# Exploring the Spatiotemporal Alterations in China's GPP Based on the DTEC Model
## 1 Introduction  
Global carbon sequestration is an essential step in the terrestrial carbon cycle [1]. For determining the state and dynamics of the terrestrial carbon cycle, Gross Primary Productivity (GPP) is an essential indicator that describes the overall amount of atmospheric carbon dioxide that plants absorb via photosynthesis [2]. Therefore, accurate modelling of GPP on a zonal or worldwide scale will contribute to understanding the stability of terrestrial ecosystems and assist in decision making in response to more climatic changes [3]. Currently, two main methods are used to estimate GPP: ground-based observations and model simulations [4]. Ground-based observational methods generally tend to use eddy correlation techniques [5]. However, as the number of flux sites is few and sparsely distributed, ground-based observations are commonly used for validation analyses of GPP models [6]. Ecosystem modelling, which primarily consists of process-based models and isbased on vegetation photosynthesis light-use efficiency (LUE) models, is a useful technique for predicting GPP on a zonal or global scale. Due to its simpler structure and fewer parameter inputs, the LUE model is widely applied on a larger scale than process-based models that require many parameters and data inputs. However, there are large differences in the variety of LUE models. For example, the MODIS GPP product was based on the big-leaf LUE model, but the fixed parameters of the model led to an underestimation of simulated GPP values, especially in farmland ecosystems, with an underestimation of approximately 30% [7]. Additionally, GPP is underestimated by the LUE model because its algorithms do not distinguish between the maximum LUE of C3 and C4 vegetation [8]. To compensate for this deficiency, [PERSON] [9] proposed a terrestrial ecosystem carbon flux model (TEC) to differentiate the LUS of C3 and C4 vegetation. C3 and C4 plants are two fundamental plant functional types (PFTs) with different responses during photosynthesis. Nonetheless, the model encountered limitations, as the LUE of scattered radiation from vegetation canopies was greater than that of direct radiation. He [10] proposed the two-leaf light-use efficiency model (TL-LUE), which distinguishes between shaded and sunlit leaves. However, the two-leaf model does not consider the influence of soil moisture stress and the diffuse fraction on vegetative photosynthesis. To address this limitation, [PERSON] [11] proposed a diffuse fraction-based two-leaf light-use efficiency model (DTEC) based on previous models. The novel model adds a diffuse fraction to recalculate the solar radiation received and the LUE by the sunny and shady leaves, allowing the model to better capture seasonal variations in the GPP [12]. The effects of climate change on the dynamics of GPP have been a hot topic in global change research [13]. Surplus solar radiation may cause an increase in soil moisture, leading to anaerobic conditions in the soil, thus reducing vegetation productivity on the Tibetan Plateau [14]. Furthermore, aerosol particles have a scattering effect on solar radiation, increasing the proportion of scattered radiation to total solar radiation, which in turn affects vegetation productivity by influencing photosynthesis [15]. In particular, climate extremes can alter the ecosystem's structure and capacity to function; this phenomenon may negatively impact the terrestrial carbon cycle [16]. Another aspect is human activity, notably grazing [17], the use of ecological restoration initiatives [18], land management strategies [19], and urbanization [20]. China has become the most important and sensitive region in global research on carbon and water cycles in terrestrial ecosystems [21]. Moreover, China, the world's largest carbon emitter, has pledged to achieve carbon neutrality by 2060 [22]. To achieve this goal, an in-depth understanding of the status of the terrestrial carbon cycle in China is needed.