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