Pteridophytes show the most pronounced latitudinal diversity gradient among plants, and the number of species increases rapidly under a strong tropical climate. In particular, tropical mountainous areas, characterized by year-round warmth, abundant rainfall, and diverse ecological environments, host the highest species diversity of pteridophytes (Moran, 2010). In the case of South Korea, Jejudo Island fits this description, with 228 taxa of native pteridophytes, accounting for 76.8% of the total 298 taxa in the country found on this island (Lee and Lee, 2018). Among these, 56 taxa are evergreen pteridophytes limited to Jejudo Island in South Korea, primarily growing in the warm-temperate forests of the submontane areas of Hallasan Mountain. Most evergreen pteridophytes have restricted distributions (categorized as phytogeographically significant species, grade III or higher), including rare and endangered species (Tables 1–4).

The warm-temperate forests of South Korea traditionally consist of species of forest trees of the Lauraceae and Fagaceae families. The evergreen broad-leaved forests in the southwestern coastal areas, including uninhabited islands, were logged for fuelwood after the Korean War. During the national reforestation efforts of the 1960s and 1970s, they were replaced by secondary forests dominated by pine or mixed pine-oak trees. Today, evergreen broad-leaved forests can be found in only a few locations, such as the valley near Daeheungsa Temple on Duryunsan Mountain in Haenam, the southwestern slopes of Sangwangbong Peak in the Wando Arboretum area, and Gyeokjabong Peak on Bogildo Island.

Fortunately, thanks to 50 years of forest protection, young evergreen broad-leaved trees are growing in the secondary forests, indicating a gradual return to evergreen forests. In the past, warm-temperate forests in South Korea were thought to be limited to Jejudo Island and the southwestern coastal regions. However, a recent survey has revealed that these forests extend as far as offshore islands in Incheon and to the Five West Sea Islands in the Yellow Sea, following the warm currents of the Yellow Sea (Lee and Choi, 2010). The dark and humid forest floor of an evergreen broad-leaved forest serves as the primary habitat for evergreen pteridophytes, and increases in species diversity and an expansion of the distribution range can be expected due to climate change and vegetation succession.

In recent years, research on domestic pteridophytes has made remarkable progress, including the reporting of new taxa and unrecorded species (Moon et al., 2002, 2014; Kim et al., 2004, 2005; Sun et al., 2012, 2019; Lee et al., 2013, 2014, 2015, 2017; Shin et al., 2017; Son et al., 2008; Choi et al., 2024), pteridophyte flora reports (Moon, 2006, 2008), and the publication of field guides (Korea National Arboretum, 2008; Lee and Lee, 2018). In this study, we review the process of the establishment and status of the distribution of the warm-temperate evergreen pteridophyte flora of South Korea based on the above-mentioned domestic literature, many non-Korean studies (Kurata and Nakaike, 1987; Iwatsuki et al., 1995; Wu and Raven, 2013), and on actual specimens (KB, KH, CNU). Information on the domestic distributions of significant species mentioned in this text includes unpublished information from the authors. A phytogeographical overview of the origin of Jejudo Island’s seed plant flora, based on distribution information from Northeast Asia, Southeast Asia, and the Himalayas, has been conducted (Im, 1992), and given the current focus on changes in biodiversity accompanying climate change, a holistic approach to evergreen pteridophyte flora is needed. The scientific names and list arrangement presented in this study follow Lee and Lee (2018).

Generally referred to as southern plants, these species are primarily distributed in the tropical and subtropical regions of Southeast Asia, and their range is extended to the warm-temperate regions of southwestern Japan through southern China and Taiwan due to the influence of warm ocean currents. For South Korea, Jejudo Island is known as the only distribution area, and representative examples among these seed plants include Myrica rubra (Lour.) Siebold & Zucc. and Sarcandra glabra (Thunb.) Nakai (Im, 1992). Cyclosorus dentatus (Forssk.) Ching is widely distributed across tropical regions worldwide, while Crepidomanes latealatum (Bosch) Copel. and Colysis elliptica (Thunb.) Ching have extended ranges from India, Southeast Asia, and southern China to Jejudo Island. Loxogramme salicifolia (Makino) Makino exhibits a distribution pattern from Southeast Asia to Jejudo Island (Table 1, Fig. 1A). These plants are likely to expand their distribution range due to the effects of global warming.

Among tropical and subtropical plants, relatively cold-tolerant groups have expanded their distribution range to colder regions, including the inland areas of the Korean Peninsula (Table 2). Psilotum nudum (L.) P. Beauv., distributed throughout tropical regions worldwide, is known to have northern limits in Tongyeong and Haenam. Pteris dispar Kunze ranges from Southeast Asia to Wando Island and Tongyeong, while Pteris vittata L. is found from India and Australia and along the coast of Korea to Ulsan and Incheon. Microlepia marginata (Panz.) C. Chr.is reported in New Guinea and Southeast Asia, as well as in Ulsan, Ulleungdo, and even inland areas such as Gwangju in Jeollanam-do (the inland distribution in Korea is unpublished information).

Originating from the Himalayan mountainous region, these plants have spread across the East Asian floristic region, extending through Sichuan and Yunnan in China to Japan, and they are primarily distributed on Hongdo Island, Jejudo Island, and the southern coastal islands in Korea. Representative examples among seed plants include Pittosporum tobira W. T. Aiton, Rubus sorbifolius Maxim., and Ilex rotunda Thunb. (Im, 1992), and notable species among evergreen ferns include Crepidomanes schmidtianum (Zenker ex Tasch.) K. Iwats. (Jejudo Island and Mt. Jirisan), Asplenium yoshinagae Makino (Gageodo Island; Lee and Lee, 2018), and Cyclosorus penangianus (Hook.) Copel. (found on Jejudo Island; Lee and Lee, 2018) (Table 3, Fig. 1B).

These plants have evolved in the warm-temperate regions of Northeast Asia, including Japan, Taiwan, and southern China. Representative species include Huperzia cryptomeriana, Athyrium otophorum, and Asplenium antiquum, which are distributed from southern China to southwestern Japan (Table 4, Fig. 1C).

Haenam County, located at the southwestern tip of the Korean Peninsula, has had an average annual temperature of 13.4°C over the past five years (2018–2022), with a winter low of −5.8°C (annual average minimum temperature of 8.2°C) and average annual precipitation of 1,230 mm (Korea Meteorological Administration, 2023). It has also experienced 78.9 frosty days per year (Korea Meteorological Administration and Gwangju Regional Meteorological Administration, 2015) over the past decade (2001–2010). The Haenam Peninsula belongs to a warm region characterized by warm-temperate evergreen broad-leaved forests dominated by species from the laurel family (Lauraceae) and evergreen species in oak family (Fagaceae). However, after the Korean War, most of the evergreen forests in the southwestern coastal region, including Haenam, were logged for fuel wood. During the national reforestation campaigns of the 1960s and 1970s, these forests were replaced with secondary forests of pine or mixed pine and oak. Today, only a few large evergreen broad-leaved forests remain in the southwestern coastal region, such as the Daheungsa Valley on Duryunsan in Haenam and the area around Sangwangbong in the Wando National Arboretum. Fortunately, owing to forest protection efforts over the past 50 years, young evergreen broad-leaved trees are now growing within these secondary forests, showing signs of recovery toward their original state. The dark and humid forest understory of these evergreen broad-leaved forests provides a key habitat for warm-temperate evergreen ferns, necessitating more detailed studies of the fern flora of the southwestern coastal region, including the Haenam Peninsula.

Crepidomanes latealatum (Fig. 2) is a small, evergreen fern distributed from India, Nepal, Sri Lanka, and Vietnam to the Tohoku region of Japan (Kurata and Nakaike, 1987), and Jejudo Island is the only known distribution site in Korea. Crepidomanes latealatum grows on the surfaces of rocks or the lower trunks of trees in the dark and moist understory of evergreen broad-leaved forests. Although similar in appearance and ecology to other species in the family Hymenophyllaceae, such as Hymenophyllum wrightii Bosch, Hymenophyllum barbatum (Bosch) Baker, and Vandenboschia nipponica (Nakai) Ebihara, C. latealatum can be distinguished by its slender rhizomes covered in dense black hairs, developed veins on its leaves, and sporangium cups that split into two parts (Lee and Lee, 2018).

In a recent survey, a new distribution site for C. latealatum was confirmed in the upper reaches of a valley on the western slope of Mt. Duryunsan, within an evergreen broad-leaved forest. This forest is primarily composed of species such as Cinnamomum camphora (L.) J. Presl., Cinnamomum chekiangense Nakai, Machilus japonica Siebold & Zucc., Machilus thunbergii Siebold & Zucc., Neolitsea sericea (Blume) Koidz., Actinodaphne lancifolia (Blume) Meisn., Quercus acuta Thunb., Quercus salicina Blume, Castanopsis sieboldii (Makino) Hatus, Dendropanax trifidus (Thunb.) Makino ex Hara, Zanthoxylum ailanthoides Siebold & Zucc., Camellia japonica L., and Daphniphyllum macropodum Miq.

Crepidomanes latealatum grows on rock surfaces scattered along the valley, and its surroundings are associated with species such as Crepidomanes minutum (Blume) K. Iwats., Lepisorus onoei Franch. & Sav, and Lemmaphyllum microphyllum C. Presl. The surrounding forest understory includes ferns such as Dryopteris championii (Benth.) C. Chr. ex Ching, Dryopteris pacifica (Nakai) Tagawa, Dryopteris saxifraga (Fée) E. H. Crane, Dryopteris erythrosora (D. C. Easton) Kuntze, Cremastra appendiculata var. variabilis (Blume) I. D. Lund, and Lophatherum gracile Brongn..

Haplopteris flexuosa (Fée) E. H. Crane (Fig. 3) is a subtropical and warm-temperate evergreen fern widely distributed from the lowlands of the Himalayas through the Indochinese Peninsula to southern China and central Japan. In Korea, it is sparsely distributed in the warm-temperate forests of the lower slopes of Mt. Hallasan on Jejudo Island, which represents the northern limit of its range (Korea National Arboretum, 2021). Haplopteris flexuosa is classified as Vulnerable (VU) on both the Red List and the Rare Plants Vulnerable (VU) category (National Institute of Biological Resources, 2021). It is confined to Jejudo Island, where only a few populations and individuals have been confirmed, making conservation efforts regarding this rare species urgent. On Jejudo Island, the Donneko Valley is a well-known habitat for H. flexuosa. Here, the typical warm-temperate evergreen broad-leaved forest is dominated by trees of the laurel family (Lauraceae)—including Cinnamomum camphora, Cinnamomum chekiangense, Neolitsea sericea, Machilus thunbergii, and Actinodaphne lancifolia—and evergreen oaks (Fagaceae) such as Quercus acuta, Quercus salicina, and Castanopsis sieboldii. The understory, which is humid and has limited light, supports a variety of evergreen ferns, along with species that include Euchresta japonica Hook. F. ex Regel, Damnacanthus indicus C. F. Gaertn., and Sarcandra glabra (Thunb.) Nakai. In Japan, the distribution of H. flexuosa is also limited to warm-temperate forests west of Mount Fuji (Kurata and Nakaike, 1987), with winter cold being a presumed limiting factor.

In recent surveys, H. flexuosa was found on a rock wall on the southeastern slope of a ridge at around 400–500 meters above sea level in a series of connected peaks running across the Haenam Peninsula. The surrounding area is a deciduous forest dominated by oaks such as Quercus serrata Murray, Quercus variabilis Blume, and Quercus aliena Blume, with accompanying species such as Pourthiaea villosa (Thunb.) Decne., Sorbus alnifolia (Siebold & Zucc.) Decne., Ligustrum obtusifolium Thunb., Mallotus japonicus (L. f.) Müll. Arg., Toxicodendron trichocarpum (Miq.) Kuntz, Prunus verecunda Rupr,, Carpinus turczaninowii Hance, Carpinus tschonoskii Maxim., and Styrax japonicus Siebold & Zucc.. Evergreen broad-leaved trees, such as Eurya japonica Thunb., Camellia japonica, Ligustrum japonicum, Hedera rhombea (Miq.) Bean, Trachelospermum asiaticum (Siebold & Zucc.) Nakai, and Kadsura japonica (L.) Dunal, grow in mixed stands. Young trees of evergreen broad-leaved species that include Machilus thunbergii, Cinnamomum chekiangense, Castanopsis sieboldii, and Quercus acuta are emerging in this area, indicating a transition of the forest toward its original warm-temperate forest composition.

On the rock surfaces where Haplopteris flexuosa grows, other ferns such as Asplenium boreale (Ohwi ex Sa. Kurata) Nakaike, Crepidomanes minutum, Lemmaphyllum microphyllum, Davallia mariesii H. J. Veitch, and Asplenium incisum Thunb. can be found, while the surrounding forest includes ferns such as Dryopteris championii, Dryopteris erythrosora, Dryopteris bissetiana (Baker) C. Chr., Dryopteris chinensis Koidz., Dryopteris lacera (Thunb.) Kuntze, Thelypteris japonica (Baker) Ching, Athyrium yokoscense (Franch & Sav.) H. Christ, Thelypteris glanduligera (Kunze) Ching, Deparia japonica (Thunb) Fraser-Jenk., and Polystichum tripteron (Kunze) C. Presl.

Unlike the Donneko population, where only a few scattered individuals of H. flexuosa grow in the dark, moist understory of an evergreen broad-leaved forest, the Haenam population consists of multiple individuals growing in clusters on rock surfaces within a deciduous broad-leaved forest with better light conditions. The region’s climatic characteristics, including annual precipitation of 1,230 mm and frequent fog, likely contribute to the favorable growth conditions for Haplopteris flexuosa.