4.1. Ecological Sensitivity Characteristics of Pingtan Island
The results of the ecological sensitivity evaluation of Pingtan Island reveal that the elevation, slope, soil, and water on Pingtan Island exhibit a low level of ecological sensitivity, while vegetation coverage demonstrates high ecological sensitivity. The wind environment is categorized as medium-low ecological sensitivity and the land use type as medium-high ecological sensitivity, and the distribution of the slope direction is relatively uniform. The results of the single-factor weight analysis indicate that the soil, wind environment, and land use type are the more influential factors, aligning with previous research findings that attribute the vulnerability of Pingtan Island’s ecological environment to a combination of natural conditions and human activities [
17]. It is recommended that ecological protection and improvement efforts on Pingtan Island prioritize the enhancement of soil conditions, the wind environment, and the optimization of land use types. Specifically, the spatial distribution analysis of soil sensitivities on Pingtan Island highlights severe salinization and sanding in the four vents, necessitating targeted prevention measures. In the subsequent stages of plant landscape construction, the ecological protection role of windbreaks should be reinforced. Considering the prevailing northeasterly winds on Pingtan Island, due north and northeasterly slopes emerge as the primary windward areas. Among them, the Changjiang’Ao windbreak, where the east and west of the stream have the maximum wind frequency and the dominant wind flow through them, is the location of the concentrated distribution of perennial wind resources. This area should be judiciously utilized for wind energy and plant configuration planning. Conversely, lower wind speeds are observed in the southern and western parts of the island, most of the southern part of Jun Mountain, the northern mountainous areas, and some of the lower-elevation areas, indicating that major mountainous terrains play a significant role in reducing wind speeds [
37]. Water is the most sensitive land use type on Pingtan Island due to topographical constraints, as is common for many islands globally, given their limited freshwater resources [
2]. Despite this limitation, water is indispensable to the ecosystem. Woodland ranks as the second most sensitive type due to the inherent difficulty in restoring mountains once destroyed. Moreover, woodlands contribute significantly to ecosystem regulation and supporting services. This is particularly crucial for islands, where plants with specialized functions such as wind protection and sand fixation hold heightened importance.
From the perspective of comprehensive ecological sensitivity, Pingtan Island is dominated by three categories: slightly ecologically sensitive areas (35.72%), moderate ecologically sensitive areas (33.99%), and extreme ecologically sensitive areas (18.02%). The ecologically insensitive areas are primarily situated in regions with notable human activity, featuring low elevation, flat terrain, sparse vegetation, and robust resistance to disturbances. These areas permit relatively intense urban development and construction and are also the best areas for the diversification of plant landscape construction. The slightly ecologically sensitive areas encompass some mountainous and green regions where human activities introduce certain disturbances. Nevertheless, the ecosystems in these areas are more stable than those in insensitive regions. Consequently, a combination of plant and landscape development with human activities conducted in a moderate manner is feasible. The moderate ecologically sensitive areas include sporadic rural settlements with less human interference, featuring higher vegetation density compared to areas with lower sensitivity. Plant landscapes in these zones can be adjusted accordingly, with options such as forest phase modifications to enhance landscape effects. Additionally, localized forest recreational activities can be conducted in areas with suitable microclimatic environments. The high ecologically sensitive areas boast high ecological quality and are abundant in botanical landscape resources. These areas warrant improvements grounded in ecological protection and the enhancement of ecological landscapes. The vegetation in extreme ecologically sensitive areas experiences minimal human disturbance but is more susceptible to natural environmental factors such as wind and sun, leading to the stunted growth and desiccation of vegetation. These zones are primarily designated for ecological restoration and protection, with the goal of constructing a distinctive ecological landscape on the island.
The wind environment simulation in this study was conducted by simulating island-wide near-surface wind conditions of NNE 13.9 m/s (gale force 7) without accounting for the topography of plants and buildings. However, upon comparing these simulation results with field measurement data, a notable similarity is observed, affirming the feasibility of the wind environment simulation method employed in this study. This finding is intended to provide methodological support for future related studies. In practical applications, the presence of wooded areas and buildings across more extensive parts of the island, along with the potential for improved localized microclimatic environments in various ecologically sensitive areas due to ground plants and shading by buildings, necessitates further in-depth investigations into these localized microclimatic habitats. Future efforts should focus on refining both the model and observation conditions to enhance the overall reliability of the conclusions drawn. Given the variations in habitat characteristics among islands, those with limited conditions can consider referring to this model for the study of island habitats.
4.2. Typical Plant Communities and Characteristics in the Mountains of Pingtan Island across Various Ecologically Sensitive Areas
(1) Exploration of Composition and Dominant Species of Typical Mountain Plant Community
The survey revealed that the typical plant community in the mountains of Pingtan Island follows the order herb layer > shrub layer > tree layer, aligning with the stage dominated by the shrub–grass succession stage. This pattern is consistent with previous findings that herbs are generally considered to be more adept at colonization than woody plants [
38]. This phenomenon can be ascribed to the impact of strong winds, rendering low-growing herbaceous plants more adaptable to this environment in comparison to taller trees and shrubs [
21]. Soil conditions, climate, and other factors also contribute to these observed patterns. Among the various ecologically sensitive areas, the high ecologically sensitive areas exhibit characteristics such as a greater number of species and higher species diversity. However, the overall mountainous plant community on Pingtan Island remains relatively homogeneous, featuring a significant proportion of pure forests. There exists a certain degree of repetition and similarity in the plant communities across different ecologically sensitive areas. This phenomenon can be attributed to the unique habitat conditions of islands, where natural selection favors more adaptable native plant communities. Consequently, the plant community similarity is more pronounced.
The dominant species vary across different sensitive areas. The group species in moderate ecologically sensitive areas are all forests, dominated by A. confusa and S. superba forests. The high and extreme ecologically sensitive areas comprise both arborvitae forests and scrub. The high ecologically sensitive areas are dominated by A. confusa, P. thunbergii, P. elliottii, and C. equisetifolia forests, while the extreme ecologically sensitive areas are dominated by P. thunbergii forests and E. emarginata scrub. Observing the different ecologically sensitive areas, it is evident that with increasing elevation, there is a shift in dominance from A. confusa to P. thunbergii and finally to E. emarginata, indicating a successional pattern.
In general, the current montane plant community on Pingtan Island is less diverse, with a single structure and lower species diversity. This is due to the fact that, because of the need to adapt to habitats with persistent high winds and low soil nutrients, Pingtan Island has more wild herbaceous plants, and more plants tend to have smaller flowers, leaves, and fruits [
21,
39], with small and hairy, highly lignified, and fleshy leaves and more thorny shrubs and vines [
40]. In the future, we should continue to screen suitable plants for mountainous areas with suitable local characteristics, strengthen the adaptability of plants to windy environments and infertile soils through domestication and introduction in order to increase the diversity of mountainous plants, and further strengthen the research on typical plant patterns in mountainous areas so as to enhance the significance of practical guidance.
(2) Characterization of Typical Mountain Plants
There are variations in the community characteristics of plant communities across different ecologically sensitive areas, which are characterized by the dominance of differentiated tree–shrub–herb layers. Trees exhibit superior growth in moderate ecologically sensitive areas, while high ecologically sensitive areas transition toward a tree–shrub community. Herbs grow better in extreme ecologically sensitive areas compared to other areas. These differences are associated with the habitat conditions in distinct sensitive areas, wherein shrubs, grasses, and other lower plants show better adaptation to the environment in higher-elevation areas [
41].
The analysis of the different plant communities revealed that the tree growth of
C. equisetifoli forests was more robust in both the moderate and high ecologically sensitive areas. However, the prominence of this dominant species has led to an excessive canopy density, creating unfavorable growing conditions for other smaller trees or lower plants. Consequently, shrub growth is minimal, and herbaceous growth is suboptimal. In the case of the
P. thunbergii forest, its performance is moderate in both the moderate and high ecologically sensitive areas. It exhibits the lowest level of tree canopy density, but this characteristic facilitates the provision of necessary light and other conditions for the growth of other plants. Therefore, shrubs and grasses within this forest exhibit better growth. The
E. emarginata scrub predominantly occurs in high and extreme ecologically sensitive areas, where the absence of the tree layer allows shrubs and grasses to thrive due to increased sunlight availability and more favorable growing conditions. The findings from the species diversity study indicate that increased ecological sensitivity contributes to the species diversity of plant communities. However, the disparities in their species diversity were not statistically significant. This is attributed to the influence of environmental factors, including elevation, precipitation, wind speed, and soil, on island species diversity [
42,
43,
44,
45]. In the study area, the overall habitats do not vary significantly, which subsequently leads to small differences in overall species diversity.
Species diversity indicators exhibited more pronounced variations among plant communities. The Shannon–Wiener, Pielou, Margalef, and Simpson indices within
A. confusa plant communities, particularly in
A. confus scrub, are elevated. This phenomenon may be attributed to the interspersion of scrub with young trees. Young trees in the community increase the diversity of the shrub layer, and the plants are more light-adapted [
29,
46] The moderate and high ecologically sensitive areas exhibited a heightened degree of diversity in the context of
A. confusa. This phenomenon may be attributed to the successional stage of the
A. confusa community on the middle and lower slopes, encompassing both sun- and shade-loving plants. Furthermore, certain anthropogenic disturbances have created forest gaps in
A. confusa forests, where the understory of shrubby
S. superba and
Ficus formosana receives increased light exposure, gaining a competitive advantage and developing into trees. This dynamic contributes to an augmentation in the diversity of this community. Additionally, the leeward slope location of
A. confusa forests favors the growth of various plant types, as they are less affected by high winds. Pielou and Simpson indices were also high in
P. thunbergii forests, indicating structural stability within the plant community [
47], a characteristic linked to community succession patterns. The relatively low canopy density in
P. thunbergii forests plays a role in supporting the growth of other sun-loving plants. The low species diversity of
C. equisetifolia forests stems from a lack of diversity or very low diversity in the arboreal shrub layer. Although individual plant growth is robust, the extremely high canopy density creates unfavorable conditions for the growth of other sun-loving plants or lower scrubs, resulting in low species diversity. Additionally, the reduction in plant numbers can impede the provision of sufficient apoplastic material for microbial decomposition, subsequently leading to habitat deterioration and a decline in species diversity indices [
29]. As ecological sensitivity increases, it signifies more pronounced ecological challenges. Based on the distribution patterns, it is observed that
P. thunbergii forests on Pingtan Island are predominantly situated in uphill positions and northeast slope directions, implying their preference for more windward locations. This suggests that
P. thunbergii forests exhibit enhanced adaptability to windy conditions and the overall environment. On the other hand,
P. thunbergii forests also offer some shaded spaces, facilitating growth conditions for semi-sun-loving plants like
Melastoma candidum and subsequently enhancing their overall diversity.
In terms of the amalgamation of characteristics and species diversity, the ideal ecological communities that are more stable in the mountains of Pingtan Island include A. confusa forests and scrubs, E. emarginata scrubs, P. thunbergii forests, etc. Conversely, those with lower composite characteristic indices were predominantly C. equisetifolia forests, P. elliottii forest communities, etc., consisting of pure forests with diminished species diversity and consequently exhibiting lower stability. Based on these findings, landscape construction in different sensitive areas can be implemented by referencing the corresponding plant community model, thereby providing a model reference for the landscape construction of other analogous islands and mountains.