Results

Flora extraction and taxon distribution

After the coordinates of the plant specimens were converted to floristic composition data for the 771 grid cells, the phytogeographic structure of the southern part of the Korean Peninsula was divided into four zones (maximum silhouette coefficient: 0.7968, Table 1, Figs. 3 and 4). The peninsula was divided into three inland zones—the cold floristic zone (hereafter, Zone I), corresponding to the high-altitude regions in the central part of the peninsula; the cool floristic zone (hereafter, Zone II), corresponding to the high-altitude regions in the southern part of the peninsula; and the warm floristic zone (hereafter, Zone III), corresponding to the lowlands in the central and southern parts of the peninsula—and the maritime warm floristic zone (hereafter, Zone IV), including Jejudo and Ulleungdo.
Although these zones formed large patches and occupied large areas, smaller patches were scattered within the other floristic zones (Fig. 4). This is because Zone III, which was connected to Zone I, contained most of the major cities in a given region or broadscale production (forestry and agriculture) regions. Our study region also included the military demarcation line between North and South Korea. The region near the military demarcation line (near the 38th parallel, length, 248 km; area, 907 km2) is characterized by a high level of disruption and environmental management activities.
Zone III—broadly covering the lowlands of the southern part of the Korean Peninsula—occupied the largest area (72.0%), followed by Zones I (12.3%) and II (11.5%) in regions with high-altitude mountains, and Zone IV (4.2%), which included coastal and island regions (Table 1, Fig. 4). Unlike the area gradient, the order of species abundance (from highest to lowest) was: Zone III (2,379 taxa), Zone IV (2,200 taxa), Zone I (1,700 taxa), and Zone II (1,668 taxa). The species abundance per classified grid cell was highest in Zone I (464.7 ± 9.9 taxa), followed by Zone II (331.7 ± 11.5 taxa), Zone IV (298.6 ± 14.0 taxa), and finally Zone III (79.3 ± 3.4 taxa), which contained a large number of developed regions.

Geographic range and climatic environment in each floristic zone

All the floristic zones showed significant differences in the mean longitude and latitude, mean annual temperature, annual precipitation, WI, and CI per grid cell (p < 0.001, Fig. 5a–f). While Zone I was focused in the central part of the Korean Peninsula, Zone II was mostly situated in the center of the southern part of the peninsula, although there were scattered small patches within the central region adjacent to Zone I or inside Zone III (Figs. 4 and 5a–b). Zone IV was mostly located on the coast and islands in the west and south of the peninsula, as well as some inland areas in the west. Zone III showed a relatively broad longitudinal and latitudinal range, since it was distributed throughout the study region across the whole southern part of the Korean Peninsula.
When the climatic environments were compared between the floristic zones, differences were observed in mean annual air temperature, mean precipitation, WI, and CI (Fig. 5c–f). Mean annual air temperature showed a gradually increasing trend from Zones I to IV. Annual precipitation showed the opposite trend, but there was no clear difference between Zones I and II. As indirect climate factors, mean WI and CI also showed clear differences between zones. In accordance with its wide geographic distribution, Zone III showed the largest ranges for these climate factors.

The physical environment in each floristic zone

The parent materials in the southern part of the Korean Peninsula are predominantly acidic rocks (48.5%) and metamorphic rocks (26.3%), and the ratio of plateaus is very low. The most common soil depth is 50–100 cm (39.1%), and 93.7% of the soil consists of silt and sandy loam (51.6%) or clay loam (42.1%) (Ministry of Land Infrastructure and Transport, 2016) (Appendix A4).
The parent materials across the four floristic zones consisted of over 70% acidic and metamorphic rocks; however, Zone IV, which includes the volcanic island of Jejudo and the oceanic island of Ulleungdo, showed an especially high ratio of volcanic ash (23.7%) among its parent materials (Table 2). In addition, Zone IV showed a far higher proportion of lava terrace as its terrain type and clay loam for its soil type, and the soil depth classes were more evenly distributed compared to the other zones (Fig. 6).

The relationships between species composition in each floristic zone

A total of 1,099 common taxa was found in all zones (Fig. 7). Interestingly, the zone with the most specific taxa was Zone IV (404 specific taxa, 18.4%), which had the smallest area. This was followed by Zone III (192, 8.1%), Zone I (72, 4.2%), and Zone II (25, 1.5%).
When specific taxa were analyzed at the family level (Fig. 8), Zone III—generally consisting of low-lying hilly terrain heavily affected by human activity and development—showed higher diversity of Poaceae (18 out of 192 taxa) compared to Zones I and II, while the marine and coastal Zone IV was characterized by a high diversity of Orchidaceae (27 out of 404 taxa), Asteraceae, and Rosaceae. Among the common species appearing in all zones, the families with the highest diversity, in descending order, were Asteraceae (113 taxa), Poaceae (98 taxa), Cyperaceae (68 taxa), Fabaceae (55 taxa), and Rosaceae (47 taxa) (Fig. 8).