Abstract:
Dryland area accounts for approximately 40% of worldwide land area, which plays a significant role in regulating the carbon sequestration capacity of land. Vegetation restoration in drylands adopted to prevent land degradation, and may also serve as a carbon sink in the earlier stage. However, the persistence of the carbon sink for the revegetated ecosystem in drylands is still unknown. Can the well-established restoration vegetation in dryland areas serve as a carbon sink in long-run? To address this question, we investigated the carbon sequestration capacity of planted vegetation in dryland areas with 13 years of observation (2009–2021) for established vegetation restoration, which began in 1989. We found that the revegetation area serves as a carbon sink in all years. The mean annual net ecosystem productivity (NEP) is 91.61 ± 36.17 gC m−2 yr−1 (mean ± standard deviation). Annual NEP showed a significant increasing trend over the study period with a rate of 5.65 gC m−2 yr−1 yr−1 (p<0.05). The increase in spring temperature, the earlier start of net carbon uptake, and the longer duration of net carbon uptake contribute to the gradual trend of NEP. The amount of annual NEP is predominantly determined by summer precipitation. Meanwhile, our results revealed that the increase in net carbon uptake by revegetation did not lead to excessive consumption of water resources. Our results have suggested that appropriate vegetation restoration in arid areas can increase ecosystem carbon sequestration over longer timescales and mitigate climate change, with relatively low environmental consequences and risks. Considering the vast area of degraded land in the global drylands, the carbon sequestration effect of this model should be given more attention.
Keywords: dryland; revegetation; carbon sequestration capacity; net carbon uptake period; Tengger Desert
Introduction
The terrestrial biosphere serves as a carbon sink that can sequestrate a large portion of emitted CO2 due to human activities (Pan et al., 2011). The net global carbon uptake by the terrestrial biosphere has increased significantly over the past decades and will likely increase future shortly (Ballantyne et al., 2012; Keenan et al., 2016; Cheng et al., 2017). It is worth highlighting that afforestation and ecological restoration play a significant role in sequestrating carbon dioxide in China and India (Piao et al., 2009; Lu et al., 2018; 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 have focused on arid areas (Yang et al., 2014; Liu et al., 2022).
The sustainability of vegetation restoration is largely uncertain due to several factors in dryland areas (Huang et al., 2016) while dryland afforestation can prevent desertification and increase carbon sequestration in a short period (Yosef et al., 2018; Wang et al., 2020). Shrub planting in drylands can increase the carbon sequestration capacity of these areas (Wocheslander et al., 2016; Chen et al., 2018; Wang et al., 2020) as more carbon is stored in the soil (Yang et al., 2014), including increasing organic carbon (Huang et al., 2012; Li et al., 2022) and inorganic carbon (Jia et al., 2019). These studies were based on ground-based surveys (e.g., vegetation or soil samples) over a short period but did not reflect the effects of long-term climate change on the carbon balance. As the interannual variability of climate is the main driver of carbon flux variability, long-term observation of the former is essential to assess its influence on the latter (Baldocchi, Chu, & Reichstein, 2018). Neither short-term nor manipulated experiments can represent the impact of the interannual variability of climate on ecosystems (Piao et al., 2019).
The role of vegetation restoration in dryland, in terms of carbon sequestration, is largely unknown in the long-term (Liu et al., 2022). This uncertainty is mainly due to the following reasons. First, planted vegetation could lead to the overconsumption of water resources (Jackson et al., 2005; Zhang et al., 2018), especially in arid and semi-arid areas. This would lead to some negative ecological consequences, such as soil drying (Huang & Shao., 2019), groundwater decline (Cao, 2008; Wilske et al., 2009), and runoff loss (Cao et al., 2011). In the sandy areas of northern China, a large area of woody sand-fixing vegetation is expected to degrade or become extinct owing to the depletion of soil moisture and the reduction of groundwater (Cao, 2008; Wang et al., 2010). Second, there is an increasing trend of drought in dryland areas because drylands have a much faster warming trend than the global average (Lian et al., 2021). Drylands are the region most vulnerable to the effects of climate change (Huang et al., 2017). This poses a challenge to the sustainability of vegetation restoration in dryland area (Lü et al., 2021) and carbon sequestration. Some studies have reported that the potential expansion of drylands may reduce carbon sequestration (Huang et al., 2016). Sand-fixing plantations (e.g. H. ammodendron) were found to be potential carbon sources due to climate change and the effects of decay and mortality of the planted forests according to the Carbon Benefits Project-Modelling Tools (Ma et al., 2021). However, other studies have found little change in the global dryland area in a warming context (Berg et al., 2021). There has been a more productive trend in the warmer and CO2-enriched drylands since the 1980s based on satellite and modelling results (Lian et al., 2021). Therefore, the carbon sequestration capacity of dryland vegetation restoration in the long run does not reach a consensus (Liu et al., 2022).
Marginal ecosystems, semiarid savannas, and shrublands play an essential role in explaining the variability of terrestrial carbon dioxide sinks (Poulter et al., 2014; Ahlstrom et al., 2015; Sha et al., 2022). The variability of carbon sequestration in arid areas is mainly due to the increasing variability of precipitation in the warming world (Poulter et al., 2014). Meanwhile, the change in the growing season or net carbon uptake period due to climate change also has a significant influence on carbon sequestration (Wu et al., 2012). The impact of climate warming on plant phenology has been 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 net carbon uptake period (CUP) (Churkina et al., 2005). The correlation between annual net carbon uptake and CUP is stronger than that between annual net carbon uptake and of GSL (Wu et al., 2012). The period of vegetation activity will be prolonged with the increase in spring and autumn temperatures (Wolf et al., 2016), which leads to an increasing net carbon uptake period (Fu et al., 2017); this usually stimulates net carbon uptake (White & Nemani, 2003). Longer periods of net carbon uptake increase net carbon uptake in forest ecosystems, but may not affect non-forest ecosystems (Wu et al., 2012). Most of these studies have focused on forests or grasslands, but few studies have focused on desert ecosystems.
Dryland accounts for 40% of worldwide land area and vegetation in dryland areas could play a significant role in sequestrating atmospheric carbon dioxide according to previous studies (Nosetto, Jobbágy, & Paruelo, 2006; Nolan et al., 2019; Sha et al., 2022). Vegetation restoration in dryland areas is a popular strategy for land degradation mitigation and prevention. The net primary productivity (NPP) of sparse vegetation for the planted vegetation in Xinjiang (2001–20018) showed a gradual increase under global warming, and annual NPP changes were regulated by both temperature and rainfall (Yan, 2020). However, field warming experiments found that warming reduced photosynthetic rates (Zhang et al., 2017) and also the release of greenhouse gases such as CO2 and methane from soils (Guan et al., 2021; Hu et al., 2022) for the planted vegetation in arid region. Climate change in arid zones is characterised by an increase in temperature (Lian et al., 2021), and an increase in the aridity index (Huang et al., 2016). The impact of these projected climate changes on carbon flux in arid areas is uncertain. Hence, this study aims to investigate the carbon sequestration potential of planted vegetation in drylands and the impact of climate change on the carbon sequestration of planted vegetation in arid regions, by taking advantage of long-term eddy flux measurements over stable planted vegetation in arid area.