Investigation of the impact of diverse climate conditions on the cultivation suitability of Cinnamomum cassia using the MaxEnt model, HPLC and chemometric methods in China

Evaluation of model accuracy

The predictive accuracy of the MaxEnt model is expressed as an AUC value between 0 and 1. Accuracy is measured by how close the area AUC is to 1 and values closer to 1 indicate greater accuracy16. The results showed that all average training AUC values and test set (10 replications) of the model were between 0.98 -1(Current, Rcp26-2050s and Rcp85-2050s: 0.988; LGM, MH, Rcp26-2090s and Rcp85-2090s: 0.987) (Fig. S3). The findings demonstrate that the MaxEnt model calculation attained excellent accuracy and that the reliability of the experimental data was high.

Environmental variables analysis

Some surveys and research reports examined where C. cassia was distributed in China, but they didn’t consider environmental variables that may have affected its distribution Using the Spatial Analyst Tools module in ArcGIS, the main ecological niche parameters of main environmental variables for determining the optimal distribution area were extracted and analyzed (Table 2). The analysis results show that bio_4 (Temperature seasonality), bio_18 (Precipitation of warmest quarter), t_clay (Topsoil clay content), bio_11 (Mean temperature of coldest quarter), bio_14 (Precipitation of driest month), awc_class (Soil available water content), s_oc (Substrate soil organic carbon), bio_15 (Precipitation seasonality), and bio_2 (Mean diurnal range) were the primary influencing factors on the distribution of C. cassia. Their percent contribution cumulative values and permutation importance reached 97.8% and 87.2%, respectively. Additionally, bio_18, bio_4, t_clay, and bio_14 were important environmental variables (Table 2), with bio_18 and bio_4 having more weight in the analysis of only one variable (Fig. S4), indicating that precipitation, soil, and temperature were significant elements influencing the potential distribution of C. cassia.

Based on the response curve of four important environmental variables (bio_18, bio_4, t_clay, and bio_14) to the probability distribution of C. cassia distribution (Fig. S5), it can be observed that the environmental variables suitable for C. cassia growth are: precipitation of warmest quarter (Bio_18) of 367.20–1697.81 mm and the optimal value of 775.55 mm, temperature seasonality (Bio_4) of 402.85-870.67 and the optimal value of 564.36, topsoil clay content (t_clay) of 41.40-51.88% w and the optimal value of 47.44% weight, and precipitation of driest month (bio_14) of 17.92–60.76 mm (Table 3). Guangxi and Guangdong, especially the Xijiang River Basin, have suitable temperatures and abundant rainfall, making them authentic production areas for C. cassia. Similar to the research results of Panax notoginseng and Zanthoxylum nitidum, bioclimatic variables, especially temperature and precipitation have a profound effect on the geographical distribution of herbal plants10,16. In addition, soil factors (topsoil clay content and soil available water content) contribute significantly to C. cassia distribution. Different climatic conditions, CO2 concentration, acid rain and CO2 fixation by plants cause alterations in soil pH and organic matter content18. In addition, the topsoil clay is higher in the south than in the north of China, and the topsoil clay affects its fertility and drainage performance19. It is reported that the soil conditions and microorganisms were different in different regions, which affected the composition and content of secondary metabolites of medicinal Citrus20. Therefore, in future research, we should further study the relationship between C. cassia quality and soil microorganisms in different suitable habitats, to better reveal the localization mechanism of C. cassia.

Distribution prediction in the current environmental conditions

After the Qin Dynasty (about 1700 years ago), the cultivation method of C. cassia shifted from wild growth to artificial cultivation, resulting in an expansion of the production areas. Figure 2a, b demonstrates that C. cassia, under the present-day climatic scenario, is found primarily in southern China between 18 and 33 °N, especially between 20 and 26 °N. There was a total of 1,606, 718.01 km2 of suitable habitat (containing both highly and moderately suitable habitats) (Fig. 3; Table 4). The highly-suitable habitat area was 706,129.08 km2, mainly distributed in two provinces of Guangxi and Guangdong, as well as in the south of Guizhou and Chongqing, the east of Taiwan, the southwest of Hubei, and the northeast of Hainan and Fujian Province. The moderately suitable area was 900,588.93 km2 (Fig. 3; Table 4), mainly distributed in most areas of Hainan, Hunan, Jiangxi, Fujian, Guizhou, and Zhejiang, the south of Yunnan and Anhui, the east of Sichuan and Hubei, and the central region of Taiwan, as well as a few areas in the south of Tibet (the southern part of Motuo County) and Jiangsu Province (Fig. 2b). It can be observed that the northern part of Yunnan and the southwestern part of Sichuan are within the 30°N latitude, but they have a relatively low distribution of C. cassia. This may be because these areas are more than 1,000 m above sea level. C. cassia is most suitable for growth at an altitude of 300–350 m, and it is not suitable for C. cassia to grow above 1000 m21. This suggests that altitude is also a limiting factor for the distribution of C. cassia. It was reported that C. cassia is widely distributed in China, with production areas in Guangxi, Guangdong, Yunnan, Fujian, and other regions1,7. Throughout history, the preferred production areas were mainly located in the Guangxi-Guangdong region. Now the authentic production areas in Guangxi and Guangdong have officially formed a pattern, accounting for more than 80% of the total area of C. cassia distribution in China2. These areas mainly have a subtropical monsoon climate and abundant water and heat resources. The results also showed that precipitation in the driest and wettest quarters had significant effects on the distribution of cinnamon. The water requirement in the dry season is more than 26 mm, and the water in the wettest season is more than 449 mm, so in the daily field management, attention should be paid to ensuring adequate water. Good concordance between predicted and actually observed distributions suggests that the MaxEnt-simulated suitable area may be utilized as a guide in selecting cultivation locations for C. cassia planting planning.

Distribution prediction in the past environmental conditions

The distribution of C. cassia under past climate scenarios is shown in Fig. 2c-d. Compared with the current situation, suitable habitats of C. cassia were limited during the Last Glacial Maximum (LGM) (Fig. 2c). The distribution of highly suitable habitats was decreased to 22–23 °N, mainly in the southern of Guangxi and a few areas in the southern part of Guangdong with only 58,423.00 km2 (Figs. 2c and 3; Table 4). This is probably due to the extremely cold climate during the LGM, in which the global average temperature dropped by approximately 5–10℃ compared to the present, and the precipitation levels also decreased16,22. According to previous research and records, wild C. cassia resources were abundant in ancient times. During the Han and Wei dynasties, most C. cassia was naturally distributed in Guangxi, with scattered distribution in Guangdong as well23, which agreed with the findings of this research, and further proved the reliability of our findings.

Compared to the LGM, the suitable habitat of the Mid-Holocene (MH) was extended to the surrounding areas (Fig. 2d). The areas of moderately (898,945.59 km2) and highly suitable habitats (521,486.20 km2) were both significantly increased (Fig. 3; Table 4). It covered almost the entire regions of Guangdong and Guangxi provinces, and the eastern regions of Fujian, Zhejiang, and Sichuan provinces. This was because of the dramatic temperature rise that occurred throughout the MH period24, which was suitable for the growth of C. cassia. Palaeoclimate research showed that the MH period was a relatively warm and humid period since the Last Glacial Maximum. The temperatures were approximately 1-1.5℃ higher than current levels, and the southern regions experienced 100–300 mm more rainfall compared to the present25,26. In particular, the average annual temperature in the Zhejiang and Fujian provinces, which are close to the East China Sea, will be about 2–3℃ higher than the current. This makes these regions more suitable for the growth of C. cassia27. In the late Holocene, there was a decreasing trend in temperature26. Thus, temperature and precipitation are important key variables affecting C. cassia’s habitat suitability dispersion.

Predictions of distribution in the future environmental conditions

Future C. cassia distribution as well as suitable habitat regions are depicted in Figs. 2e-h and 3. Both SSP126 and SSP585 future climate scenarios for the 2090s predict a significant reduction in the low-suitable habitat regions for C. cassia compared to the two 2050s future climate scenarios (Fig. 3; Table 5). The high-suitable and total habitat areas both exhibit an upward trend, particularly in the 2090s SSP585 scenario, and the newly expanded total area of suitable habitat is the highest (312,963.53 km2), and the distribution of total suitable habitat has expanded to 18–32 °N. (Fig. 2e-h; Table 5). As shown in Fig. 2h and Fig. S6, the highly suitable habitat areas of C. cassia have expanded to the south of the Yangtze River. Moreover, the highly suitable habitat areas had increased significantly in Fujian, Jiangxi, Hunan, Hubei, Zhejiang, Hainan, and Taiwan provinces, as well as in the south of Yunnan and Anhui provinces and the east of Sichuan. This demonstrates the potential for future scientific expansion of C. cassia planting areas. But it also means that the environment is changing and the temperatures are rising. According to reports, the yearly average surface temperature in southern China rose by 2.7 to 2.9℃, with warming increasing by 1.8 to 2.5℃. Additionally, the warming rose from south to north by > 2.5℃ with the annual precipitation increasing by 1.5–2.0%28,29. Changes in precipitation are closely related to temperature changes. Global climate warming has led to a general increase in atmospheric humidity and precipitation intensity30. This could potentially expand the range of suitable habitats for C. cassia. Changes in the environment will affect the range and distribution of many medicinal plants9,16,31. This suggests that temperature and precipitation may be key factors influencing the southward expansion of suitable habitat for C. cassia towards the Yangtze River region.

Quality evaluation of C. cassia in different suitable habitatsHPLC fingerprint of C. cassia

To reveal the variations in the content of active constituents in natural medicinal materials from different suitable habitats, the trans-cinnamaldehyde content of 31 C. cassia samples collected from various suitable environments was analyzed by HPLC. According to the results, trans-cinnamaldehyde levels varied among 31 samples collected from various suitable habitats (Fig. 4A; green column). In general, the high-suitable habitats of Guangxi (GX1-GX16) and Guangdong (GD1-GD8) have relatively higher levels of trans-cinnamaldehyde, while the medium- and low-suitable habitats of Yunnan (YN1-YN5), Hunan (HN) and Hubei (HB) have relatively lower levels of trans-cinnamaldehyde. In high suitable habitats, the average level of trans-cinnamaldehyde (Fig. 4A; red columns, 0.85%) is notably higher than that in medium- and low- suitable habitats (Fig. 4A; blue columns, 0.30%). Previous studies have also shown that C. cassia produced in Guangxi and Guangdong is considered to be of the best quality23,32, this is related to the environmental parameters like sunlight, temperature, soil composition, and precipitation have a remarkable influence on plant secondary metabolite synthesis and accumulation2,33,34. It has been reported that the suitable growth temperature for C. cassia is between 22 °C and 26 °C. When temperatures exceed 20 °C, the content of trans-cinnamaldehyde increases; however, when temperatures exceed 30 °C, the growth rate of C. cassia slows down, and trans-cinnamaldehyde synthesis decreases. In regions with annual precipitation between 1200 mm and 1800 mm, the average content of trans-cinnamaldehyde reaches 5.2%35. Guangdong and Guangxi provinces are the primary high-suitability areas for C. cassia, where the climate is warm and suitable (with average temperatures in 2022 of 26.2 °C and 22.5 °C, respectively) and rainfall is abundant (with average precipitation in 2022 of 2114.3 mm and 1696.7 mm, respectively). These conditions make them ideal for C. cassia cultivation, and they are also the main production areas for C. cassia in China. The C. cassia grown in these regions has a high trans-cinnamaldehyde content, making it suitable for use in the pharmaceutical field. The moderate-suitability areas are mainly concentrated in the northern and western parts of the subtropics. Due to colder winter temperatures and insufficient rainfall, the trans-cinnamaldehyde content in places like Yunnan and Hubei is lower than that in Guangdong and Guangxi. Therefore, it is suggested that the medicinal materials produced in these regions can be used as food additives or spices. Our results further supported that the medicinal plants in suitable habitats contain higher levels of active constituents9,10,31.

To accurately and visually observe the differences between samples, an OPLS-DA model was constructed with 22 aroma components as the dependent variables and different geographical origins as the independent variables (Fig. 4B). In this analysis, the fixed index for the independent variables (R2x) was 0.513, the fit index for the dependent variables (R2y) was 0.714, and the model prediction index Q2 was 0.581. R2 and Q2 values greater than 0.5 indicate acceptable model-fitting results36. From Fig. 4B, it can be observed that the high-suitable habitats (green box) and the medium- and low-suitable habitats (blue box) samples of C. cassia clustered into distinct groups. GX13 (Chongzuo, Guangxi) and GX16 (Baise, Guangxi) are located in the western part of Guangxi, close to Yunnan, which makes them similar to the samples from the medium- and low-suitable habitats. OPLS-DA can effectively discriminate C. cassia samples from high-, medium- and low-suitable habitats.

Quality evaluation based on FTIR fingerprint of C. cassia

The power of FTIR as a quantitative tool lies in its ability to easily perform multicomponent analyses37. This technique is usually considered to be non-destructive, simple, and fast with good repeatability and accurate analysis of traditional Chinese medicine38. Figure 5a illustrates that the FTIR fingerprint spectra peaks of 31 batches of the C. cassia leaves had many similar peaks with characteristic infrared absorption spectra ranging between 1800 –600 cm−1. Several characteristics of the typical C. cassia spectrum seen in Fig. 5a were determined via analysis. The aldehyde carbonyl’s C = O stretching vibration shows up as the strongest peak at 1665 and 1628 cm−1, and the peak is strong and broad, which is consistent with the high content of cinnamaldehyde and other aldehydes in C. cassia leaves. The absorption peak at 1451 cm−1 during bending vibration absorption is highly indicative of a C-OH alcohol. Bands at 1250 cm−1 indicate the presence of the aromatic acid ester C-O-C symmetric expansion and the stretching vibration of the phenolic C-OH groups, whereas the frequency at 1070 cm−1 is associated with C-O, with stretching vibration mode. Absorption due to C-H bending vibration absorption is attributed to the peak at 780 cm−1. Finally, the band at 680 cm−1 is an olefin vibration absorption, and these vibrational characteristics are consistent with C. cassia volatile oil containing olefinic substances.

Because there is a significant overlap in the chemical composition fingerprints, distinguishing between the components of the samples is challenging. The IR spectra data of the 31 samples was chosen for HCA and OPLS-DA to more directly detect the variations in the components of the C. cassia leaves. The OPLS-DA model has a good fit with an R2x of 0.901 for the independent variables, an R2y of 0.575 for the dependent variables, and a Q2 of 0.961 for the model prediction, indicating the reliability of this model36. As illustrated in Fig. 5b, c, both HCA and OPLS-DA analyses show the same result. All samples could be clearly divided into two categories. Samples (GX1-GX12, GX14-GX15, and GD1-GD7) from the high-suitable habitats were distinguished as a cluster, and samples (YN1-YN5, HN, HB) from the medium- and low-suitable habitats were classified as a cluster. In these two methods, GX13 (Baise, Guangxi), GX16 (Chongzuo, Guangxi), and GD8 (Puyang, Guangdong) are classified together with the samples from the medium- and low-suitable habitats. C. cassia is a tropical and subtropical plant that thrives in warm environments. It has been mainly distributed in the warm Xijiang River Basin from ancient times to the present39. Baise, Chongzuo, and Puyang are far away from the Xijiang River Basin and have different climatic and soil environments, which may lead to large differences in chemical composition between the three samples and other samples from high-suitable habitats. It can be seen that FTIR, combined with HCA and OPLS-DA, is an effective and accurate method for identifying different suitable habitats of C. cassia, proving once again that the MaxEnt model used in this study is accurate.