3.1 Ecological factors of different populations
The key thresholds of the ecological factors of species were extracted from ArcGIS following the sampling sites of P. veitchii. The violin plots of the ranges of the ecological factor values of PV_Daodi and PV_non-Daodi are shown in Fig. 2, which shows that the ecological factor values varied considerably. The isothermality (Bio3), annual temperature range (Bio7), precipitation of the wettest month (Bio13), precipitation of the driest month (Bio14), mean global UV radiation in January (UV_JAN), and mean global UV radiation in July (UV_JUL) of PV_Daodi and PV_non-Daodi showed significantly different values (p<0.05). This result indicated that the populations of Daodi- and non-Daodi-producing areas require different environmental and climatic conditions.
3.2 Population Grouping
The distribution of the 226 presence sites was drawn based on the first two principal components, which explained 78.6% of all variance (Fig. 3). Despite some overlap between the two populations, PV_Daodi and PV_non-Daodi were located separately in PCA maps: PV_non-Daodi occupied a niche featured by annual mean air temperature (Bio1), Bio3, high Bio13, Bio14, UV_JAN, and UV_JUL, whereas PV_non-Daodi was featured by mean diurnal range (Bio2) and Bio7. The HCA result showed that all sites of occurrence can be grouped into three groups: two groups for PV_non-Daodi and one group for PV_Daodi. The combined results from HCA and PCA indicated that divergence in climate adaptation exists between the two populations despite the little overlap between these populations. In other words, the population from Daodi-processing area experienced a different climate compared with those in other areas.
3.3 Modeling performance and important environmental variables influencing the current habitat.
The data of 114 and 112 sites currently owned by PV_Daodi and PV_non-Daodi, respectively, were selected to build the separate SDMs. Six bioclimate variables and two UV variables were used in the MaxEnt model. The AUC values of the training and test data sets were 0.986 and 0.986 for PV_whole, 0.994 and 0.994 for PV_Daodi, and 0.991 and 0.991 for PV_non-Daodi, respectively. These values indicate that the model predicted the suitable habitat of P. veitchii with excellent performance.
Percentage contribution values (Table 1) revealed that the top three contributing environmental variables were UV_JUL (31.3%), Bio13 (22.3%), and UV_JAN (15.5%) for PV_Daodi, which account for 69.1% of the variation. The three most dominant contributing environmental variables for PV_non-Daodi were Bio1 (20.4%), Bio3 (17.3%), and UV_JAN (23.9%), which account for 61.6% of the variation. The three most dominant variables for PV_whole included Bio1 (16.0%), UV_JAN (26.6%), and UV_JUL (21.4%), which account for 64.2% of the variation.
Fig. 4 shows the thresholds (presence probability > 0.2) for each variable, which were calculated through separate response curves. For PV_Daodi, Bio13 had a range of 116.68–250.73 mm, UV_JAN had a range of 1151.32–2962.71 J/m2/day, and UV_JUL had a range of 7402.44–9368.21 J/m2/day. For PV_non-Daodi, Bio1 varied from −1.47 °C to 14.57 °C, Bio3 ranged from 27.69 °C to 40.24 °C, and UV_JAN ranged from 994.76 J/m2/day to 1933.83 J/m2/day. For PV_whole, Bio1 varied from −1.93 °C to 15.61 °C, UV_JAN ranged from 1019.93 J/m2/day to 2657.71 J/m2/day, and UV_JUL ranged from 4174.56 to 4785.13 and 5216.04 J/m2/day to 10021.21 J/m2/day.
3.4Predicting distribution under current conditions
Figures 5a–c show the potential distributions of PV_Daodi, PV_non-Daodi, and PV_whole, respectively, according to the MaxEnt model under current climatic conditions. The logical output generated by the MaxEnt software is expressed in probability and ranges from 0 to 1. The model results were divided into four levels using the reclassification tool of ArcMap 10.2: 0–0.2 is not suitable, 0.2–0.4 is generally suitable, 0.4–0.6 is moderately suitable, and 0.6–1 is highly suitable [32].
The suitable areas for PV_Daodi were China, India, Bhutan, the United States, Afghanistan, and Syria with smaller distributions in Lebanon, Israel, Nepal, and Burma. The highly suitable areas were concentrated in Eastern China (Fig. 5a). The current areas of the generally, moderately, and highly suitable habitats for PV_Daodi were 3.8×105, 2.6×105, and 1.1×105 km2, respectively. The suitable areas for PV_non-Daodi were distributed in China with sporadic distributions in Afghanistan, Bhutan, India, Iran, Iraq, Lebanon, Morocco, Spain, Syria, Tajikistan, and the United States. The highly suitable areas were greatly distributed in China (Fig. 5b). The current areas of the generally, moderately, and highly suitable habitats for PV_non-Daodi were 6.4×105, 4.0×105, and 2.0×105 km2, respectcively. The suitable areas for PV_whole were distributed in China, Afghanistan, India, Bhutan, United States, Lebanon, Syria, Morocco, Burma, and Vietnam, and the highly suitable area is in China (Fig. 5c). The current areas of the generally, moderately, and highly suitable habitats for PV_whole were 8.6×105, 5.4×105, and 2.7×105 km2, respectively.
The SDM of PV_non-Daodi predicted 28 countries, whereas that of PV_whole failed to predict two of these countries, namely, Colombia and Chile. The SDM of PV_whole predicted Cyprus, Vietnam, and France, which the two separate models (PV_Daodi and PV_non-Daodi) did not forecast. This finding suggested that model-based prediction should consider the local adaptation of a species to predict a species’ potential distribution area.
3.5 Suitable habitat distribution under past and future climate changes.
The distributions of P. veitchii under future (RCPs 2.6, 6.0, and 8.5) and past climate scenarios (LGM and MH) are shown in Supplementary Figs. 3–6. The distributions of PV_Daodi, PV_non-Daodi, and PV_whole from past to future scenarios are different. The PV_Daodi population was predicted to have an expansion of its potential distribution in 2050 and a slight contraction in 2070 under RCP 2.6 and 6.0 scenarios. The PV_non-Daodi and PV_whole populations will have consistent expansion under RCP 6.0 and 8.5 scenarios. The suitable area of PV_Daodi was approximately 1.37×106 km2 in LGM and declined to 1.04×106 km2 in MH and only 7.49×105 km2 at present. This area might increase in the 2050s under RCP 2.6 (9.60×105 km2), RCP 6.0 (1.00×106 km2), and RCP 8.5 (9.01×105 km2) and decrease in the 2070s except under RCP 8.5, in which the area will increase to 9.89×105 km2. The suitable area of PV_non-Daodi was about 1.99×106 km2 in LGM and decreased to 1.63×106 km2 in MH and 1.24×106 km2 at present. This area might increase in the 2050s and 2070s under RCP 6.0 (2.11×106 km2) and RCP 8.5 (1.93×106 km2) but might decrease in the 2070s under RCP 2.6 (1.89×106 km2). The suitable area of PV_whole was about 7.00×106 km2 in LGM and 8.69×106 km2 in MH but decreased to 1.67×106 km2 at present. This area might increase in the 2050s under RCP 2.6 (2.47×106 km2), RCP 6.0 (2.63×106 km2), and RCP 8.5 (2.37×106 km2) and keep on increasing in the 2070s under RCP 2.6 (2.46×106 km2), RCP 6.0 (2.78×106 km2), and RCP 8.5 (2.51×106 km2).
3.6 suitable habitat centroid shift from past to future climate.
The centroid was calculated to fully understand the suitable distribution shifts, and the centroid migration path was drawn to illustrate the direction and distance shift of the population under different climate scenarios (Fig. 7). The centers of the suitable distribution area are from Libya and Gaza in LGM to Iran in MH and to China and India at present. The centers will move northward toward Central Asia under the three future climates. The magnitude of migration varies slightly: the future centers of the PV_Daodi population are Iran and Afghanistan, the future centers of the PV_non-Daodi population are Turkmenistan and Afghanistan, and the center of the PV_whole population is Afghanistan. Notably, PV_Daodi will move a smaller distance to the north than PV_non-Daodi under the future climate environments. This difference may be caused by the high temperature of the carbon dioxide emission model, which PV_non-Daodi will not be able to adapt.