# _forests_
## 1 Introduction  
As one of the most widely distributed vegetation types on the global land surface [1], forests not only contribute most of the world's gross primary product (GPP), but also store more carbon (C) in the biomass and soil of the forest than in the atmosphere [2]. Thus, forest ecosystems play an important role in the global C cycle, providing feedback into theclimate system [3]. Forests are one of the most important net C sinks in the atmosphere [4]; however, whether this C sink will continue in terms of climate change remains largely uncertain. Therefore, studying forest C dynamics is of great significance for understanding the role of forests in global C cycling. Currently, most of the studies are focused on the dynamics and decomposition of soil organic carbon (SOC) in topsoil within the range of 0-0.2 m [5; 6]. However, subsoil (below 0.2 m) has a better C sequestration capacity than topsoil [7]. The dynamics of SOC in the subsoil may be as sensitive as topsoil to environmental changes [8], but this has not currently been proved [9]. SOC turnover time (\(\tau\), years) is not only an indicator of soil C stability, but it also determines the C sequestration capacity of soil [10]. SOC turnover time is the average time from the initial photosynthetic fixation to the respiratory or non-respiratory loss of C atoms in terrestrial ecosystems, which can be calculated from C stocks relative to C inflow or outflow [11]. However, \(\tau\) has an important steady-state assumption; that is, under the steady-state assumption, C output is equal to input [12]. In fact, few soils maintain a stable state because of the effects of natural and anthropogenic factors and climate change [8]. Compared with topsoil, the soil organic C in the subsoil is more stable and less susceptible to disturbance [13], indicating that subsoil can be considered to be in a stable state [8]. Previous studies have already explored the C turnover time of soil or vegetation at regional or global scales [14; 15]. A recent study showed that, using the modeling data at global scales [8], soil property was the main factor driving subsoil \(\tau\) (0.3 to 1 m). However, the result was estimated based on the Harmonized World Soil Database (HWSD) and remote sensing data, and this study did not determine which environmental variable was more important for subsoil \(\tau\) in different forest types and climate zones. Therefore, fundamental problems related to subsoil \(\tau\) remain: firstly, questions regarding whether conclusions from field observations support those from global model data arise because the mismatch between the observed data and global grid data is a well-known problem in ecological studies due to differences in spatial resolution [15]. Secondly, there is a question concerning whether the subsoil \(\tau\) of different forest types or climate zones is governed by the same environmental variable. Thirdly, because vegetation is one of the sources of soil C, the determination of how vegetation variables (such as NDVI, GPP, EVI) influence subsoil \(\tau\) is a major concern. Therefore, it is important to evaluate subsoil \(\tau\) and identify the main driving factors by using field observations for a reliable evaluation of subsoil C cycle-climate feedback. Although temperature and precipitation are the most important climatic factors affecting soil \(\tau\)[16], soil and vegetation variables may also influence \(\tau\). For example, the leaf area index (LAI) is the main vegetation factor affecting C turnover [17; 18], and soil bulk density (BD), soil silt, and clay indirectly affect subsoil \(\tau\) by affecting fine roots and C storage, respectively [19; 20]. In addition to climatic and soil factors, biological traits also affect subsoil \(\tau\) (such as vegetation longevity and types, as well as biomass) [12]. Different vegetation types may be accompanied by different climatic conditions, which greatly affect vegetation productivity, impacting \(\tau\)[20].