There are two opposing views on forest responses to the Quaternary climatic changes in East Asia (Qian & Ricklefs, 2000; Harrison et al., 2001). Qian & Ricklefs (2000) suggested that multiple refugia for temperate forests might have existed in coastal areas in the north of China during the Last Glacial Maximum (LGM), which may have promoted the current species diversity through allopatric speciation.
Moreover, temperate forests would have extended across the continental shelf to link populations in China, Korea and Japan during glacial periods, whereas higher sea levels during interglacial periods isolated these regions. Conversely, palaeovegetation data from east Asia show that temperate forests in these regions were considerably more restricted than today and would have retreated southward to c. 30°N during the LGM, calling into question the existence of northern refugia and the coalescence of tree populations required by the hypothesis of Qian & Ricklefs (Harrison et al., 2001). It is not possible to determine whether temperate forests of eastern Asia coalesced or fragmented during the LGM without more detailed information (Qian & Ricklefs, 2001). Fortunately, molecular evidence has provided an effective approach, independent of fossil information, for testing the range dynamics of most organisms during the Quaternary (Avise, 2000), as climatic oscillations have left genetic signatures in current populations (Hewitt, 2000, 2004).
Tian et al. (2009) examined the phylogeographical pattern of a temperate deciduous shrub species (Ostryopsis davidiana) in northern China. They found multiple refugia were maintained across the range, contrasting with the conclusions of Harrison et al. (2001) that temperate forests would have retreated southward to 30°N during the LGM. Chen et al. (2008) reached similar conclusions concerning a cold-resistant conifer species, Pinus tabulaeformis. Until now, there were no independent phylogeographical studies of temperate deciduous tree species in eastern Asia to test the two hypotheses, especially in northeastern and northern China as well as Japan and Korea. Northeastern China (NEC) is a megadiversity area with complex topography within 40–50°N, adjacent to the Korean Peninsula, including the NEC plain and the major mountain ranges, such as the Daxing’anling Mountain range and the Changbai Mountains (China EPA, 1998; Xu et al., 1999) (Fig. 1). Northern China is an important part of the north–south vegetation transect, south to the Qinling Mountains and north to the Yanshan Mountain, including the northern China Plain and the Taihang Mountains (Ren, 1985) (Fig. 1). Both regions are a mosaic of mountains and were characterized by a relatively mild Pleistocene climate (Weaver et al., 1998; Ju et al., 2007), potentially hosting microclimatic zones capable of supporting a variety of habitats in relative stability (Qian & Ricklefs, 2000). Thus, glacial refugia may have been available in these regions for East Asian species. However, another possibility that cannot be ruled out is that a refugium might have been present in the southern Korean peninsula and/or Japan, and the species colonized NEC from there during postglacial times. More phylogeographical studies in these regions would contribute to a resolution of this issue.
Most phylogeographic studies of plants have been based on chloroplast DNA (cpDNA) and have revealed genetic heterogeneity throughout the range of a species and allowed an inference of historical range shifts and recolonization routes (Taberlet et al., 1998; Wares & Cunningham, 2001; Petit et al., 2003). Although the merits of the nonrecombinant and maternally inherited cpDNA for phylogeographic studies in most angiosperms have been demonstrated, the genetic structure only reflects the history of a single gene (Hey & Machado, 2003). In addition, cpDNA can only reveal seed gene flow, providing no information about pollen gene flow, a main component shaping the organization of genetic diversity within and among populations. Liepelt et al. (2002) showed, using cpDNA and mtDNA markers, that a high level of pollen-mediated gene flow between refugia in a conifer species (Abies alba) eliminated the genetic imprints of Pleistocene refugial isolation. Recently, it has been shown that very different phylogeographic structures can be resolved by DNAs that contrast in rates of gene flow (Currat et al., 2008; Du et al., 2009; Zhou et al., 2010). In view of these findings, it is desirable that plant phylogeographic studies should employ both nuclear and cpDNA markers, as use of both types of marker could reveal more information than a single type of marker about the history of range shifts and postglacial gene flow among refugia during Pleistocene climatic oscillations (e.g. Schonswetter et al., 2005; Alsos et al., 2007; Edh et al., 2007).
In this study we examined the phylogeographic patterns of Juglans mandshurica , a temperate deciduous tree distributed in northern and northeastern China, and locally scattered in the Russian Far East, Korea and Japan. This species is wind-pollinated, and its fruits (walnuts) typically fall in the vicinity of parental plants, so seed dispersal distance is generally much more limited than pollen. Flowering is heterodichogamous (Bai et al., 2006), that is there are two temporal morphs (protandry and protogyny) within a population, with the flowering periods of the two morphs reciprocal and synchronous. We used cpDNA, which is maternally inherited in Juglans species (Potter et al., 2002), as well as10 nuclear microsatellite loci, which were biparentally inherited. Data from the two marker systems were compared at the population level in the phylogeographic analysis of J. mandshurica. The main goals of the study were: to infer the existence and locations of past glacial refugia for J. mandshurica; to reconstruct its postglacial history and gene flow according to the two different molecular marker systems; and to ascertain whether temperate forest communities merged or fragmented during glacial periods.