## 1 Introduction  
Terrestrial ecosystems have acted as a substantial sink for atmospheric carbon dioxide, and sequestered about 15%-30% of fossil fuel emissions every year [1]. The gross ecosystem productivity (GPP) is the net amount of carbon dioxide fixed by land plants' photosynthesis [2]. Previous studies showed a significant increase in the magnitude of GPP over the past decades [3, 4]. The CO\({}_{2}\) fertilization effect [5, 6], nitrogen deposition [7], climate warming [8], and land-use change (e.g., aforestation, grassland restoration) [9, 10] have been proven to have positive effects on the GPP increase. Specifically, [PERSON] et al. (2015) pointed out that the rising atmospheric CO\({}_{2}\) concentration explains 85% of the average growing season vegetation growth trend in China from 1982 to 2009 [11]. [PERSON] al. (2011) showed that nitrogen deposition and fertilizer applications account for 61% of the net carbon storage in recent decades in China's land ecosystems [12]. Moreover, [PERSON] et al. (2018) found that the annual aboveground biomass carbon showed significantly positive trends post-2000 in Southwest China, due to the implementation of ecological engineering [13]. [PERSON] et al. (2021) indicated that the appropriate climatic conditions, and the implementation of ecological engineering, caused an accelerated increase in vegetation carbon sequestration in China after 2010 [14]. It should be noted that climate change can affect the implementation benefits of ecological restoration projects [15]. Therefore, it is crucial to study the effect of climate factors on the GPP, which can better reveal the GPP's spatial-temporal changes. Forests, covering approximately one-third of the global land area, are the dominant terrestrial ecosystem type on the global scale, and play an essential role in the carbon and water cycles [16]. Deciduous forest ecosystems usually experience a pronounced seasonality compared to evergreen forest ecosystems. A recent study showed that the temporal variability of the global land carbon sink is mainly determined by non-evergreen ecosystems [17]. Global changes are composed of a series of regional changes, with different processes and phenomena [18]. Hence, understanding the long-term trends in the GPP in deciduous forest ecosystems at a regional scale was of great importance, to evaluate the contribution of vegetation to climate change mitigation. The mean GPP was 1453 gC/m\({}^{2}\)/yr in CBS during 2003-2005 in China's deciduous forest ecosystems [19]. Harvard Forest's mean GPP (1690 gC/m\({}^{2}\)/yr) was similar to the GPP value in China's deciduous forest ecosystems [20]. Additionally, the annual GPP increased at a trend of 3.56 gC/m\({}^{2}\)/yr from 1997 to 2014 in 12 deciduous forest sites in Europe, Asia, and North America [21]. In addition, based on whether they are highly managed by humans, the forests could be further divided into natural and planted forests [22]. Planted forests differ from natural forests in many aspects, such as species richness, plant density, and stand age [23]. Planted forests respond quickly to climate change [24; 25]. Thus, trends in the GPP in natural and planted forests might be different. A previous study pointed out that the mean GPP value in natural forests is slightly lower than that in planted forests, due to their high sensitivity to rising temperatures [26]. However, a comprehensive analysis of the natural and planted forests' GPP trend is still lacking, especially in deciduous forest ecosystems. Changes in the annual GPP can be associated with the growing season length (GSL) and the maximum value of gross primary productivity (GPPmax) (Figure 1) [27; 28]. The GSL can be used as an indicator of vegetation phenology, and the GPPmax represents the characteristics of photosynthesis, which can be used as an indicator of physiology. The temporal variations in the GSL and GPPmax due to climate change could affect trends in the GPP [27; 28].