Introduction
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.
Materials and methods