1. Introduction
The concept of ecosystem services was first proposed by Wilson [
1], and several other researchers have since further developed the study of ecosystem services [
2,
3]. Ecosystem services are benefits that are directly or indirectly derived through the structure, process, and function of the ecosystem, which are divided into provisioning, regulating, supporting, and cultural services, and include economic, ecological, and social values related to the ecosystem [
4]
More and more attention has been paid to the study of ecosystem services of plantations [
5,
6]. Understory vegetation, as an indispensable part of the plantation ecosystem, is not only conducive to maintaining forest plant species diversity and optimizing community structure, but also plays an irreplaceable role in promoting the health of the soil–vegetation system and accelerating community succession [
7], which is an important aspect of plantation ecosystem research. Species diversity can reflect the competition and coordination mechanism of each species in the community in terms of the resource space and living environment, and plays an important role in measuring the stability of the forest community structure and ecosystem function [
8].
Pinus massoniana, as a suitable tree species and main cultivated tree species of water retention forest in most areas, has also been deeply studied by many scholars [
9]. Cheng Changjin et al. [
10] studied the interception effect of
Pinus massoniana and other vegetation on nitrogen and phosphorus by simulating the surface runoff. Ding Xia et al. [
11] analyzed the soil, litter and forest canopy from vertical level to discuss the water retention service function of
Pinus massoniana forest. Among them, the maximum water holding capacity, effective water holding capacity, and soil effective water holding capacity of the litter layer were the highest in the medium density stand. Therefore, the results showed that the water retention capacity of the forest under medium density was the best. At present, most of the research on
Pinus massoniana plantations have focused on the impact on carbon storage, soil quality, and nutrient cycling of the plantation [
12,
13,
14], but there are relatively few studies on the community structure characteristics and diversity of the understory vegetation.
Danjiangkou Reservoir is located at the junction of Hubei, Henan, and Shaanxi Provinces. It is located in the middle and upper reaches of Hanjiang River, the largest tributary of the Yangtze River, connecting the Hanjiang River and its tributary Danjiang River. As the water source of the Middle Route Project of South-to-North Water Transfer, it is also an important local wetland and water source protection area. Since its official storage, the Danjiangkou Reservoir has continuously delivered high-quality, abundant drinking water to Northern China, benefiting 53 million residents in the provinces (cities) of Beijing, Tianjin, and Hebei, and effectively solving the water shortage problem of people along the route (
https://www.ndrc.gov.cn/ accessed on 17 September 2022). Its ecological construction has largely determined the success or failure of the South–North Water Transfer Project [
15]. Some scholars have studied the water and soil conservation capacity and water retention function of vegetation types such as conifer and cypress mixed forest [
16] and oak and broadleaf mixed forest [
17] in the Danjiangkou reservoir area. The representative plantations are
Cunninghamia lanceolata,
Larix gmelinii, and
Pinus massoniana.
Pinus massoniana is a typical coniferous tree species in the subtropical region of China. It has the characteristics of a large area, wide distribution, strong adaptability to climate and soil, rapid growth, and is easy to grow into forests. It is one of the afforestation species widely distributed in the Danjiangkou reservoir area. Taking the
Pinus massoniana plantation in the Danjiangkou reservoir area as an example, this paper aimed to: (1) Discuss the impact of stand density change on the community structure and species diversity of
Pinus massoniana plantation; (2) analyze the stand density most suitable for the local community environment; (3) investigate the trade-offs and synergies between the two ecosystem service functions of water content function and species diversity in the study area, with a view to providing theoretical references for the management and improvement of the comprehensive benefits of plantation forests in the reservoir area to further optimize the forest community structure, protect species diversity, and promote ecosystem health.
4. Discussion
In this study, with the increase in the stand density, the species diversity decreased gradually, and the species richness and Shannon–Wiener diversity index of the shrub layer also decreased with the increase in the stand density. The diversity of shrub vegetation was also different among the three densities, indicating that the understory vegetation composition is sensitive to the change in the canopy density and microenvironment, which is consistent with the research results of many scholars [
34]. The research results of Lu et al. [
35] showed that the stand density will limit the species and quantity of undergrowth plants by changing the soil, temperature, and light intensity in the forest and the distribution of soil nutrient elements in the forest. With the increase in stand density, the amount of light penetrating through the forest decreases, which affects the decomposition of litter [
35] and there is insufficient light in the forest, which directly leads to the decrease in soil fertility, thus resulting in the decrease in the species diversity of the understory plants. Unlike herb plants, the species richness of shrubs increased slightly with the increase in stand density, indicating that the survival ability of shrubs in high-density stands is obviously better than that of dwarf grasses, indicating that with the increase in the stand density, the canopy density increased, and the competition for resource space of the tree layer species intensified, which inhibited the survival of the understory vegetation species. In the long succession process, some species without competitive advantage are gradually eliminated.
The calculated results of the ecosystem service relationship between the water retention capacity and understory plant species diversity of the Pinus massoniana plantation in the Danjiangkou reservoir area showed that there is a trade-off relationship between the two services, and the higher the stand density, the greater the trade-off. As far as the study area is concerned, the larger the stand density, the higher the canopy density, and the larger the canopy interception. With the increase in the stand density, the living environment under the canopy deteriorates, and the species number of understory plants decreases due to the lack of sufficient water and light. Therefore, water retention and biodiversity services present a trade-off relationship.
In contrast, China’s plantation structure is a single tree-layer, plant species diversity is low, ecological stability and stress resistance are poor, and productivity and pollution resistance are low. In forestry management, intensive afforestation technical measures are not fully implemented, and soil erosion and soil fertility degradation are common [
36]. Research on trade-offs and synergies among ecosystem services is an important means to clarify the relationships among various services, and is also the premise for managers to put forward targeted strategies. Plant species diversity is the basis of maintaining ecosystem health and stability, and indirectly reflects the quality and sustainable management level of the plantation. The species diversity of the understory vegetation plays an important role in improving forest productivity, maintaining forest community stability, and exerting ecosystem functions [
37]. Appropriate stand density is beneficial to improving the stand structure, maintaining higher species diversity of each community, and promoting the natural regeneration of stands. Therefore, it is very necessary to control reasonable stand density in the process of watershed ecological environment management. Therefore, it is suggested that the medium-density forest should be taken as a reference, the over-dense forest should be tended and selectively cut, and the over-sparse forest should be replanted with fast-growing, shade-tolerant and stress-resistant broad-leaved species. In the investigation of the study area, it was found that there are many natural regeneration broadleaf seedlings under the forest, which can be considered to protect them, increase the mixing ratio, and improve soil fertility and ecological function.
5. Conclusions
In this study, a total of 69 species of shrubs and herbs were investigated and counted including 32 species of shrubs and 37 species of herbs, belonging to 33 families and 62 genera. The data in
Table 3 show that the species richness of the shrub layer increased with increasing stand density, while the opposite was true for the herbaceous layer. The Shannon–Wiener diversity index and Simpson dominance index of the shrub layer showed a pattern of high density > low density > medium density, while the herbaceous layer gradually decreased with increasing density. The Pielou evenness index was the highest in the high-density and medium-density stands for the shrub and herbaceous layers, respectively. The trunk stem flow and soil layer interception are synergistic with biodiversity at medium–high densities. Except for trade-offs, litter layer interception with biodiversity at low–medium densities in the shrub and herb layers also showed a synergistic relationship.
Sustainable development particularly emphasizes the importance of forests in social and economic development in China [
38]. In order to better maintain the plant species diversity of
Pinus massoniana plantations, give full play to the ecological benefits of the plantation, and promote the healthy development of habitat quality in the reservoir area, comprehensive analysis of the plant species diversity of understory vegetation and its water retention capacity was carried out. Among the ecosystem services of
Pinus massoniana plantations in the Danjiangkou reservoir area, there was a trade-off relationship between the water retention capacity and community plant species diversity. Comprehensive consideration of the relationship between multiple ecosystem services, combined with site conditions, focusing on selecting key ecosystem services for governance and improvement, is a necessary condition to promote the value of ecosystem services in the basin.