1. Introduction
Over the last several decades, China has been carrying out two ecological restoration programs: the Natural Forest Protection Project (NFP) and the Grain for Green Programme (GGP). These two projects have increased the area of forest (Zhang et al., 2020), restored degraded environments (Ren et al., 2017), and enhanced carbon stocks in natural and planted forests (Pawson et al., 2013). Furthermore, the Chinese government has implemented stricter protection measures, which included the prohibition of commercial logging in natural forests, especially in ecological sensitive areas. The competition between different uses of land such as agricultural production, timber production, and ecological services has increased. Optimizing the sustainability of the supply of competing services from forests requires consideration of potential trade-offs between them. The goal of planted forest restoration in China has been shifted from increasing productivity and coverage to developing the ecological function of approximate natural forest with diversity species, high water and soil conservation function, natural succession and sustainable carbon sequestration function. This goal is consistent with the target of “close-to-nature-management” (Wang et al., 2018), which will not only develop mixed and uneven-aged, structurally diverse forests, restore forest productivity but will also focus on the stand stability and reliance of forest ecosystems. The design of plantations that complement the services provided by natural forest is particularly important in the face of future environment change, as plantations are often monocultures with alien species (Verheyen et al., 2016). Generally, the quality of plantations needs to be improved while balancing productivity and other ecological services.
The definition of biomass temporal stability is the ratio of mean value to the standard deviation of biomass across time, which is an important indicator of ecological stability (Hautier et al., 2015; Morin et al., 2014). The insurance hypothesis, which postulates that biodiversity improves ecosystem stability, has been widely studied (García-Palacios et al., 2018). To improve the quality of planted forest, we need to know what are relationship among biodiversity, productivity and ecosystem stability (Domínguez-García et al., 2019). Because planted forests are thought to have lower diversity, productivity, and ecological stability compared with natural forests during the long-term recovery process (Ke et al., 2020; Zhang et al., 2020). Previous experiments have shown that more tree species can increase productivity and provide more ecosystem services than monoculture (Aussenac et al., 2019). However, whether such an effect applies to natural ecosystems remains unclear. Some studies have found a positive concave relationship between biodiversity and productivity change within a sampling unit and spatial scale in a global data set (Liang et al., 2016; Luo et al., 2019). Forest biomass increased over time in forest with high tree diversity, while which decreased with time in poor species forest in Canada (Hisano et al., 2019). In Europe, the stability of biomass was more stability in mixed forests (Jucker et al., 2014). While, one to two stability indicators at a time and response to a single disturbance were usually quantified in the existing studies (Domínguez-García et al., 2019).
In natural ecosystems, the effects of environmental context (Ratcliffe et al., 2017; Mori et al., 2018), climate change (García-Palacios et al., 2018), stand structure (Guo et al., 2021; Ouyang et al., 2019) and diversity have effect on forest productivity and ecosystem stability, the effects also may vary in time and space. Furthermore, these factors interact and become more complex. It’s difficult to weigh these effects against each other in natural ecosystems. (Isbell et al., 2017; Isbell et al., 2019). While, Hautier et al. (2015) think the ecosystem stability is mainly altered by factors that affect biodiversity in the global drought gradient. For example, climate change may transform the certain relationship between biodiversity and ecosystem stability (García-Palacios et al., 2018). Therefore, Yao et al. (2018) thought that mean annual temperature was the main effect factor for forest productivity change. While, Ouyang et al. (2019) suggested that forest structure, for example stand age and density manipulated forest productivity in subtropical in China. Elevation and soil fertility only had significant effects on biomass in forests less than 100 years old, and which were mainly affected by biological interaction in older forest (Satdichanh et al., 2019). The change rate of biomass would decrease with stand age (Chen and Luo, 2015). The NFP and the GGP have been in operation for nearly 20 years. Most forest plantations are young and were only established between 1999 and 2014 on sloped terrain. Most studies have focused on the productive and ecological benefits of increased forest cover (Brown et al., 2020; Cook-Patton et al., 2020; Deng et al., 2017; Tong et al., 2020). Temporal and spatial variation in productivity and ecological stability between planted and natural forests have received less attention by comparison. In addition, how environmental context, climate change and forest structure factors affect forest productivity and ecological stability in a spatially heterogeneous mountain ecosystem remains unclear, and this knowledge gap increases the difficulty of restoring and reconstructing planted forests in mountain ecosystems.
Here, we studied spatio-temporal productivity and ecological stability patterns, identified the main factors that explained changes in the productivity and ecological stability of planted and natural forests, and explored the coupling effects between the most important factors. It is relevant and timely to boost sustainable forest management policies according to environmental conditions and forest structures, especially in planted forest. Generally, this study provides a new insight that will aid the improvement of quality, functionality and productivity planted forest, and help balance productivity and ecological stability in the future.
Materials and Methods