2.1 | Common gardens
Two common gardens were established in an inland site (44.7˚N, 142.1˚E, 20 m elevation) and a coastal site (45.3˚N, 141.6˚E, 5 m elevation) in northern Hokkaido (Figure S1 in Supporting Information). In each site, the three taxa, Quercus mongolica var. crispula(Qc ), Q. dentata (Qd ), and their hybrids, Q.× angustilepidota (Qa ), were transplanted from natural forests in 32 provenances in Hokkaido (Figure 1, Table S1). Acorns (seeds) of several maternal trees were collected from a forest stand in each provenance, and their seedlings were reared in a nursery in the Dohoku Station of the Forestry Research Institute of the Hokkaido Research Organization. In the inland site located in the Dohoku Station, seedlings were planted at 1.4 m intervals along plantation rows spaced 2.8 m apart in three blocks in 1992–1993, and half of trees in two blocks and a few trees in one block were thinned in 2006, when tree crowns became crowded (Figure S2). In the coastal site on sand dunes at 500 m distance from the seashore, seedlings were planted at 0.8 m intervals along plantation rows spaced 2.8 m apart in two blocks in 1986, and trees were not thinned because tree crowns had been sparse (Figure S3).
We observed living trees in both sites and identified the taxa of trees in 2017–2019 (Figures S2, S3). Because taxonomic identification ofQc and Qd is ambiguous due to intermediate phenotypes (Ohba 2006; Aizawa et al. 2021), we classified the taxa using the following criteria based on hairs on shoots according to our previous studies (Nagamitsu et al. 2019; Nagamitsu et al. 2020). Trees with pubescent one-year-old shoots with trichomes (stellate hairs) were identified as Qd . Among trees with glabrous one-year-old shoots, trees from provenances in coastal habitats, sand dunes and coastal cliffs, were identified as Qa , which was recognized as the coastal ecotype of Qc in the previous studies (Nagamitsu et al. 2019; Nagamitsu et al. 2020), and trees from provenances in inland habitats, hills and mountains, were identified as Qc (Table S1). In addition to living trees, we observed dead trees that had died recently. We were not able to record individuals that had died when they were saplings. We confirmed the provenances of living and dead trees based on plantation records of the common gardens.
The following measurement and collection were conducted in both sites in 2017–2019. We measured the girth (cm) of every stem of each living tree to evaluate tree size. The girth at the breast height in the inland site and that at the ground level in the coastal site were measured, because trees in the coastal site were crooked and branching near the ground due to strong wind. The locations of living and dead trees and the girths of living trees of Qc , Qa , and Qd were recorded in both sites (Figures S2, S3). We collected some branches of trees sampled from living trees in July to obtain DNA and to measure phenotypes. Some flesh leaves on current-year shoots were stored at –20˚C, and DNA was extracted from leaf tissues using DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). Three leaves on current-year shoots and three one-year-old shoots selected from collected branches of each sampled tree were dried and preserved for morphological measurement.