Humans have a significant influence on future climate change, primarily reflected in the generation of anthropogenic greenhouse gas emissions (Neukom, Barboza, Erb, Feng, & Gunten, 2019; T. F. Stocker et al., 2013). The Intergovernmental Panel on Climate Change (IPCC) estimates that greenhouse gas emissions will result in a decadal 0.2°C global average temperature increase, and global temperatures will rise by, at most 2.64.8°C, and at least 0.3–1.7°C in the 21st century (T. F Stocker et al., 2014). The resulting variations in extreme temperature and precipitation patterns threaten the stability and diversity of forest ecosystems globally (Eastman, Florencia, Elia, John, & Assaf, 2013; Y. Liu, Yan, Li, & Motesharrei, 2015). The extinction rate of species will subsequently increase, and warmer temperatures may affect plant growth and yield (Hatfield & Prueger, 2015; Lawler, 2009; Ray, Gururaja, & Ramchandra, 2011). The Earth has experienced several climatic upheavals, and plants tend to alter their bioecological characteristics, phenology, or resilience response mechanisms through natural selection and genetic evolution to gradually spread or migrate to more suitable habitats to avoid being affected by adverse climatic conditions (R. T. Corlett & Westcott, 2013; Hewitt, 2000; Thomas et al., 2003). However, in the face of rapid climate change, it is unrealistic for plants to evolve adaptation strategies in such a short period, especially for arbour species. Therefore, exploring the potential habitat distributions of cherished species under future climate change is crucial to their diversity and conservation.
Sapindus is an evergreen, or deciduous, tree in the Sapindaceae family, which comprises 13 species with a global distribution in warm-temperate to tropical regions, found mainly in southeast Asia and North and South America (J. Liu et al., 2017). Among these, Sapindus mukorossi (S. mukorossi), Sapindus delavayi (S. delavayi), and Sapindus rarak (S. rarak) are concentrated in east and southeast Asia (J. Liu et al., 2017). Owing to the high yield of its seed oil (26.69–44.69%), as well as the high medium-chain monounsaturated fatty acid content (Sun, Jia, Xi, Wang, & Weng, 2017), Sapindus seed oils are suitable for biodiesel production according to American Society for Testing and Materials (ASTM) D6751 and the European EN 14214 standards (Chakraborty & Baruah, 2013; Pelegrini et al., 2017; Sun et al., 2019). Crude extracts from the fruit pericarps of Sapindus are also rich in triterpenoid saponins (4.14–27.04%) and sesquiterpenoids (J. Liu et al., 2019; Xu, Jia, Chen, & Gao, 2018), particularly in S. rarak, which exhibit excellent surface activity as well as antibacterial (Basu, Basu, Bandyopadhyay, & Chowdhury, 2015), elution (Mukherjee, 2015; Mukhopadhyay, Hashim, Sahu, Yusoff, & Gupta, 2013; Mukhopadhyay et al., 2016), antibacterial (Shinobu-Mesquita et al., 2015; Smu?ek et al., 2016), insecticidal (Shinobu-Mesquita et al., 2015; Smu?ek et al., 2016), pharmacological (Rodriguez-Hernández, Demuner, Barbosa, Csuk, & Heller, 2015), and physiological (Singh & Singh, 2008) properties. Saponin serves as an efficient natural surfactant in commercial soaps, shampoos, and cosmetic cleansers (Muntaha & Khan, 2015), and the root and fruit of Sapindus trees are used in traditional Chinese medicine. Therefore, Sapindus is regarded as an important biodiesel, biomedical, and multifunctional economic forest species (J. Liu et al., 2017; Sun, Jia, Ye, Gao, & Weng, 2016). Sapindus germplasm resources are widely distributed; however, they are generally scattered in the form of single plants or extremely small populations. Further, with global deforestation and the rapid anthropogenic development, Sapindus germplasms have been persistently damaged or lost in recent decades, especially in China, India, and Nepal (Jia and Sun, 2012; Liu et al., 2017). We doubt that Sapindus will experience a greater loss of diversity under future climate change. It is imperative to conserve the diversity of Sapindus, and natural populations at risk of destruction should be protected through in or ex situ conservation. However, there is the question of where Sapindus germplasm collection should be prioritised: where should the germplasms we have collected be preserved and cultivated under current environment and future climate change? Further, how should we preserve and cultivate different Sapindus germplasm species?
Zhang et al. (Zhang, Jing, Li, Liu, & Fang, 2019) and Peng et al. (Peng et al., 2019) explored suitable habitats for Cinnamomum camphora and Paeonia ostia under current and future climate conditions, respectively, using ecological niche models and offered suggestions for their respective targeted collection and preservation. Ashish (Pal, Vaishnav, Meena, Pandey, & Singh, 2020) also simulated the adaptability and limiting factors of Sapindus emarginatus Vahl in the face of future climate regimes through two ecological niche models. Therefore, ecological niche model appear one of the optimal solutions to establishing how species will adapt to future climate scenarios. Plant niches are habitats with a minimum threshold necessary for a plant’s survival (Barry, Moore, & Cox, 1980). The forest niche is strongly affected by the environment, and its niche changes or moves with environmental changes. The principle of ecological niche models is to infer the ecological needs of species through mathematical models based on occurrence data and environmental factors to draw a statistical or mechanistic model of their potential distributions (Araújo & Peterson, 2012; Elith & Leathwick, 2009; Zhu, Liu, Bu, & Gao, 2013). Benefitting from the simplicity of niche models and data accessibility (Merow et al., 2014), researchers can often estimate the potential habitat distribution and range shifts of species under future climate change. At present, the most commonly used niche models are the GARP (Stockwell, 1999), MaxEnt (Elith et al., 2006), Bioclim (Beaumont, 2005), random forest (Cutler et al., 2007), and boosted regression tree (Elith, 2008) models. Many model intercomparison studies have reported that the MaxEnt model, which is based on the principle of maximum entropy (Elith & Leathwick, 2009; Elith et al., 2011; Steven J. Phillips, Anderson, & Schapire, 2006), typically outperforms other species distribution models (SDMs) in terms of high tolerance and high predictive accuracy (Li, Du, & Wen, 2016; Merow et al., 2014; Merow, Smith, & Silander, 2013). Over the past decade, research teams have achieved rich results in the study of rare animal and plant diversity protection (Guo, Guo, Shen, Wang, & Wang, 2019; Huang et al., 2019; Rong et al., 2019; W. Wang et al., 2019), invasive species risk prediction (Ramos, Kumar, Shabani, & Picanco, 2019; Sultana, Baumgartner, Dominiak, Royer, & Beaumont, 2020), marine ecosystem protection (Melo-Merino, Reyes-Bonilla, & Lira-Noriega, 2020; Sterne, Retchless, Allee, & Highfield, 2020), disaster distribution prediction (Convertino, Annis, & Nardi, 2019), and disease propagation (Ardestani & Mokhtari, 2020; Hanafi-Bojd, Vatandoost, & Yaghoobi-Ershadi, 2020) using the MaxEnt model. However, S. mukorossi is the most widely distributed species of Sapindus in Asia, and thus far, there are no reports exploring its habitat distribution using ecological niche models. Adopting ecological niche models to explore the distribution of Sapindus habitat distribution and their responses to future climate change is likely an effective strategy to address the conservation of Sapindus germplasm diversity.
In this study, we combined principal component analysis, the MaxEnt model, and ArcGIS for the three major Sapindus species in east and southeast Asia, namely, S. mukorossi, S. delavayi, and S. rarak. Based on 2,041 occurrence records, we drove models using current climate normals (averages between 1970 and 2000) and future climate projections (averages between 2021 and 2100) to investigate the habitat distribution and centroid shifts of S. mukorossi, S. delavayi, and S. rarak. The three objectives for this study were as follows: (1) to determine the spatial heterogeneity of the distribution of S. mukorossi, S. delavayi, and S. rarak; (2) to project suitable habitats for these three Sapindus species under current environmental conditions; and (3) to reveal the potential habitat redistribution patterns of these three Sapindus in response to future climate change, and identify hotspots of habitat degradation/expansion to facilitate climate change-adaptive biological conservation.