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.