Eco-environmental preferences
The climatic, edaphic and topographic characteristics of known
A. annua L. habitats are listed in Table
2. While low CV values for RH (CV: 0.33), TAP (1.28), AST (3.33), ATJU (4.60), AAT (4.69), ATA (6.23), ATJ (6.77) and ATMA (6.81) suggested that these could be the major limiting factors affecting the distribution of high quality
A. annua L., high CV values for AL (29.79), ATJA (21.46) and ATF (21.43) suggested otherwise. According to the CV values, weighting value for each parameter was divided into levels I (0.15), II (0.08), III (0.06) and IV (0.03) and weighting values should add up to one. In addition, datasets of eco-factors from known habitats in Youyang County were as follows: ATJA = 1.2-5.6°C, ATF = 2.0-6.0°C, ATM = 4.0-10.0°C, ATAP = 10.0-16.0°C, ATMA = 14.0-20.0°C, ATJ = 18.0-24.0°C, ATJU = 21.6-27.3°C, ATA = 20.0-26.0°C, AAT = 15.9-21.0°C, AST = 1048-1200 h, TAP = 1169-1267 mm, RH = 79.2-80.6%, AL = 498-1010 mm. Soil types were mainly yellow soil, yellow sandy soil, limestone soil, paddy soil and brown soil with pH value at 6-7 and organic matter content ≥1.3%. Thus, we assumed that these conditions were optimal for the growth of high artemisinin-yielding
A. annua L.
A. annua L. is a short-day plant. Non-juvenile plants are very responsive to short photoperiodic stimuli and flower about two weeks after induction. They require about 1000 hours of sunlight per year. Our results suggest that annual sunlight time is a critical factor for the growth of
A. annua L., which is consistent with previous studies [
5,
38]. Previous findings that
A. annua L. requires a strict watering regime during the preliminary growth stages [
5,
39] are also consistent with our results.
Predictive maps
Figures
2 and
3 are the maps derived from the TCM-GIS analyses. The predicted areas were primarily located in the Wuling Mountain region in central China, covering Guizhou, Chongqing, Hunan, Hubei and Sichuan (25°14'-31°38' N to 104°31'-111°51'E). The predicted habitat density was high in northeastern Guizhou, southeastern Chongqing, northwestern Hunan, southwestern Hubei and parts of southern Sichuan.
The total favorable regions (SI 98%-99%) made up 1.60% of China's total land area covering 162 counties and cities (a total of 60,292 km
2), among which Guizhou took the lead with 31,150 km
2 including 68 counties and cities. The most favorable region for
A. annua L. (SI 99%-100%) was in the 58 counties and cities in Guizhou Province with a predicted area of 54,350 km
2. The second largest predicted area (14,330 km
2) was in the 12 counties and cities in Chongqing, followed by Hunan, Hubei and Sichuan (Figure
4). The counties and cities with significant areas of potential habitat are listed in Table
3. The data indicated that Youyang County contained the largest favorable area with more than 4000 km
2. Unexpectedly, the total predicted areas in Wuchuan and Zunyi Counties in Guizhou exceeded 2000 km
2.
One of the world's largest artemisinin manufacturers and its affiliates operate
A. annua L. farms in the Chongqing Wulingshan Mountain Range [
40,
22]. Apart from this, Guizhou may be another important region for
A. annua L. cultivation, particularly in the northeastern part of the province. Our model predicted that 13% of this area is potential
A. annua L. habitats [
41,
42]. Our model did not predict Guangxi Province, known for its habitats of
A. annua L. of relatively low quality, as a region for
A. annua L. cultivation possibly due to the subtropical climate, low altitude and red soil in Guangxi which are very different from those in other
A. annua L. regions in China [
9].
Interviews with the locals suggest that the Guizhou region and Youyang County have comparative advantages for A. annua L. growth with a high-yield variety and minimal pests. Furthermore, the northeastern Guizhou is home to wild populations of A. annua L. which may be an alternative source for artemisinin.
Using the TCM-GIS, we aimed to determine the optimal ecological factors from known habitats and the results showed that RH, TAP, AST, STJU, AAT and SP were important limiting factors. We also aimed to map the distribution of potential regions for the development of A. annua L. in China based on selected climatic, soil and topographical values. Using bioclimatic similarity theory and the TCM-GIS, we predicted the potential growing areas at the county level, particularly in northeastern Guizhou Province. The TCM-GIS is adequate for predicting and identifying potential areas for A. annua L. cultivation.
Using a higher resolution raster and vector spatial databases, we improved the resolution of species distribution considerably on the national surveys conducted in the 1960s, 1970s and 1980s. While most of the survey data were based largely on personal experiences and rough estimates, the model used in the present study is relatively objective.