Effects of Urban Park Construction Period on Plant Multidimensional Diversities, Landscape Patterns of Green Spaces, and Their Associations in Changchun City, Northeast China

Abstract

Understanding the characteristics of urban plant multidimensional diversity and urban green spaces (UGSs) landscape patterns is the central theme of urban ecology, providing theoretical support for UGSs management and biodiversity conservation. Taking Changchun, a provincial city, as an example, a total of 240 plots were surveyed using the stratified random sampling method. We studied the effects of the urban park construction period on plant multidimensional diversities, landscape patterns of green spaces, and their associations in Changchun City, Northeast China. The results indicated that total woody species and tree species diversity attributes were both the highest in the construction period of 2001–2020 and lowest in the construction period before 1940. However, shrub species diversity attributes were completely the opposite. Diameter at the breast height (DBH) diversity index (H_d) was the highest in the construction period before 1940 and lowest in the construction period of 2001–2020. However, the height diversity index (H_h) showed the opposite trend. Phylogenetic structures of total woody species and tree species showed divergent patterns in parks constructed before 1940 and 1940–2000 period, while that in 2001–2020 period could not be determined. In contrast, the phylogenetic structure of the shrub species clustered across all construction periods. Landscape pattern metrics varied significantly among different construction periods. Total Area (TA) was the highest in the construction period of 2001–2020. The structural equation model (SEM) revealed that construction periods exerted significant direct effects on both multidimensional diversities and landscape patterns of green spaces. Specifically, construction periods indirectly affected tree species diversity through structural diversity and influenced shrub species’ phylogenetic diversity through shrub species diversity. What is more, Patch Density (PD), Edge Density (ED), and Aggregation Index (AI) correlated with H_h, which had a direct effect on the Shannon–Wiener diversity index of tree species (H′_t). Overall, the results indicated that species diversity can be enhanced through regulating landscape patterns, rationally selecting tree species, and optimizing plant configuration. These above results can provide scientific references for the configuration of plant communities and selection of tree species in urban parks, and offer important guidance for urban biodiversity conservation and enhancement.

1. Introduction

Humanity’s relentless demand for the Earth’s resources is accelerating the rate of species extinction, disrupting the global ecological balance and leading to continued loss of biodiversity [1,2]. For example, in the tropical regions, where biodiversity is rich, 32 million hectares of primary or recovering forests were lost between 2010 and 2015 [1]. The Earth may experience an extinction, with an estimated 20–50% of animal species facing extinction [3]. Biodiversity holds significant value, including consumptive use value, productive use value, social value, ethical value, esthetic value, option value, and ecosystem service value [4]. As a result, a decline in biodiversity can lead to a series of problems, such as threats to food security, economic losses, loss of cultural values, the breaking of ecological balance, and a decline in human well-being. Therefore, the protection of biodiversity is essential for maintaining the balance of ecosystems and the long-term prosperity of human societies.

Urban green spaces (UGSs) are an important part of the urban ecosystem, which have the functions of soil and water conservation, climate regulation, air purification, environmental protection and beautification, and the protection of biological resources [5,6,7]. With the acceleration of urbanization, habitats of many plants and animals have been replaced by impervious surface areas [8]. Therefore, UGSs are assuming an increasingly critical role in the conservation and maintenance of biodiversity. A large number of studies have mainly focused on the assessment of tree species composition and diversity [8], which is essential to protecting the biodiversity of forests [9]. In addition, some studies have suggested that the effects of tree species diversity may be partly attributed to structural diversity [10,11,12,13,14,15]; Wang et al. considered that large plants and the top 50% of the tallest trees contribute to enhancing the carbon sequestration capacity of UGSs [16]. Notably, the structural diversity of UGSs is not static in space and time, and it is often altered by human interference, such as different planning and management, human outdoor activities, and land use [17]. Unlike species diversity, phylogenetic diversity takes into account the evolutionary history of species. Li et al. considered that phylogenetic structure has an important reference value for biodiversity conservation [18]. What is more, previous studies have shown that combining species diversity with phylogenetic diversity can more accurately reflect changes in community biodiversity and the mechanisms of community assembly [19]. Therefore, integrating phylogenetic diversity with studies on species diversity and structural diversity can reveal the ecological functions and evolutionary potential of biodiversity from multidimensional perspectives [20]. Landscape pattern refers to the spatial arrangement and composition of different elements within the landscape, which is a specific expression of landscape heterogeneity, and, at the same time, is the result of the effects of various ecological processes at different scales [21]. Previous studies have shown that the regulation and construction of sustainable landscape patterns is a possible way to improve biodiversity [22], and landscape characteristics can also be used as an index to measure biodiversity [23,24,25]. With the rapid development of urbanization and the increase in urban population density, the proportion of UGSs has been changing [26], which changes the urban forest landscape patterns [27]. However, most of the previous studies have focused on landscape patterns and their driving factors [28]. In UGSs studies, most focused on the impact of socioeconomic factors on species diversity [29], while comprehensive research on landscape patterns and multidimensional diversities is scarce.

As the main component of the UGSs, urban parks are important resources that are different from other types of UGSs [30]. Urban parks have a large green area and rich species composition, and perform ecological functions such as habitat provision for plants and animals, and biodiversity conservation. With the continuous development of cities, the status and role of parks in the city are becoming increasingly significant, and the connection between urban public space and social life is also strengthening. In addition, urban parks have become a comprehensive reflection of politics, economy and culture [31]. There are many parks in the city, and the parks constructed in different periods have significant differences. They have experienced different historical evolutions, have different design concepts, have different service facilities, and play different social functions. Therefore, it is of great significance to study the multidimensional diversities and landscape patterns of urban parks constructed in different periods.

During the Puppet Manchukuo period, Changchun was designated as “Xinjing”. Since the reform and opening-up era, Changchun has gradually developed into a major national industrial base and transportation hub. In recent decades, urban population and urban impervious areas have accelerated swiftly [32,33]. As the geographical center of Northeast Asia, Changchun continues to write a new chapter in its development with the vision of an ecologically livable “Forest City”. Given its historical trajectory, Changchun’s Park planning and construction have been shaped by distinct developmental stages. The study aimed to achieve the following objectives: (1) determine whether species diversity, structural diversity, and phylogenetic diversity are significantly different in the parks constructed during different periods; (2) determine whether green space landscape patterns are significantly different in urban parks constructed during different periods; (3) explore the associations among multidimensional diversities, landscape patterns, and park construction periods.

2. Methods

2.1. Study Area

The study area is located in Changchun, the provincial capital of Northeast China’s Jilin Province, which is also known as “Spring City of the North”. Changchun has a continental monsoon climate in the northern temperate zone, with four distinct seasons: windy spring, rainy summer, cool autumn, and cold and long winter. The annual minimum temperature is −39.8 °C, the maximum temperature is 39.5 °C, and the average temperature is 4.8 °C. The annual average precipitation is 567 mm [34]. The precipitation is mainly concentrated in July and August. Changchun is bordered Songyuan City, Siping City, Jilin City, and Harbin City. The study area is located within the ring expressway of Changchun City (Figure 1b), with a land area of 524 square kilometers (125°07′–125°26′ E, 43°44′–44°02′ N) [35].

2.2. Field Survey

In this research, the stratified random sampling method was used to determine the number of sample plots, and a total of 240 plots from 29 parks were investigated. According to the construction period of the parks, 240 plots were divided into three categories (

Table 1. Urban parks during different construction periods.

Construction Period	Park Name
before 1940	South Lake Park, Children Park, Chaoyang Park, Shengli Park
1940–2000	Daishan Park, Labour Park, Junzilan Park, Forest Park, Zoo and Botanical Garden, Mudan Garden, Xinghuacun Park, Changchun Park
2001–2020	Friendship Park, Sculpture Park, Jinjiang Park, Tianjia Park, Guanlan Lake Park, Baihua Park, Car Park, Beihai Park, Shuiwenhua Park, Changchun Industrial Trajectory Park, Kuancheng central Park, Changchun Youth Park, Nanxi Wetland Park, Yuehe Cultural Park, Binhe Park, North Lake Wetland Park, Baimu Park

): the construction period before 1940, the construction period of 1940–2000 and the construction period of 2001–2020, with the number of plots being 90, 52 and 98, respectively, and the size of the plot was 20 m × 20 m. The species name, species richness, the abundance of each species, tree DBH (the diameter at breast height), tree height, crown width and health status of the woody plants in each plot were recorded.

2.3. Calculation of Species Diversity Indices

Species diversity indices serve as core indicators of the health and stability of an ecosystem. The Species Richness (SR), Margalef richness index (d_Ma), Shannon–Wiener diversity index (H′) and Pielou Evenness index (J′) of each sample plot were calculated by using the data of field investigation. The calculation formulas for the indices are as follows (Equations (1)–(4)).

Species Richness: SR = S

(1)

$$\mathrm{Margalef} \mathrm{richness} \mathrm{index}: d_{Ma}=(S-1)/lnN$$

(2)

$$\mathrm{Shannon}\text{–}\mathrm{Wiener} \mathrm{diversity} \mathrm{index}: H^{\prime }=-{\sum }_{i=1}^S{RA}_i\left(ln{RA}_i\right)$$

(3)

$$\mathrm{Pielou} \mathrm{Evenness} \mathrm{index}: J^{\prime }=H^{\prime }/ln\left(S\right)$$

(4)

In the above formulas, S represents the Species Richness; N represents the sum of the individuals of all species; RA_i represents the relative abundance of species i [35]. In this study, SR_w, SR_t and SR_s represent the Species Richness of woody plants, trees, and shrubs; d_Maw, d_Mat and d_Mas represent Margalef richness indices of woody plants, trees, and shrubs; H′_w, H′_t and H′_s represent Shannon–Wiener diversity indices of woody plants, trees, and shrubs; J′_w, J′_t and J′_s represent Pielou Evenness indices of woody plants, trees, and shrubs.

2.4. Calculation of Structural Diversity Indices

Structural diversity was classified into horizontal and vertical components, which are represented by the DBH structural diversity index and tree height structural diversity index, respectively. The value of the DBH structural diversity index (H_d) reflects the community age structure [36]. A higher tree height structural diversity index (H_h) indicates that the community has a multi-layered structure, forming complex vertical niches [20]. According to the field survey data, the tree DBH was divided into 6 levels, which were 0~10.00 cm, 10.01~20.00 cm, 20.01~30.00 cm, 30.01~40.00 cm, 40.01~50.00 cm, and >50.00 cm. The tree height was divided into 4 levels, which were 0~4.99 m, 5.00~9.99 m, 10.00~15.00 m, and >15.00 m. H_d and H_h of each plot were calculated. The formulas are as follows (Equations (5) and (6)).

$$\mathrm{DBH} \mathrm{diversity} \mathrm{index}: H_d=-{\sum }_{j=1}^d{P}_j\mathit{ln}\left(P_j\right)$$

(5)

$$\mathrm{Height} \mathrm{diversity} \mathrm{index}: H_h=-{\sum }_{k=1}^h{P}_k\mathit{ln}\left(P_k\right)$$

(6)

P_j represents the ratio of the DBH area of grade j, and P_k represents the ratio of the DBH area of height grade k, respectively [37].

2.5. Calculation of Phylogenetic Diversity Indices

Phylogenetic diversity reflects the phylogenetic relationships and evolutionary distances among species. Three phylogenetic diversity indices were calculated, and they were Faith’s phylogenetic diversity index (PD), net relatedness index (NRI) and net nearest taxon index (NTI), respectively. A larger PD indicates that species are more distantly related phylogenetically. Positive values of NRI and NTI suggest clustered phylogenetic structures, whereas negative values indicate overdispersed structures [20]. NRI focuses on measuring phylogenetic similarity among species, while smaller NTI values indicate closer evolutionary relationships [38,39]. The formulas are as follows (Equations (7)–(11)).

$$PD=\sum _{j=1}^m{length}_i$$

(7)

$$MPD={\displaystyle \frac{{\sum }_n^k{\sum }_n^k{\delta }_{\mathit{mn}}}{k}}, m\ne n$$

(8)

$$NRI=-1\times (MPDs-MPDr)/SD\left(MPDr\right)$$

(9)

$$MNTD={\displaystyle \frac{{\sum }_n^k\mathit{min}{\delta }_{\mathit{mn}}}{k}}, m\ne n$$

(10)

$$NTI=-1\times (MNTDs-MNTDr)/SD\left(MNTDr\right)$$

(11)

where m is the number of species within the plot, and length_i denotes the branch length of species i in the phylogenetic tree. k represents Species Richness; δ_mn represents the phylogenetic distance between species m and n; minδ_mn represents the minimum phylogenetic distance between species m and all other species within the community. MPDs and MNTDs are the observed values of each plot. MPDr and MNTDr are the average of MPD and MNTD from the null model (999 randomizations), and SD is the standard deviation [20,40]. In our study, PD_w, PD_t and PD_s represent Faith’s phylogenetic diversity indices of woody plants, trees and shrubs; NRI_w, NRI_t and NRI_s represent net relatedness indices of woody plants, trees and shrubs; NTI_w, NTI_t and NTI_s represent net nearest taxon indices of woody plants, trees and shrubs.

2.6. Analysis of UGSs Landscape Patterns

The landscape patterns of green spaces of 29 parks were mapped using Google Earth images. A series of landscape pattern metrics were calculated in the software of FRAGSTATS (version 4.2). According to the characteristics of landscape pattern metrics, the following metrics were selected for this study: area-edge metrics (TA, AREA-MN and ED), shape metrics (PARA-MN, SHAPE-MN and CONTIG-MN) and aggregation metrics (PD, AI, DIVISION and COHESION) [35,41,42,43].

2.7. Statistical Analysis

One-way ANOVA and non-parametric tests were used in SPSS 27.0 to verify whether there were significant differences in species diversity attributes (SR_w, SR_t, SR_s, d_Maw, d_Mat, d_Mas, H′_w, H′_t, H′_s, J′_w, J′_t, and J′_s), structural diversity attributes (H_d and H_h), phylogenetic diversity attributes (PD_w, PD_t, PD_s, NRI_w, NRI_t, NRI_s, NTI_w, NTI_t, and NTI_s) and landscape pattern metrics (TA, AREA-MN, ED, PARA-MN, SHAPE-MN, CONTIG-MN, PD, AI, DIVISION, and COHESION) in urban parks across different construction periods. The lavaan package in RStudio (version 4.3.3) was used to establish a structural equation model (SEM) to determine the relationships and effects among complex variables [44].

3. Results

3.1. Species Composition

We investigated 7689 plants, which belonged to 25 families, 56 genera, and 116 species (

Table 2. The number of family, genera, and species classification in different construction periods of urban parks in Changchun city.

Construction Period	Before 1940	1940–2000	2001–2020
Family (tree)	20	20	17
Genera (tree)	34	35	33
Species (tree)	62	64	73
Family (shrub)	11	9	9
Genera (shrub)	20	19	19
Species (shrub)	24	23	25

). Among them, 6723 trees belonged to 21 families, 39 genera, and 90 species, and 966 shrubs belonged to 13 families, 27 genera, and 38 species. Tree species in urban parks constructed before 1940 contained 20 families, 34 genera, and 62 species. In the construction period of 1940–2000, tree species belonged to 20 families, 35 genera, and 64 species. And tree species belonged to 17 families, 33 genera, and 73 species in the construction period of 2001–2020. Shrub species in urban parks constructed before 1940 contained 11 families, 20 genera and 24 species. In the construction period of 1940–2000, shrub species belonged to 9 families, 19 genera, and 23 species. And shrub species belonged to 9 families, 19 genera and 25 species in the construction period of 2001–2020. The top three most common woody plant species in different construction periods of urban parks in Changchun city are shown in

Table 3. The top three most common woody plant species in different construction periods of urban parks in Changchun city.

Construction Period	Tree Species	Shrub Species
before 1940	Pinus tabuliformis var. Mukdensis, Larix olgensis, Quercus mongolica	Sambucus williamsii, Lonicera japonica, Forsythia mandschurica
1940–2000	Pinus sylvestris var. Mongolica, Abies holophylla, Fraxinus mandshurica	Ligustrum quihoui, Lonicera japonica, Weigela florida
2001–2020	Pinus sylvestris var. Mongolica, Picea koraiensis, Betula platyphylla	Lonicera tatarica, Lonicera japonica, Prunus salicina

.

3.2. Multidimensional Diversity Attributes of Urban Parks Across Different Construction Periods

There were significant differences in species diversity attributes, structural diversity attributes, and phylogenetic diversity attributes in different construction periods of urban parks. For total woody and tree species diversity (Figure 2), SR_w, d_Maw, H′_w, J′_w, SR_t, d_Mat, H′_t, and J′_t were all the highest in the construction period of 2001–2020 and lowest in the construction period before 1940. From before the 1940 construction period to the 2001–2020 construction period, SR_w, d_Maw, H′_w, J′_w, SR_t, d_Mat, H′_t, and J′_t increased by 13.42%, 17.62%, 17.33%, 12.74%, 38.04%, 44.92%, 40.84%, and 25.24%, respectively. In contrast, shrub species diversity (Figure 2) followed an opposite trend. SR_s, d_Mas, H′_s, and J′_s were all the highest in the construction period before 1940 and lowest in the construction period of 2001–2020. From before 1940 construction period to 2001–2020 construction period, SR_s, d_Mas, H′_s, and J′_s decreased by 53.30%, 61.19%, 60.57%, and 61.02%, respectively.

For structural diversity attributes (Figure 3), H_d was the highest in parks constructed before 1940 and lowest in those constructed during 2001–2020, and there were significant differences between before 1940 and 2001–2020 period, and also between 1940 and 2000 and 2001–2020 period (p < 0.05). H_h was the highest in the construction period of 2001–2020 and lowest in the construction period before 1940, and there were significant differences between before 1940 and 2001–2020 period (p < 0.05).

Regarding phylogenetic diversity (Figure 4), NTI_w, NRI_W, NTI_t, and NRI_t in the construction period of 1940–2000 and before 1940 were all negative. In the construction period of 2001–2020, NTI_w, NTI_t were positive, and NRI_w, NRI_t were negative. For shrub species phylogenetic diversity attributes, NRI_s and NTI_s were all positive in different construction periods. PD_w was the highest in the construction period of 2001–2020 and lowest in the construction period of 1940–2000, and there was no significant difference among construction periods (p > 0.05). PD_t was the highest in the construction period of 2001–2020 and lowest in the construction period before 1940, and there was also no significant difference among construction periods (p > 0.05). PD_s was the highest in the construction period before 1940 and lowest in the construction period of 2001–2020, and there were significant differences between before 1940 and 1940–2000 period, and also between before 1940 and 2001–2020 period (p < 0.05).

3.3. Landscape Pattern Metrics of Urban Parks Across Different Construction Periods

As shown in Figure 5, landscape patterns analysis demonstrated significant differences in urban parks across different construction periods (p < 0.05). TA was the highest in the construction period of 2001–2020 and lowest in the construction period of 1940–2000 (Figure 5a), and the construction periods before 1940 and 1940–2000 showed significant differences (p < 0.05), while the 2001–2020 period also demonstrated significant differences compared to the 1940–2000 period (p < 0.05). ED was the highest in the construction period of 1940–2000 and lowest in the construction period before 1940 (Figure 5b), and significant differences were found among all construction periods (p < 0.05). AREA-MN was the highest in the construction period before 1940 and lowest in the construction period of 1940–2000 (Figure 5c), and significant differences were also found among all construction periods (p < 0.05). In addition, CONTIG-MN (Figure 5d), SHAPE-MN (Figure 5e), and PARA-MN (Figure 5f) were significantly different among all construction periods (p < 0.05). COHESION and AI were the highest in the construction period before 1940 and lowest in the construction period of 1940–2000 (Figure 5g,j), and there were significant differences between before 1940 and the 1940–2000 period, and also between before 1940 and 2001–2020 period (p < 0.05). DIVISION was the highest in the construction period before 1940 and lowest in the construction period of 2001–2020 (Figure 5h), and there were significant differences between before 1940 and 1940–2000 period, and also between before 1940 and 2001–2020 period (p < 0.05). PD was the highest in the construction period of 1940–2000 and lowest in the construction period before 1940 (Figure 5i), and significant differences were found among all construction periods (p < 0.05).

3.4. Associations Among Construction Period, Multidimensional Diversities and Landscape Patterns: Decoupling by SEM

Model fit was evaluated using indices including df (35), GFI (0.956), CFI (0.981), RMSEA (0.057), and srmr (0.077). Structural equation model (SEM) indicated that construction periods had significant effects on woody species diversity (−0.14, p < 0.05), tree species diversity (−0.24, p < 0.001), shrub species diversity (0.22, p < 0.01), woody species phylogenetic diversity (−0.19, p < 0.01), tree species phylogenetic diversity (−0.14, p < 0.05), shrub species phylogenetic diversity (0.02, p > 0.05), structural diversity (−0.20, p < 0.01) and landscape patterns (−0.27, p < 0.001) (Figure 6). As shown in

Table 4. The SEM analysis of the effect of different construction periods on species diversity, phylogenetic diversity, structural diversity, and landscape patterns.

Response Variable	Mediate Variable	Predictor Variable	Pathway	Standard	p-Value	Significance/Total
Direct effects
Woody species diversity		Different construction periods	Direct	−0.14	0.024	90% (9/10)
Tree species diversity		Different construction periods	Direct	−0.24	0.000
Shrub species diversity		Different construction periods	Direct	0.22	0.001
Woody species phylogenetic diversity		Different construction periods	Direct	−0.19	0.003
Tree species phylogenetic diversity		Different construction periods	Direct	−0.14	0.032
Shrub species phylogenetic diversity		Different construction periods	Direct	0.02	0.738
Structural diversity		Different construction periods	Direct	−0.20	0.002
landscape patterns		Different construction periods	Direct	−0.27	0.000
Tree species diversity		Structural diversity	Direct	0.04	0.045
Shrub species phylogenetic diversity		Shrub species diversity	Direct	0.21	0.003
Indirect effects
Tree species diversity	Structural diversity	Different construction periods	Indirect	−0.0080	0.024	100% (2/2)
Shrub species phylogenetic diversity	Shrub species diversity	Different construction periods	Indirect	0.0462	0.024
Total effects
Tree species diversity		Different construction periods	Total	−0.2480	0.024	50% (1/2)
Shrub species phylogenetic diversity		Different construction periods	Total	0.0662	0.291

, construction periods had an indirect effect on tree species diversity through structural diversity (−0.2 × 0.04 = −0.008, p < 0.05). Construction periods indirectly affected shrub species phylogenetic diversity through shrub species diversity (0.22 × 0.21 = 0.0462, p < 0.05). Furthermore, there was a significant correlation between woody species diversity and tree species diversity (0.72, p < 0.001), tree species phylogenetic diversity correlated well with woody species phylogenetic diversity (−0.15, p < 0.05), and tree species diversity was related to tree species phylogenetic diversity (0.85, p < 0.001). Landscape patterns also correlated with structural diversity (0.06, p > 0.05), and structural diversity had a direct effect on tree species diversity.

4. Discussion

4.1. Effects of Construction Period of Urban Parks on Woody Plant Multidimensional Diversities

Comprehensive studies on species diversity, structural diversity, and phylogenetic diversity can offer foundational theoretical guidance for biodiversity conservation in urban parks. The results confirmed that there were significant differences in species diversities of woody plants, trees, and shrubs across different construction periods (Figure 2). Previous studies have shown that habitat age was related to plant diversity. In Zhanjiang, the earlier the construction period, the higher the cultivated species richness. Additionally, there is a positive correlation between spontaneous species richness and construction age [45,46]. However, our study found that the later the construction period, the higher the diversity attributes of total woody species and tree species. It may be due to more elaborate planning and more adequate financial support [47]. Alien species introduced by human activities may further increase urban species diversity [31]. Moreover, accelerated urbanization [48] might also contribute to higher species diversity of total woody species and tree species, as people’s changing lifestyles and values result in higher SR and d_Ma. Our results also indicated that H′_t and H′_s had significant differences across different construction periods, which can be ascribed to urban management policies, such as fertilization and watering frequency [35,45,49]. Wang et al. [50] suggested that there were no significant differences in the Pielou Evenness index among vegetation types across different restoration years. Figure 2 showed that J′_w, J′_t, and J′_s had significant differences across different construction periods, which indicated that the numbers of total woody species, tree species, and shrub species in urban parks varied with construction periods. J′_w and J′_t were the lowest during the construction period before 1940 since the forest stand types of urban parks constructed before 1940 were predominantly mature pure stands. Wang et al. [51] considered that, compared with trees, shrubs contribute relatively less to UGSs, but they also play an indispensable role in UGSs. In our study, the earlier the construction period, the higher the SR_s, d_Mas, H′_s, and J′_s. This is because “old” parks were constructed earlier, and the forest stand types in these early-constructed parks were mainly single-tree-species; landscape management departments replanted shrubs to enhance the vertical structure of plant communities. In contrast, newly constructed parks paid more attention to the multi-layered combination of arbor tree species, resulting in a lower variety of shrub species.

Structural diversity indices exhibited significant differences among parks of different construction periods. Our research indicated that H_d was the highest in the construction period before 1940 (Figure 3a). Park management departments adopted reasonable renewal methods based on the growth of old tree species. For areas that needed to increase the greenery coverage or improve the ecological environment, supplementary planting or dense planting was carried out. Therefore, the age structure of trees was more diverse and H_d increased during the construction period before 1940. H_h was the highest in the construction period of 2001–2020 (Figure 3b), and tree species diversity was also the highest in this construction period (Figure 2), which may be because of the rational selection of species and plant allocation in urban parks. In line with the principle of year-round seasonal diversity, flowering and fruit-bearing small arbor species were strategically combined in the planting design.

Our results indicated that the phylogenetic relationship of total woody species and tree species was the most distant in the construction period of 2001–2020 (Figure 4), and total woody species and tree species diversity attributes were also the highest in this construction period (Figure 2). The phylogenetic relationship of total woody species and tree species was the closest in the construction period of 1940–2000 and before 1940, respectively. In addition, the phylogenetic relationship of shrub species was the most distant in the construction period before 1940, and shrub species diversity attributes were also the highest in this construction period. The phylogenetic relationship of shrub species was the closest in the construction period of 2001–2020, and shrub species diversity attributes were also the lowest in this construction period. These results suggested that the greater the phylogenetic distance among species, the higher the species diversity attributes, which is consistent with previous studies [52]. NRI_w, NRI_t, NTI_w, and NTI_t demonstrated that there were woody species and tree species phylogenetic dispersion in the construction periods of 1940–2000 and before 1940, which indicated that the phylogenetic structure was over dispersed. NRI_s and NTI_s indicated that there were cases of shrub species phylogenetic clustering in all the construction periods, which indicated that phylogenetic structure was clustered. Phylogenetic diversity can reflect plant functional traits; when the phylogenetic relationship between plant species is close, it is likely that their functional traits will be similar [53]. Previous studies have shown that correlations between leaf traits of shrubs were significantly stronger than those of trees [54]. This finding is consistent with our research. Our results underscored that the phylogenetic structure of shrubs was more clustered than that of trees.

4.2. Effects of Construction Period of Urban Parks on Landscape Pattern Metrics

Landscape pattern metrics were significantly different in urban parks across different construction periods (Figure 5). TA in urban parks constructed during 2001–2020 was significantly higher than in other construction periods. On the one hand, the ever-increasing demands of the people have led to rapid urbanization, and the citizens’ demand for public places to rest, exercise, and socialize has risen rapidly. This has promoted the rapid growth of UGSs and driven the construction of parks [55]. On the other hand, the Chinese government has rolled out a series of policies to strengthen ecological environment construction and biodiversity protection, such as ecological civilization construction, land and spatial greening initiatives, and “Forest City” development [56,57]. ED was the highest in urban parks constructed during 1940–2000, which demonstrated that parks constructed in this period were subjected to more human interference. Parks constructed in the construction period of 1940–2000 had the lowest AREA-MN, COHESION, and AI and the highest PD. Consequently, they not only need to expand the area of green patches, but also establish corridors between green patches [34] and improve the degree of patch aggregation. AREA-MN was the highest in the construction period before 1940. This is because the construction of “old” parks featured a large-scale spatial layout, with a large area of continuous green space reserved, resulting in the formation of giant patches. Moreover, “old” parks might have focused on natural conservation, with fewer artificial facilities and functional partitions, thus avoiding the fragmentation of patches. DIVISION was the highest in the construction period before 1940, and it may be due to various design concepts of urban parks and urbanization intensity [58]. This could lead to habitat fragmentation and reduce the available survival spaces of animals and plants [59]. Therefore, we suggested that enhancing the integrity of landscapes in parks constructed before 1940 is essential.

4.3. Associations Among Construction Period, Multidimensional Diversity Attributes and Landscape Patterns Decoupling by SEM

Analysis of species diversity, structural diversity, landscape patterns and construction periods can provide scientific guidance for enhancing species diversity. Our results provided evidence that different construction periods had significant effects on species diversity, structural diversity, phylogenetic diversity, and landscape patterns (Figure 6). SEM indicated that the later construction period corresponded to higher indices of woody species diversity, tree species diversity, woody species phylogenetic diversity, tree species phylogenetic diversity, and structural diversity, while landscape pattern was also significantly affected by the construction period. In the study of typical mixed broadleaved-Korean pine (Pinus koraiensis) forests, it has been found that the species diversity indices would be higher with a more complex community structure [60], which is consistent with our research results; our results showed that structural diversity (H_h) affected tree species diversity (H′_t) positively. It may be because the higher the structural diversity indices are, trees and other plants can utilize spatial resources more effectively, thus improving the species diversity of the community [11,61,62,63]. What is more, UGSs are places for people to relax and enjoy leisure time, especially for the elderly and children, and the increase in vegetation coverage and biodiversity can reduce the occurrence of negative emotions [59,64,65]. Regarding species diversity and phylogenetic diversity, our results suggested that tree species diversity (H′_t) was significantly correlated with tree species phylogenetic diversity (PD_t). In addition, shrub species diversity (SR_s, J′_s) affected shrub species phylogenetic diversity (NRI_s) positively, which demonstrated that species diversity was positively correlated with phylogenetic distance. This result is likely due to the fact that with the increase in species diversity, different species occupy different ecological niches, thereby increasing the branches of the phylogenetic tree, resulting in the dispersal of phylogenetic structure [19]. In addition, our results indicated that the lower the patch aggregation, the greater the fragmentation, and the more complex the patch shape of green spaces in urban parks, the higher the H_h and H′_t. This is because complex patch shapes and fragmented patches create more diverse habitats, providing living spaces for tree species with different ecological niches. Additionally, decentralized small green spaces may be designed as diverse plant communities (e.g., themed areas with distinct compositions). This phenomenon suggests that under specific urban park management contexts, moderate landscape fragmentation and complex patch shapes can enhance H_h and species diversity (H′_t) by increasing habitat heterogeneity and balancing competitive relationships. Consequently, H_h can be enhanced by moderately increasing landscape fragmentation and patch shape complexity while reducing green space patch aggregation in urban parks, thus improving the species diversity of trees.

5. Conclusions

The construction periods of urban parks have significant impacts on the species diversity, structural diversity, and phylogenetic diversity of woody plants, trees, and shrubs, as well as the landscape pattern of green spaces. Later construction periods correlate with higher woody and tree species diversity but lower shrub species diversity. Earlier construction periods are associated with higher H_d and lower H_h. Furthermore, higher phylogenetic diversity corresponds to higher species diversity. Landscape patterns (PD, ED, and AI) also correlate with structural diversity (H_h). Based on these findings, recommendations are proposed for urban parks in Changchun City and other urban parks with similar characteristics:

(1)

For “old” parks constructed before 1940, supplementary planting of small-diameter tree species is recommended to enhance species diversity while enriching the vertical layers of plant communities. For “new” parks constructed in the 2001–2020 period, reducing human intervention in shrub management combined with targeted replanting to increase shrub species diversity is suggested, and the management should replant medium-diameter trees to optimize the horizontal structure of plant communities.

(2)

From the perspective of multidimensional diversities, planting species with distant phylogenetic relationships to enhance species diversity is recommended. Additionally, increasing the H_h of communities can not only improve the species diversity of trees but also enhance the species diversity of woody plants.

(3)

Regulating landscape patterns can enhance both species diversity and structural diversity. Rational design of patch layout in green spaces of urban parks (e.g., decentralization, diverse shapes) may be more beneficial for biodiversity conservation than traditional centralized green spaces. This conclusion provides a novel insight into UGSs planning.