Keywords: dryland; revegetation; carbon sequestration; carbon uptake period

Terrestrial biosphere serves as a carbon sink which can sequestrate a large portion of emitted CO₂ due to human activities (Pan et al., 2011). The net global carbon uptake by terrestrial biosphere increased significantly over the past decades and seems to increase in the near future (Ballantyne et al., 2012; Cheng et al., 2017; Keenan et al., 2016). It is worthwhile to highlight that the afforestation and ecological restoration plays a significant role in sequestrating the carbon dioxide in China and India due to vegetation restoration (Lu et al., 2018; Piao et al., 2009; Yang et al., 2022). Studies on the impact of revegetation on carbon sinks have been reported mainly in wet or semi-humid areas, and very few studies are focusing on arid areas (Yang et al., 2014; Liu et al., 2022).

The role of vegetation restoration in dryland, in terms of carbon sequestration, is largely un-known in long term (Liu et al., 2022). Although dryland afforestation can prevent desertification and increase carbon sequestration in short period in drylands (Wang et al., 2020; Yosef et al., 2018), the sustainability of vegetation restoration is largely uncertain due to a number of factors. Shrub planting in arid and semi-arid areas regions can increase the carbon sequestration capacity of these areas (Chen et al., 2018; Wang et al., 2020) and more carbon is stored in the soil (Yang et al., 2014). The above study is based on ground-based surveys (vegetation or soil sample scales) on short period and do not reflect the long-term climate change impacts on the carbon cycle. However, the carbon sequestration in long run is not sure due to the following reasons. First, the planted vegetation could lead to over-consumption of water resources (Jackson et al., 2005), especially in arid and semi-arid areas. This would lead to some negative ecological consequences such as drying of the soil, decline in groundwater (Cao S X, 2008) loss of runoff, etc. (Wilske et al., 2009). Numerous studies predicted that the large area of woody sand-binding vegetation will degrade or become extinct due to soil water depletion and underground water decreasing in sandy areas of northern China (Cao, 2008). Second, dryland is the most vulnerable regions to the effects of climate change (Huang et al., 2017). There is an increasing trend of drought in dryland area as dryland has much faster warming trend than the global average (Lian et al., 2021). This poses a challenge to the sustainability of vegetation restoration in dryland area and also to the carbon sequestration. Therefore, the stability and sustainability of the carbon sink effect of dryland vegetation restoration does not reach consensus (Liu et al., 2022).

The marginal ecosystems, semiarid savannas and shrublands, play an essential role in regulating the variability of the land CO₂ sink (Ahlstrom et al., 2015; Poulter et al., 2014; Sha et al., 2022). The variability of carbon sequestration in arid area is mainly due to the increasing variability of precipitation in the warming world (Ahlstrom et al., 2015; Poulter et al., 2014). Meanwhile, the change of growing season or carbon uptake period due to climate change also has significant influence on the carbon sequestration (Wu et al., 2013). The impact of climate warming on plant phenology was extensively investigated, especially focusing on the length of the growing season (GSL) (Piao et al., 2007; Piao et al., 2020) or the prolongation of the carbon uptake period (CUP) (Churkina et al., 2005). Overall, the relationship between annual carbon uptake and CUP is more strongly correlated than GSL (White and Nemani, 2003; Wu et al., 2012). Warmer temperatures in spring and autumn extend the period of vegetation activity, which leads to the increasing net carbon uptake period, and usually stimulates NEP (Fu et al., 2017). Longer periods of carbon uptake increase NEP in forest ecosystems, but may have no effect on non-forest ecosystems (Wu et al., 2012). Most of the above studies on the effects of climate change on NEP due to warming have been conducted in forests or grasslands, with very few studies in desert ecosystems.

As dryland accounts for 40% of land area and vegetation in dryland area could play a significant role to sequestrate carbon dioxide of atmosphere according to previous studies. Meanwhile, vegetation restoration in dryland area is a popular strategy for land degradation prevention. Hence, this study aims to investigate the carbon sequestration potential of planted vegetation in dryland and also the impact of climate change on the carbon sequestration of these planted vegetation in arid region, by taking the advantage of long-term eddy flux measurements over the stable planted vegetation in arid area.