Climate change may accelerate the decline of desert riparian forest in the lower Tarim River, Northwestern China: Evidence from tree-rings of Populus euphratica

Highlights

• Sensitivity of P. euphratica to climate depended on the groundwater environment.

• Groundwater depth was the direct driver for P. euphratica growth.

• However, warming might accelerate the decline of the desert riparian forest.

Abstract

Although climate change has emerged as a major threat to biodiversity, few assessments exist of the sensitivity of desert riparian forests to climate in extremely arid plain areas. We mapped tree growth-climate correlations (time period: 1957–2015) to identify the sensitivities of Populus euphratica (P. euphratica), a dominant and important tree species in the desert riparian forest in the lower Tarim River, an extremely arid area in China, to climate change. The results indicated that groundwater depth was the key factor for controlling tree growth in the study area. In order to accurately assess sensitivity to climate change of the desert riparian forest, it was necessary to consider the effects of hydrological fluctuations on P. euphratica because the sensitivities of tree growth to precipitation and temperature depended on changes in the groundwater environment. The groundwater depth threshold to distinguish the sensitivity of climate change to P. euphratica growth was 6–7 m. Moreover, warming would accelerate desert riparian forest decline when the groundwater depth was more than 6 m because an anomalously high temperature would intensify evapotranspiration and the shortage of soil moisture during the growing season, and offset the benefits from precipitation, which would cause more drought if groundwater was not compensated by sufficient runoff. Therefore, global warming would bring great disadvantages to desert riparian forests if the current mode of the ecological water conveyance project (EWCP) continues to be implemented without a fixed time and water amount in the lower Tarim River, which could not steadily decrease and actively maintain shallow groundwater depth.

Introduction

Climate change has emerged as a major threat to biodiversity globally (McCullough et al., 2017) because it results in species change, extinction, turnover, and alterations in communities’ structure and population through a profound impact on habitat (Bakker, 1983, Bakker and Coetzee, 1988). Moreover, as of 2010, climate change has caused approximately 20% of all plant species to be in danger of extinction (Hamann and Wang, 2006, Brummitt and Bachman, 2010, Burbano, 2013, Yi et al., 2017). In view of the importance of climate to biology, rapid warming will dramatically alter ecosystems (Elmendorf et al., 2012, Gałka et al., 2018, Rogora et al., 2018). Some investigations suggest that plants can benefit from global warming because increasing concentrations of CO₂ and nitrogen deposition should augment photosynthesis rates and plant growth (Matyssek et al., 2006). However, others found that climate change will impede plant growth and development (Thomas et al., 2004, Lindner et al., 2010, Engler et al., 2011, Dullinger et al., 2012, Hällfors et al., 2016). These researches indicated that the responses of various species to climate change differed, which was further proven by Zhu et al. (2017), who reported that plants’ phenology on the Qinghai-Tibetan Plateau (QTP) exhibited divergent shifting trends due to the different responses to climate warming during the period from 1981 to 2011. Extant literature also showed that geographic variation existed in climate sensitivity of the same tree species’ growth, and the unevenness of these relationships suggested a pivotal role of local adaptation (McCullough et al., 2017).

The Tarim River Basin, located in the mid-latitudes of Eurasia, is an extreme arid region of China (Chen et al., 2012). The regional topography features a mountain–basin complex comprising mountains and a mosaic of inner-mountain basins, which makes it highly susceptible to climate change (Yang et al., 2012). Investigations determined that the air temperature in this region had been increasing by a rate of 0.33–0.39 °C/decade during 1960–2010 (Shi and Zhang, 1995, Zhang et al., 2010), which was considerably higher than the overall rate of China (0.25 °C/decade) (Ren et al., 2005) and that globally (0.13 °C/decade) (IPCC, 2007) for the same period. In addition, temperature change in winter played the most critical and direct role in this regional deviation (Li et al., 2012). The lower Tarim River is also one of the world’s most ecologically vulnerable areas (Ling et al., 2017), because it experienced markedly different periods of hydrological changes, including the period of perennial river (1957–1972), the period of complete drying-up (1973–1999), and the period of intermittent resumption of surface runoff (2000 – present). Previous researches in this area primarily focused on temperature and precipitation changes under global change and their impacts on water resources (Yang, 2003, Yuan et al., 2004, Yuan et al., 2005, Xu et al., 2004, Xu et al., 2010, Jiang et al., 2009, Yang et al., 2012, Chen et al., 2015), or focused on the effects of environmental water on physiological and biochemical indexes of tree growth (Chen et al., 2006a, Chen et al., 2006b, Zhou et al., 2010, Zhou et al., 2013, Ma et al., 2013, Fu et al., 2014, Ayup et al., 2015, Si et al., 2015, Pan et al., 2016, Yan et al., 2016, Chen et al., 2017, Zhang et al., 2018). Research about the influence of regional climate factors on plant growth is scarce (Xu and Wang, 2016).

The desert riparian forest is a main natural forest in the lower Tarim River, and Populus euphratica (P. euphratica), a rare, ancient, and endangered species (Meher-Homji, 1973, Zhou et al., 2010), is a unique, important and constructive species of the desert riparian forest. P. euphratica in the Tarim River Basin accounts for 54% of the world’s area of natural forest of P. euphratica and 89% of Chinese natural forest of P. euphratica (Wang, 1996, Hukin et al., 2005). It is commonly concluded that P. euphratica in the lower Tarim River is affected by water resources (Chen et al., 2006a, Chen et al., 2016, Chen et al., 2017, Zhang et al., 2017, Zhang et al., 2018). To date, the relationship between physiological and ecological changes of P. euphratica and groundwater or surface water in the lower Tarim River has been determined (Yan et al., 2016, Chen et al., 2017, Zhang et al., 2018), but little knowledge exists concerning the effect of local climate change on long-term growth of P. euphratica. Abrupt changes in the water environment of the lower Tarim River, i.e., the drying up of river flow and the Ecological Water Conveyance Project (EWCP) for lower reaches of the river, made it is possible to examine relationships between tree growth and climate change. Tree-rings offer a precise means to assess climate sensitivity in tree species on long temporal scales (McCullough et al., 2017, Rahman et al., 2017), since they can provide an annually resolved, continuous, dated to the precise year, and long-period natural archives of habitat environment or climatic change (Zhang, 2015). Therefore, in the present study, we reported the relationships between the tree-ring chronology of P. euphratica and main regional climate variables. Our two objectives were as follows: 1) to test whether local climatic change affected tree growth in the desert riparian forest in the lower Tarim River; and 2) to discuss the vulnerability and sensitivity of the desert riparian forest to future scenarios of climate warming.