdots u_m\right\},u_i\left(i=1,2,\cdots ,m\right)$$

where $u_i\left(i=1,2,\cdots ,m\right)$ represents various factors that affect the judgment object.

(2)

Establish a weight set:

Because the importance of each factor to the judgment object is different, the corresponding weight should be given to each factor $u_i$, that is, $A=\left(a_1,a_2,\cdots ,a_n\right)$. Thus, $a_i$ should satisfy Equation (1).

$${{\displaystyle \sum }}_{i=1}^n{a}_i=1, a_i\ge 0, \left(i=1,2,3,\cdots ,n\right)$$

(1)

(3)

Establish an evaluation set:

The evaluator needs to use some grade indicators when evaluating the judgment object, and the set composed of all grade indicators can be called the evaluation set, that is, $V=\left\{V_1,V_2,\cdots ,V_n\right\}$.

(4)

Single-factor fuzzy evaluation:

Starting from one factor, we use the evaluation set to evaluate the judgment object and then obtain the membership grade of the judgment object to the evaluation set elements, which can be called single-factor fuzzy evaluation. Specifically, based on the $i$th factor in the factor set $u_i$, using the $j$th element $V_j$ in the evaluation set to evaluate the judgment object, we can obtain a membership grade $r_{ij}$. Therefore, the evaluation result of the $i$th factor $u_i$ can be expressed as $R_i$, as shown in Equation (2).

$$R_i=\frac{r_{i1}}{V_1}+\frac{r_{i2}}{V_2}+\cdots +\frac{r_{in}}{V_n}$$

(2)

where $R_i$ is a single-factor evaluation set, which can be expressed as: $R_i=\left(r_{i1},r_{i2},\cdots ,r_{in}\right)$. Similarly, the single-factor evaluation set of all factors can be expressed as Equation (3).

$$\begin{array}{l}\begin{array}{c}{R}_1=\left(r_{11},r_{12},\cdots ,r_{1n}\right)\\ R_2=\left(r_{21},r_{22},\cdots ,r_{2n}\right)\end{array}\hfill \\ \begin{array}{c}⋮\\ R_m=\left(r_{m1},r_{m2},\cdots ,r_{mn}\right)\end{array}\hfill \end{array}$$

(3)

In addition, all single-factor evaluation sets can form a membership grade matrix $R$, as shown in Equation (4).

$$R=\left[\begin{array}{c}\begin{array}{c}{R}_1\\ R_2\\ ⋮\end{array}\\ \begin{array}{c}{R}_i\\ ⋮\\ R_n\end{array}\end{array}\right]=\left[\begin{array}{cc}\begin{array}{c}\begin{array}{ccc}{r}_{11}& r_{12}& \cdots \end{array}\\ \begin{array}{ccc}{r}_{21}& r_{22}& \cdots \end{array}\\ \begin{array}{ccc}⋮& ⋮& \ddots \end{array}\end{array}& \begin{array}{c}\begin{array}{ccc}{r}_{1j}& \cdots & r_{1m}\end{array}\\ \begin{array}{ccc}{r}_{2j}& \cdots & r_{2m}\end{array}\\ \begin{array}{ccc}⋮& \ddots & ⋮\end{array}\end{array}\\ \begin{array}{c}\begin{array}{ccc}{r}_{i1}& r_{i2}& \cdots \end{array}\\ \begin{array}{ccc}⋮& ⋮& \ddots \end{array}\\ \begin{array}{ccc}{r}_{n1}& r_{n2}& \cdots \end{array}\end{array}& \begin{array}{c}\begin{array}{ccc}{r}_{ij}& \cdots & r_{im}\end{array}\\ \begin{array}{ccc}⋮& \ddots & ⋮\end{array}\\ \begin{array}{ccc}{r}_{nj}& \cdots & r_{nm}\end{array}\end{array}\end{array}\right]$$

(4)

(5)

Fuzzy comprehensive evaluation:

The single-factor fuzzy evaluation in step 4 reflects the influence of a factor on the judgment object. However, to obtain an accurate evaluation result, the influence of all factors on the judgment object needs to be considered. Therefore, the fuzzy comprehensive evaluation set $B$ can be obtained using Equation (5).

$$B=A°R=\left[b_1,b_2,\cdots b_j,\cdots b_m\right]$$

(5)

where the symbol $“°”$ represents the fuzzy synthesis operation, and $b_j$ is the obtained fuzzy comprehensive evaluation index. Further, this study uses the synthesis rule $M\left(\wedge ,\vee \right)$ for fuzzy synthesis operation, as shown in Equation (6).

$$b_j{=}_{i=1}^{\begin{array}{c}m\\ \vee \end{array}}\left(a_i\wedge r_{ij}\right), j=1,2,\cdots ,n$$

(6)

where “∨” and “∧” indicate the meaning of taking larger and smaller, respectively.

3. Empirical Study

Section 2 elaborated on the design and evaluation methods of the tree-planting vehicle. Based on these methods, this section presents the design and evaluation processes of the desert tree-planting vehicle in detail. The design process is shown in Figure 1, and the specific execution steps are described in Section 3.1, Section 3.2, Section 3.3, Section 3.4, Section 3.5 and Section 3.6.

3.1. Step 1: Gather Information about the Tree-Planting Vehicle

Before the formal design, we collected some information about the tree-planting vehicle through literature and patent searches, Internet searches, and interviews with engineering vehicle designers. Specifically, by consulting the literature and patents, we found that many scholars focus on (1) retrofitting existing manual tree-planting tools to improve efficiency and (2) certain aspects of tree-planting activities, such as irrigation, fertilization, and sapling transportation. However, few scholars have presented a complete, systematic, and practical tree-planting vehicle. Although we searched product design websites, we found that some creative conceptual design schemes did not follow a strict and scientific design process and evaluation methods [25,27,28,29]. In addition, through interviews with arborists in desert areas, we learned that the existing tree-plantation process faces some problems, including: (1) the whole process requires a large workforce (e.g., for transporting saplings, replenishing saplings, digging holes, covering soil, and fertilizing); (2) planting efficiency needs to be improved (e.g., there is a lack of mechanization and limited ability of personnel to collaborate); (3) the performance of existing equipment needs to be improved (e.g., heat insulation and wind and sand protection); and (4) lack of humanistic care (e.g., the space environment lacks airtightness and comfort). Motivated by this, we are committed to presenting a practical and systematic desert tree-planting vehicle under the premise of following a scientific design process.

3.2. Step 2: Clarify the Design Objectives of the Desert Tree-Planting Vehicle

The designers worked out the preliminary design objectives based on the above analysis through the first discussion. Subsequently, after several discussions, three primary design objectives and seven subordinate design objectives were set. Three primary design objectives are “efficient”, “excellent operability”, and “safe”; and seven secondary design objectives are “large storage space”, “rapid planting speed”, “easy to replace saplings”, “reasonable ergonomic structure”, “not easy to make operational errors”, “could protect against wind and sand”, and “good heat insulation”. In contrast to the primary design objectives, the subordinate design objectives are more specific. Finally, an objective tree diagram was drawn, as shown in Figure 2. These design objectives should serve as design guidelines throughout the design process.

3.3. Step 3: Analyze the Functional System of the Desert Tree-Planting Vehicle

This section should consider which functions could satisfy the design goals of the desert tree-planting vehicle described above. Based on the execution steps described in Section 2.2, we systematically analyzed the functional system of the desert tree-planting vehicle, as shown in Figure 3. Specifically, to transform the function system of the desert tree planting vehicle from a “black box” to a “white box”, we used “Saplings” as the “Input” and “Finished planting” as the “Output” of this functional system. Furthermore, this functional system includes three system boundaries: simple, traditional, and complicated. To satisfy the above design objectives, we provided a complicated and efficient desert tree-planting vehicle by widening the functional system boundaries.

3.4. Step 4: Deduce the Shape of the Desert Tree-Planting Vehicle

In this section, based on the functional system, we obtained several shape layouts of the desert tree-planting vehicles using the FSM. First, according to the steps described in Section 2.3, we disassembled the primary function of the desert tree-planting vehicle into a combination of five secondary functional units, namely drive, power, storage, work, and brace units. Second, to better analyze the shape layouts, we used several geometric figures with different shapes and colors to represent each functional unit. Lastly, based on the design objectives (see Section 3.2) and the functional system (see Section 3.3), the designers presented four layouts after discussion, as shown in Figure 4. Specifically, storage unit C and work unit D of layout 1 and layout 4 are independent of each other. Thus, layout 1 and layout 4 can be equipped with larger storage space. In contrast, storage units C of layouts 2 and 3 are attached to work unit D. Thus, the size of the storage space of layout 2 and layout 3 would be affected by work unit D. In addition, both sides of layout 2 are equipped with a work unit D. Similarly, layout 3 is also equipped with two work units D. Although more work units could improve planting efficiency, it increases the difficulty of replenishing saplings. Moreover, work unit D of layout 4 is suspended externally. Although this measure could increase the space of the storage unit C, it would increase the complexity of the mechanical connection.

3.5. Step 5: Generate Several Alternatives

In this step, we constructed a morphological chart using the morphological analysis method to specify the shape of the desert tree-planting vehicle. First, we still used the five secondary functional units in Figure 4. Second, the designers provided several types to implement each secondary function through referencing the design objectives and similar products. Lastly, a morphological chart was constructed, as shown in

Table 1. The morphological chart of the desert tree-planting vehicle.

Sub-Functions	Type 1	Type 2	Type 3	Type 4	Type 5
Drive (D)	(D1) Wrapped Cockpit	(D2) Round head cockpit	(D3) Flat-headed cockpit	(D4) Reversible cockpit	(D5) Extended cockpit
Power (P)	(P1) Fuel oil	(P2) Solar energy	(P3) Electric energy	(P4) Hybrid energy
Storage (S)	(S1) Separate storage	(S2) Integrated storage
Work (W)	(W1) Traditional drill	(W2) Mechanical arm drill	(W3) Retractable drill	(W4) Split drill	(W5) Flat drill
Brace (B)	(B1) Traditional round wheel	(B2) Triangular track	(B3) Rounded rectangular track

. Theoretically, 5 × 4 × 2 × 5 × 3 = 600 design schemes could be generated using this morphological chart.

To achieve the design objectives, the designers assembled three alternatives based on several layouts (see Figure 4) and Table 1, as shown in Figure 5. Alternative 1 comprises D1, P1, S1, W1, and B3. Alternative 2 comprises D2, P4, S2, W4, and B1. Alternative 3 comprises D5, P1, S1, W1, and B1. Subsequently, the designers used the 3D drawing software Rhino to draw the 3D models of those alternatives and used the rendering software KeyShot to draw their renderings, as shown in Figure 6.

3.6. Step 6: Design Evaluation of Several Alternatives

This section uses the FCM to select an optimal solution from three alternatives (see Figure 6). The specific steps are as follows.

3.6.1. Establish the Factor Set of the Desert Tree-Planting Vehicle

As shown in Figure 2, the design objectives were divided into two levels, namely primary objectives and subordinate objectives. First, we used the primary objectives to build a level-1 factor set, denoted as U.

$$U=\left\{Efficiency \left(u_{11}\right),Operability \left(u_{12}\right),Safety \left(u_{13}\right)\right\}$$

In addition, we used the subordinate objectives to build a level-2 factor set, denoted as $U^{\prime }$.

$$U^{\prime }=\left\{\begin{array}{c}Storage capacity \left(u_{21}\right),\\ Planting speed \left(u_{22}\right),\\ Degree of difficulty of replacement \left(u_{23}\right),\\ Rationality of structure \left(u_{24}\right),\\ Inclusiveness of operation errors \left(u_{25}\right),\\ Sand-prevention performance \left(u_{26}\right),\\ Heat-shielding performance \left(u_{27}\right)\end{array}\right\}$$

3.6.2. Establish the Weight Set of the Desert Tree-Planting Vehicle

We invited 12 subjects (including engineering designers and industrial design teachers) to evaluate the importance of three level-1 factors. Specifically, the subjects were asked to evaluate these factors objectively on a scale of 0 to 1 based on their professional knowledge. After the evaluation results were normalized, the weight set of three level-1 factors could be expressed as:

$$A=\frac{a_{11}}{u_{11}}+\frac{a_{12}}{u_{12}}+\frac{a_{13}}{u_{13}}=\frac{0.34}{\mathrm{Efficiency}}+\frac{0.32}{\mathrm{Operability}}+\frac{0.34}{\mathrm{Safety}}=\left(0.34, 0.32, 0.34\right)$$

Similarly, to obtain weights for the level-2 factors, we invited 60 subjects (including engineering designers, industrial design teachers, and industrial design students) to rate seven level-2 factors using the Likert five-point scale method. Finally, a total of 50 valid questionnaires were collected. In addition, because seven level-2 factors belong to three level-1 factors (see Figure 2), the normalized weight set of the three groups of level-2 factors can be expressed as:

$$A_{11}=\frac{a_{21}}{u_{21}}+\frac{a_{22}}{u_{22}}=\frac{0.54}{u_{21}}+\frac{0.46}{u_{22}}=\left(0.54, 0.46\right)$$

$$A_{12}=\frac{a_{23}}{u_{23}}+\frac{a_{24}}{u_{24}}=\frac{0.49}{u_{23}}+\frac{0.51}{u_{24}}=\left(0.49, 0.51\right)$$

$$A_{13}=\frac{a_{25}}{u_{25}}+\frac{a_{26}}{u_{26}}+\frac{a_{27}}{u_{27}}=\frac{0.31}{u_{25}}+\frac{0.36}{u_{26}}+\frac{0.33}{u_{27}}=\left(0.31, 0.36, 0.33\right)$$

3.6.3. Establish an Evaluation Set for the Desert Tree-Planting Vehicle

In this paper, we used five levels as the evaluation set $V$, which can be expressed as:

$$V=\left\{0 \mathrm{Very}\mathrm{unsuitable}, 0.25 \mathrm{Unsuitable}, 0.5 \mathrm{Ordinary}, 0.75 \mathrm{Suitable}, 1 \mathrm{Very}\mathrm{suitable}\right\}$$

3.6.4. Single-Factor Fuzzy Evaluation for the Desert Tree-Planting Vehicle

Level-2 factors are used as evaluation criteria. Then, we invited 20 experts (i.e., industrial designers and engineering designers) to perform single-factor fuzzy evaluations on three alternatives (see Figure 6) using the five levels of the evaluation set. Finally, the averages of all expert evaluations were used as valid data, as shown in

Table 2. Results of the single-factor fuzzy evaluation.

Level 2 Evaluation Factors	Weight (W)	Evaluation Scores
		Alternative 1	Alternative 2	Alternative 3
Storage capacity $\left(u_{21}\right)$	0.54	0.285	0.885	0.465
Planting speed $\left(u_{22}\right)$	0.46	0.240	0.285	0.840
Degree of difficulty of replacement $(u_{23})$	0.49	0.420	0.570	0.330
Rationality of structure $(u_{24})$	0.51	0.285	0.465	0.285
Inclusiveness of operation errors $(u_{25})$	0.31	0.285	0.420	0.420
Sand-prevention performance $\left(u_{26}\right)$	0.36	0.615	0.375	0.420
Heat-shielding performance $(u_{27})$	0.33	0.375	0.330	0.420

.

Therefore, the evaluation sets of the three level-2 factors sets can be expressed as follows:

$$R_1=\left[0.54 0.46\right]°\left[\begin{array}{c}\begin{array}{ccc}0.285& 0.885& 0.465\end{array}\\ \begin{array}{ccc}0.240& 0.285& 0.840\end{array}\end{array}\right]\approx \left[\begin{array}{ccc}0.26& 0.61& 0.64\end{array}\right]$$

$$R_2=\left[0.49 0.51\right]°\left[\begin{array}{c}\begin{array}{ccc}0.420& 0.570& 0.330\end{array}\\ \begin{array}{ccc}0.285& 0.465& 0.285\end{array}\end{array}\right]\approx \left[\begin{array}{ccc}0.35& 0.52& 0.31\end{array}\right]$$

$$R_3=\left[0.31 0.36 0.33\right]°\left[\begin{array}{c}\begin{array}{ccc}0.285& 0.420& 0.420\end{array}\\ \begin{array}{c}\begin{array}{ccc}0.615& 0.375& 0.420\end{array}\\ \begin{array}{ccc}0.375& 0.330& 0.420\end{array}\end{array}\end{array}\right]\approx \left[\begin{array}{ccc}0.43& 0.37& 0.42\end{array}\right]$$

Finally, after normalization, the single-factor evaluation matrix $\underset{\_}{R}$ can be expressed as:

$$\underset{\_}{R}=\left[\begin{array}{c}\begin{array}{ccc}0.18& 0.40& 0.42\end{array}\\ \begin{array}{ccc}0.30& 0.44& 0.26\end{array}\\ \begin{array}{ccc}0.35& 0.31& 0.34\end{array}\end{array}\right]$$

3.6.5. Fuzzy Comprehensive Evaluation for the Desert Tree-Planting Vehicle

The weight set $A$ of the three level-1 factors and the single-factor evaluation matrix $\underset{\_}{R}$ is known. Thus, the fuzzy comprehensive evaluation result $B$ can be obtained by using Equation (5), as follows:

$$B=A\circ \underset{\_}{R}=\left[\begin{array}{ccc}0.34& 0.32& 0.34\end{array}\right]\circ \left[\begin{array}{c}\begin{array}{ccc}0.18& 0.40& 0.42\end{array}\\ \begin{array}{ccc}0.30& 0.44& 0.26\end{array}\\ \begin{array}{ccc}0.35& 0.31& 0.34\end{array}\end{array}\right]=\left[\begin{array}{ccc}0.28& 0.38& 0.34\end{array}\right]$$

The results showed that the priority of three alternatives is alternative 2, alternative 3, and alternative 1. Thus, alternative 2 is considered the optimal solution. In addition, we used a 3D printer to obtain a model of this optimal solution, as shown in Figure 7.

4. Results and Discussion

This paper presented a complex and practical tree-planting vehicle (see Figure 7) for desert areas through scientific design and evaluation methods. In contrast to traditional manual or semi-mechanized planting [23,24], this tree-planting vehicle can improve planting efficiency and liberate the workforce. In contrast to other tree-planting vehicles [25,27,28,29], the functional system of this tree-planting vehicle is more powerful, embodied in the multi-functional drill, sufficient storage space for saplings, unique sapling-transfer methods, and a closed cockpit. Specifically, the characteristics of this tree-planting vehicle are as follows:

• Since the storage unit and the work unit are independent of each other, the storage unit could store more saplings, saving time when transporting and replenishing saplings;
• The work unit adopts a split drill (see Table 1). In contrast to other drills, the split drill integrates soil turning, planting saplings, and cultivating soil, as shown in Figure 8. Therefore, it could increase the tree-planting efficiency and liberate the workforce;
• The storage unit adopts an integrated storage method for storing saplings (see Table 1). This method can not only carry more saplings at one time, but also, the saplings can be transferred from the storage unit to the work unit through the automatic transfer system. The detailed transfer process is shown in Figure 9;
• The drive unit adopts a round-head cockpit. In contrast to other cockpits, the round cockpit has a rounded shape, and there are no extra recessed edges to collect dust.

5. Conclusions

Tree planting is a crucial measure for the development of desert greening. Although people realize that a tree-planting vehicle for desert sites can promote the efficiency of tree plantation, few scholars have followed a rigorous design process to present a desert tree-planting vehicle. In this article, we integrated the objectives tree method, the function analysis method, the finite structure method, and fuzzy comprehensive evaluation to present a tree-planting vehicle. Specifically, the objective design goals, complete functional systems, reasonable shape layouts, and optimal solutions for tree-planting vehicles were obtained through these methods. In contrast to traditional tree-planting methods, this optimal solution could (1) carry and store a large number of saplings; (2) reduce the time for replenishing saplings; (3) integrate a variety of functions, thereby liberating the workforce; and (4) avoid the harsh environment affecting workers. In conclusion, this desert tree-planting vehicle could improve the efficiency of tree plantation activities, thereby promoting the sustainable development of desert greening.

The present study is subject to some limitations. First, one or several saplings suitable for the local environment could be selected due to differences in climate or soil quality in desertified areas. However, the perimeters of these saplings may differ. Thus, designers should consider the effect of various circumferences of saplings on planting and transportation actions. Second, although this design scheme integrates various functions, some work still needs to rely on manual labor. For example, to prevent wild animals from eating saplings, workers need to use grass to wrap the trunks of saplings. Thus, in further design research, we could broaden the functional system boundary of this design scheme, thereby providing a more practical desert tree-planting vehicle. Lastly, although a model of this design scheme was obtained using a 3D printer, we need to consider the aspects of mechanical automation and human factors engineering to implement its use. Much research is still needed to improve the design scheme.