4.1 Response of suitable distribution to environmental variables
It is of great significance for ecological management to explore the changes in the geographical distribution of species caused by climate change (Hamann and Wang, 2006; Paź-Dyderska et al., 2021; Varol et al., 2021). O Neill and Kriegler et al (2017) predicted that the average global temperature would be more than 4 ℃ above pre-industrial levels by 2100s, which would alter the geographic distribution of many species (Lenoir et al., 2008; Bellard et al., 2012; Hundessa et al., 2018; Bouahmed et al., 2019). The profound impact on the distribution change of species might alter the function and structure of terrestrial ecosystems in turn (Huang et al., 2021). Therefore, assessing the impact of climate change on the spatial distribution of different species will help address issues related to changes in the distribution and range of various species.
This study predicted the potential distribution pattern of S. javanica under current and future climatic conditions in China using the MaxEnt model. From the phytogeographic point of view, S. javanica was mainly located in the subtropical region of China (south of the Qinling Mountains-Huaihe River Line and east of the Hengduan Mountains). The presence of mountain ranges affected the distribution of plants and animals (Elsen et al., 2018; Odland, 2010). In this study, the Qinling-Huaihe line was found to be the northern boundary of S. javanica distribution, and the western boundary of S. javanica was the Hengduan Mountains. The Qinling Mountains-Huaihe River Line serves as the dividing line between the southern and northern regions of China, with significant differences in geography, climate and environment (González-Prieto et al., 2016). The south region of the line has a humid, warm and rainy climate, with an average January temperature above 0°C and average annual precipitation above 800mm (Li et al., 2018). This boundary is also the dividing line between the subtropical monsoon and temperate monsoon climates, and is considered to be the main parameter affecting the distribution of many species (Zhou et al., 2011; Pan et al., 2020; Xie et al., 2021). Moreover, the warm and humid airflow from the Indian Ocean is blocked by two tall east-west mountain ranges (the Himalaya and the Gondola) and enters China along the north-south Hengduan mountain range, bringing abundant rain to the Tibetan Plateau (Li et al., 2018; Kramer et al., 2010; Xu et al., 2019). Therefore, the Qinling-Huaihe line and the Hengduan Mountains are the boundaries of the 800 mm equivalent precipitation area in China, which overlap with the modern distribution boundary of S. javanica (Fig. 5a). The cumulative contribution of two precipitation factors (Prec01 and Bio18) with a value of 63.4% (Table 3) was also further evidence that precipitation was critical to the distribution of S. javanica. This result suggested that the limiting effect of precipitation on the growth of S. javanica might be one of the main reasons why its modern distribution is mainly concentrated in the subtropical region of South China.
In addition, the altitude-induced temperature change (decrease about 6℃ per 1000 meter elevation) was also an important factor affecting the distribution of plants (Rangwala & Miller, 2012). The Yunnan-Guizhou Plateau, bounded by the Wumeng Mountains, was divided into the Yunnan Plateau (altitude 3000–4000 m) and the Guizhou Plateau (altitude 2000–2400 m) (Zheng et al., 2020). In summer, the temperature of the Yunnan-Guizhou Plateau was lower than that of the same latitude region due to its higher altitude. In winter, the temperature of Yunnan-Guizhou Plateau was higher than that of the same latitude region due to the plateau is at the intersection of the Pacific and Indian Ocean monsoons (Yang, 2008). The unique geographical location of the Yunnan-Guizhou Plateau results in low temperatures in summer and high temperatures in winter, also leading to its annual temperature difference smaller than that of the same geographical latitude (Yang, 2008).
This might be the main reason why the Yunnan plateau has adequate precipitation (1000–1200 mm per year), but the distribution suitability class of S. javanica in the region is largely low suitability distribution area.
4.2 Changes of suitable habitat in future climate scenarios
Research into the impact of future climate changes on plants could be beneficial to develop strategies towards the challenges caused by climate change (Liao et al., 2020). The sixth iteration of the Climate Model Intercomparison Project (CMIP6) is widely applied to discuss the impact of future climate change on global ecology and economic development (Xu et al., 2022; Tan et al., 2022). The CMIP6 modeled future climatic conditions by combining possible future socio-economic conditions (Shared Socio-economic Pathways) and different greenhouse gas (GHG) emission scenarios (Representative Concentration Pathways, RCPs) (O'Neill et al., 2016). In this study, the climate scenarios SSP126 and SSP585 were used as a background to simulate the future distribution of S. javanica. The SSP126 scenario represented a sustainable development path. In this scenario, fossil energy dependence and global carbon dioxide emissions would be greatly reduced, and the radiative forcing level would approach 2.6Wm− 2. Carbon dioxide emissions are expected to be reduced to zero by around 2050s, and the global temperature would be about 1.8°C higher than today by around 2100s (O'Neill et al., 2016). The SSP585 scenario represented the traditional path of economic development. In this scenario, people solve social and economic problems by emphasizing self-interest and rapid development. The radiative forcing level would approach 5.8Wm− 2. Carbon dioxide emissions are expected to double relative to current levels by around 2050s, and the global temperature would be about 4.4°C higher than today by around 2100s (O'Neill et al., 2016).
In this study, the variation of the suitable distribution area of S. javanica are investigated under two different environmental strategies. The results showed that the suitable distribution area of S. javanica, under the SSP126 climate scenario, exhibited a shrinking trend relative to the current suitable distribution area. However, under the SSP126-2080s scenario, the highly suitable distribution area of S. javanica showed a significant increase, caused by the conversion of low, generally and moderately suitable distribution areas to highly suitable distribution areas. This suggested that there could be a risk of S. javanica suitable habitats loss and fragmentation under the SSP126 climate scenario. Although the suitable distribution area of S. javanica showed a shrinking trend under the SSP585-2040s and SSP585-2060s, the distribution was gradually recovered and became stable after the SSP585-2080s. Meanwhile, in the context of future global warming, the low suitability distribution of S. javanica in the Yunnan Plateau showed a trend of conversion to moderate suitability distribution, probably caused by the relatively low temperature at high altitudes that provided a potential geographic barrier for S. javanica against future climate warming (Xie et al., 2021). Summary, although the distribution areas of S. javanica might be affected to some extent under future climate scenarios due to increased precipitation and temperature, the overall distribution pattern of S. javanica was unlikely to change. This suggested that S. javanica could cope with the effects of future global warming on its distribution pattern.
4.3 Prospects and Suggestions
The active substances (mostly secondary metabolites) produced by medicinal plants serve as direct indicators for judging its quality. Previous researches showed that the contents, types and proportions of active substances are strongly related to environmental factors such as illumination, moisture and temperature (Penuelas & Llusia, 1997; Liu et al., 2016). However, a suitable growth environment does not necessarily lead to the accumulation of more active ingredients in the medicinal plants. They might also produce more active substances in response to environmental stress under unfavorable growth conditions (Wang et al. 2016; Alhaithloul et al., 2020; Toivonen et al., 1992; Jochum et al. 2007). For example, the contents of myricetin-3-O-rhamnoside in the roots of Limonium bicolor (Bag.) Kuntze were significantly increased under salt stress (Wang et al. 2016). In the medicinal plants Mentha piperita and Catharanthus roseus, the levels of tannins, terpenoids and alkaloids were significantly increased under the combined heat/drought stress (Alhaithloul et al., 2020). At present, the relationship between the environmental factors and the active ingredients of S. javanica is currently not clear. Therefore, the influence of environmental factors on the medicinal active ingredients of S. javanica is a major research focus for the future.