INTRODUCTION
The domestication of domestic native tree species and the introduction of foreign tree species are important contents of modern forestry, which contribute to the development of modern tree introduction theory is particularly important (Wang and Jiang, 1995). The remote migration, geographical variation, and demonstration mechanism of tree species are important aspects of forest introduction theory and an important basis for biodiversity conservation (Morgenstern, 1996). Understanding the geographical variation and adaptability of tree species, the stability of excellent provenances, and determining the suitable cultivation area of tree species are important scientific issues in the research of tree genetic breeding (Wang and Jiang, 1995; Morgenstern, 1996; Zenni et al., 2016).
The study of geographical variation of trees begin in the middle of the 18th century, taking Pinus sylvestris L. as the research object, the results showed that there were general differences in growth and adaptability of geographical germplasm from different distribution areas of Europe in French experimental forest, and further determined the geographical germplasm with the highest wood yield (Turnball et al., 1986). The study of the geographical variation of forest tree species in China begin in the 1980s (Xu and Fen, 1978; Deng et al., 1981), and the seed regions of the main tree species were divided around the 1990s (Chen, 2001). Before that, the allocation scope of forest seeds in each producing area had not been determined, and there was a general cross-provincial allocation of some tree species for afforestation, which brought difficulty for high-quality plantations achieving. More alarmingly, the seeds produced by these unidentified plantations and their offspring will continue to be allocated for afforestation, which will threaten the survival of tree species in the long run (Boshier et al., 2015). The geographical variation and domestication of foreign germplasm in the introduction place may lead to its gradual domestication into new germplasm adapted to local natural growth and cultivation. Therefore, it has important theoretical guiding significance to research geographical variation, germplasm traceability, domestication and adaptation of plantation, progressively reveal the germplasm source of the plantation population, and understand the suitability of foreign germplasm in the introduction place, for the follow-up correct allocation of plantation germplasm, promotion of high-quality plantation construction, regional germplasm resources collection, and plantation cultivation (Shang, 2019).
The Chinese pine (Pinus tabuliformis Carr. ) is an important native timber tree species widely distributed in northern China (Niu et al., 2022; Niu et al., 2022). The study on geographical variation (Xu et al., 1981; Xu and Tang, 1984) and provenance test (Li, 1983; Wen et al., 1986) of Chinese pine begin in 1981, and there were significant differences in adaptability and growth among different provenances, which showed the regional specificity of different provenances (Dong et al., 1991; Sun et al., 1991; Liu et al., 2008; Zhang et al., 2010). Natural and neutral selection were the main reasons for generating the variation in the natural populations of Chinese pine (Meng et al., 2013; Xia et al., 2018). Shanxi Province was the core distribution area of the natural populations of Chinese pine in China, which could be demonstrated by geographical investigation of the natural population distribution in China showed Shanxi Province with the wide distribution of natural population (Xu et al., 1981; Fu et al., 1988), and Shanxi Province was the refuges for the Chinese pine population during Last Glacial Maximum (LGM) (CHEN et al., 2008; Hao et al., 2018). Due to the long-term war in modern China and the large demand for timber in the process of revival, the natural forests in northern China represented by Beijing, Hebei, and Liaoning Provinces were excessively felled, the large-scale plantations of Chinese pine in these areas began in the 1950-1960s (these stands were sometimes mistaken for natural forests in some studies), and the stand archives showed that most of the germplasm came from Shanxi Province, but the exact provenance was not clear. Defining the exact germplasm sources of the main Chinese pine plantation populations in northern China could provide a reliable theoretical basis for the scientific allocation, management and utilization of Chinese pine germplasm, and promoting the efficient cultivation of the Chinese pine populations (Boshier et al., 2015; Shang, 2019).
In recent years, we analyzed the genetic structure and geographical variation of Chinese pine natural populations in five mountains of Shanxi Province based on SSR markers (Wu et al., 2018), and preliminarily understood the genetic relationship between the main plantation populations in Beijing and the natural populations in Shanxi Province (He et al., 2020). Recently, we had a more comprehensive understanding of the genetic background of Chinese pine in northern China for the first time, determined that Shanxi natural populations had ancestral status in northern China, and “Taiyueshan” was the core distribution area of Chinese pine natural population in Shanxi Province, confirmed that most of the germplasms of Chinese pine plantations in northern China (represented by Liaoning and Hebei province) may came from Shanxi Chinese pine natural populations, revealed that the new adaptive variations and the strong gene flow “driving force” from local natural populations had brought the opportunity for Shanxi germplasms to invade Hebei and Liaoning Provinces successfully (Zhou et al., 2022). However, due to the limitation of SSR markers, we could arduously determine the exact germplasm source of Chinese pine plantations in northern China, and the ”point-by-point” tracing of the plantation population was still a very challenging task.
Mitochondrial DNA (mtDNA) has the characteristics of conservative variation (Held and Patel, 2020), which has been widely used in understanding genetic diversity, genetic structure, and haplotype characteristics of the populations (Liu et al., 2020), determining the evolution history (Ye et al., 2019), geographical origin (Kang et al., 2021) of species, and so on. In the genus Pinus, mtDNA is maternally inherited and dispersed through seeds (Neale et al., 1989), has been widely used as a genetic marker in research, including determining the refuge of Chinese pine and Picea mariana during the LGM (Jaramillo-Correa et al., 2004; CHEN et al., 2008; Hao et al., 2018), revealing the origin of Pinus densata (WANG et al., 2011), and so on. In this study, 796 (873) individuals representing 35 (38) populations of main natural and artificial Chinese pine populations in northern China (5 groups including Shanxi Province natural populations, SX*, Hebei Province natural and plantation populations, HB* and HB, Liaoning Province natural and plantation populations, LN* and LN) were selected and analyzed based on two mtDNA markers. Our aims were to ① reveal the genetic diversity, genetic structure and haplotype characteristics of five groups, ② and further tracing the germplasm source of HB and LN plantations exactly.