Subtropical evergreen broad-leaved forests (hereafter EBLFs) refer to lauro-fagaceous evergreen forests, also known as lucidophylls or laurophylls, referring to their often glossy leaves. These forests, dominated by the genera Castanopsis (D. Don) Spach, Lithocarpus Blume, Cyclobalanopsis Oerst. (Fagaceae), Machilus Rumph. ex Nees (Lauraceae), Schima Reinw. ex Blume (Theaceae), Manglietia Blume, Michelia L. (Magnoliaceae), and Distylium Siebold & Zucc. (Hamamelidaceae) are almost exclusively confined to southern, southwestern, and southeastern mainland China, Taiwan, southwestern Japan, southern Korea, and some mountain regions of Vietnam, Laos, Thailand, Myanmar, India, Bhutan, and Nepal (Kira, 1991; Ohsawa, 1993; Tagawa, 1995; Song, 1995; Li, 1997; Tang & Ohsawa, 2009; Song, 2013; Tang et al., 2013). Climatic delimitation of Asian regions has been discussed by investigators (Wolfe, 1979; Ohsawa, 1990, 1993; Kira, 1991; Box, 1995) who have emphasized the seasonal variations in temperature of the monsoon system as the principal determinant of plant distribution patterns. Ohsawa (1995) has provided an explicit template to depict zonal climate-forest patterns along latitudinal and altitudinal gradients in East Asia. As to latitude, southwestern China (SW China including Yunnan, Sichuan, Guizhou provinces and Chongqing Municipality, 21º 08′ 32′′–34º 20′ N, 97º 30′–110º 10′ E) (Fig. 1) falls into the transitional forest zone from tropical mountain to temperate mountain zonation (Fig. 2).

Primary EBLFs once covered most of the subtropical areas of East Asia, but are now greatly diminished as a result of human activities. A recent study found only 5% of China’s total land area to be covered by natural EBLFs (Chen, 1995). In the subtropical regions of China, extensive secondary forests have recently been grown on land used for agriculture or firewood production from the 19th to the middle 20th century, but remnants of the natural EBLFs are still found in remote isolated areas, on steep slopes, in nearly inaccessible or protected areas in nature reserves, and around temples. China, especially Yunnan, is home to the most diversified subtropical EBLFs to be found in East Asia (Tang & Ohsawa, 2009; Tang, 2010; Tang et al., 2013).

In this paper the aim is to present an overview of the distribution patterns of the subtropical EBLFs across SW China as compared with all the Chinese subtropical regions to the east.

China’s subtropical regions are bounded to the north by a line running between the Huai River and the Qinling Mountains (35º N), with a southern limit approaching the Tropic of Cancer. Eastward the subtropics extend to the coastlands and the islands of the East China Sea, the South China Sea and Taiwan; the westward limit is the Chinese national border, extending from the eastern slope of the Tibetan Plateau to southern Yunnan. The subtropical zone thus spans 11–12º from north to south, 28º from east to west (Wu, 1980). China’s subtropical region is divided into three different zones: (1) the southern subtropics (transitional tropics), extending from latitude 22º to 24º or 25º N; (2) the middle subtropics, which lie between 24º or 25º N and 31º or 32º N; and (3) the northern subtropics (transitional subtropics) which cover from 31º or 32º N to 34º or 35º N (Hou, 1983). SW China is located at subtropical latitudes often distinguished as north, middle, and south subtropical (Fig. 3). A small part of southernmost Yunnan falls into the tropical zone. According to altitude, climates in SW China include tropical, subtropical, temperate, alpine and frigid. The climate is dominated by the Asian monsoon system, including the southwestern monsoon (the summer monsoon of India), the East Asian summer monsoon, and the East Asian winter monsoon, with dry continental winds in winter and moist oceanic winds in summer (Fig. 4). In China, generally the oceanic monsoon season begins earlier and ends later in the south than in the north, giving the south a longer rainy season and milder winter winds (Guo & Werger, 2010). In general, Chinese regions east of ca. 103º E are strongly influenced by the East Asian monsoon, while the regions to the west are affected by the Indian monsoon.

Song (2013) offers the general information on the climate of the EBLFs. The annual temperature difference between the warmest and coldest months is smaller in the west (10–14ºC) than in the east (14–27ºC). In Holdridge’s life zone system (1947), bio-temperature (the sum of temperatures >0ºC, if mean monthly temperature >30ºC is calculated as 30ºC) ranges from 12 to 24ºC for the EBLF. In the subtropical EBLF region of China, the bio-temperature is 15–22ºC.

Kira (1945) has proposed a warmth index (WI) and a coldness index (CI) for vegetation zones in East Asia, as follows: WI (ºC) = Ʃ(t − 5) ∙ months; CI (ºC) = Ʃ(5 − t′) ∙ months; t is the mean monthly temperature ≥5ºC and t′ is the mean monthly temperature ≤5ºC. In Japan, a WI of 85‒180ºC ∙ months and a CI of −10‒0ºC ∙ months correspond to the EBLF distribution, and a CI of −10ºC is a decisive northern limit for the EBLF (Kira, 1945; Hattori & Nakanishi, 1985). In China, a WI of 125ºC and a CI of −5ºC determine the northern limit of the EBLF distribution; a WI of 135ºC, and a CI of −2ºC correspond to the northern limit of the EBLF zone (Song, 2013).

In China, the annual precipitation for the EBLF in the eastern region ranges from 1100 to 1700 mm, and is between 800–1000 mm in most areas of the western region (such as central and northern Yunnan and northwestern Guizhou). However, due to the complexity of the topography and varying elevations, the annual rainfall is ca. 1500–1800 mm in southern Yunnan, though it reaches 1500–2800 mm (but 3673 mm in Bapo, Dulongjiang) in some localities of western Yunnan, but in deep river valleys and on lee slopes it is just ca. 600–800 mm. In the western region, 80–90% of the annual precipitation falls from May to October, and the dry season is much more pronounced from November to April as compared with the east. The mean annual potential evapotranspiration for the subtropical EBLF region is ca. 800–1200 mm, and it is greater in the west than in the east. The Sichuan Basin area has the lowest evapotranspiration (Gao et al., 2006).

Li (1997) divided the EBLF of East Asia into four regions: the Japanese, eastern Chinese, western Chinese, and western Chinese sclerophyllous distribution regions. The evergreen sclerophyllous forest should be considered apart from the subtropical EBLF, because the morphology, physiognomy, and structure of their leaves are different. The subtropics from 33º to 35º N constitute a transitional zone from EBLFs to deciduous broad-leaved forests. Zhu (2013) considered a line at ca. 22º 30′ N to be the northern biogeographic boundary of the tropical zone in southern China. The evergreen broad-leaved forest of southernmost Yunnan is considered to be a tropical montane evergreen broad-leaved forest (Zhu, 2007; Zhu & Yan, 2009). The subtropical EBLFs exclude northern tropical rain forests, which are basically tropical in nature but with some obvious transitional characteristics from tropical to subtropical floras, and are found at low altitudes in southernmost Yunnan (e.g. Xishuangbanna in S Yunnan, Xilongshan and Daweishan in SE Yunnan, Nangunhe in SW Yunnan). But here I do include the tropical montane evergreen broad-leaved forest of S, SE and SW Yunnan so as to take into account the altitudinal transition from the tropical rain forest to the subtropical EBLF. Song (2013) asserted that the subtropical EBLF of China corresponds to the region from 23º to 33º N and from 98º to 123º E. In fact, within Yunnan the forest extends west to ca. 97º 32′ E (e.g. Dulongjiang), and within Tibet it extends to 95º E (e.g. Chayu and Motuo in Yarlung Zangbo River). The most eastern longitude is 122º E in Taiwan. In sum, 95º–122º E is the longitudinal range of the EBLF within China. The distribution range of EBLFs in China is proposed in Fig. 5.

A detrended correspondence analysis (DCA) (Fig. 6) for the plots of natural mature EBLFs of SW China, including Yunnan, Guizhou and the Sichuan Basin, that have been free of major disturbance during the past 60 years, indicates that along Axis 2 from the low to the upper side with increasing moisture, four forest types are roughly distinguished: the semi-humid EBLF (SH), the monsoon EBLF (MS), the humid EBLF (HE) and the mid-montane moist EBLF (MM). Axis 1 may indicate a gradient of topography or soil properties.

In SW China, most significantly, four forest types are representative of the major subtropical EBLFs:

The montane mossy dwarf forest as classified by former researchers (e.g. Wu, 1980; Wu et al., 1987; Shi & Zhu, 2009), dominated by species of Rhododendron L. along with some dwarf trees of the Fagaceae, Vacciniaceae, Rosaceae, Aceraceae, grows in special topo-habitats such as near mountaintops. This is not considered a type of the subtropical EBLF in this paper.

Southwestern China

Yunnan

The spatial patterns of distribution of dominant evergreen broad-leaved communities along latitudes, longitudes, and altitudes in Yunnan, as represented by 22 mountains, are depicted in Fig. 7. In Yunnan, secondary forests such as coppice woods and fast-growing Pinus L. (e.g. Pinus yunnanensis Franch. and Pinus armandii Franch.) and Eucalyptus (e.g. Eucalyptus smithii R. T. Baker and Eucalyptus globulus Labill.) plantations, as well as agricultural fields are common at low altitudes, while remnants of the natural subtropical EBLF are only found in isolated mountains, on steep slopes, and temple grounds. The semi-humid EBLFs (SH) dominated by Castanopsis orthacantha Franch., Cyclobalanopsis glaucoides Schottky, Cyclobalanopsis delavayi (Franch.) Schottky, or Lithocarpus dealbatus (Hook. f. & Thomson ex Miq.) Rehder are found at (1500)1900‒2400 m in central, south-central, and eastern Yunnan, including Jizhushan, Huafushan, Shizishan, Qiongzhushi, Wuliangshan, eastern Ailaoshan, Zhujiangyuan, Laojunshan of Wenshan, Wumengshan, and Yaoshan (shan means “mountain”).

The monsoon EBLFs (MS) are mainly distributed at (800)1000‒1800 m in southern Yunnan (e.g. Tongbiguan, Nangunhe, Beihualin, Yongde Daxueshan, Wuliangshan, Ailaoshan, Xilongshan, Daweishan, and Xishuangbanna) and in the province’s north-west (e.g. Dulongjiang, and Fugong). Most of them are dominated by species of Castanopsis and Lithocarpus with some Trigonobalanus Forman and Cyclobalanopsis, as well as Lauraceae and Theaceae, such as Castanopsis hystrix Hook. f. & Thomson ex A. DC., Castanopsis fleuryi Hickel & A. Camus, Castanopsis calathiformis Rehder & E. H. Wilson, Lithocarpus truncatus (King ex Hook. f.) Rehder & E. H. Wilson, Lithocarpus polystachyus (Wall. ex A. DC.) Rehder, Lithocarpus fenestratus (Roxb.) Rehder, Trigonobalanus doichangensis (A. Camus) Forman, Cyclobalanopsis augustinii (Skan) Schottky, Cyclobalanopsis kerrii (Craib) Hu, Cryptocarya calcicola H. W. Li, Cryptocarya densiflora Blume, Beilschmiedia yunnanensis Hu, Schima wallichii (DC.) Korth., and Anneslea fragrans Wall.

The mid-montane moist EBLFs (MM) are distributed at (1600)1800‒2500(2800) m from the east to the west of Yunnan, and from southern to south-central, and northern Yunnan, but not in central Yunnan, as on Jizushan, Huafushan, Shizishan, and Qiongzhusi, where no such moist EBLFs grow. Compared with the former two forest types, in the mid-montane moist EBLFs are to be found, besides dominant species of Fagaceae [e.g. Cyclobalanopsis lamellosa (Sm.) Oerst., Cyclobalanopsis oxyodon (Miq.) Oerst., Cyclobalanopsis myrsinifolia (Blume) Oerst., Lithocarpus variolosus (Franch.) Chun, Lithocarpus hancei (Benth.) Rehder, Lithocarpus pachyphyllus (Kurz) Rehder, Lithocarpus xylocarpus (Kurz) Markgr., Lithocarpus echinotholus (Hu) Chun & C. C. Huang ex Y. C. Hsu & H. W. Jen, Castanopsis echidnocarpa A. DC., Castanopsis wattii (King ex Hook. f.) A. Camus, and Castanopsis remotidenticulata Hu], Lauraceae [Machilus longipedicellata Lecomte, Machilus viridis Hand.-Mazz., Cinnamomum iners Reinw. ex Blume, Phoebe faberi (Hemsl.) Chun], Theaceae (Schima khasiana Dyer, Schima argentea E. Pritz., and Schima villosa Hu) and Magnoliaceae [Manglietia kungshanensis Law, Manglietia insignis (Wall.) Blume, and Alcimandra cathcartii (Hook. f. & Thomson) Dandy] also appear as co-dominants. The mid-montane moist EBLF is characterized by abundant lianas, ferns and mosses on tree trunks and branches.

Sichuan, Guizhou, and Chongqing Municipality

Figure 8 shows the spatial patterns of distribution of dominant evergreen broad-leaved communities along latitudes, longitudes, and altitudes in Sichuan, Guizhou, and Chongqing Municipality, as represented by 21 mountains. In these areas, natural EBLFs at low altitudes have been even more severely reduced by human activities than in Yunnan. Here secondary forests, shrublands, and plantations of pines (Pinus yunnanensis in the west of Sichuan and Guizhou, Pinus massoniana Lamb. in the east of Sichuan and Guizhou as well as Chongqing) or Cunninghamia lanceolata (Lamb.) Hook. or Cryptomeria japonica (Thunb. ex L. f.) D. Don var. sinensis Miq. (formerly Cryptomeria fortunei Hooibr.) prevail. Some semi-humid EBLFs dominated by Castanopsis delavayi Franch. or Cyclobalanopsis glaucoides are found in western Sichuan (e.g. Jinyangxian, Xiaoxiangling). In these areas, the typical humid EBLFs (HE) prevail. The dominants are Castanopsis, Lithocarpus, and Cyclobalanopsis [e.g. Castanopsis carlesii (Hemsl.) Hayata var. carlesii, Castanopsis fargesii Franch., Castanopsis eyrei (Champ. ex Benth.) Tutch., Castanopsis carlesii (Hemsl.) Hayata var. spinulosa W. C. Cheng & C. S. Chao, Castanopsis platyacantha Rehder & E. H. Wilson, Lithocarpus corneus (Lour.) Rehder, Lithocarpus cleistocarpus (Seemen) Rehder & E. H. Wilson, Cyclobalanopsis oxyodon, and Cyclobalanopsis multinervis W. C. Cheng & T. Hong], and many species of Lauraceae [e.g. Phoebe neurantha (Hemsl.) Gamble, Phoebe zhennan S. K. Lee & F. N. Wei, Lindera pulcherrima (Nees) Hook. f. var. hemsleyana (Diels) H. P. Tsui, Lindera megaphylla Hemsl., Lindera pulcherrima (Nees) Hook. f. var. pulcherrima, Cinnamomum longepaniculatum (Gamble) N. Chao ex H. W. Li, Machilus pingii W. C. Cheng ex Yen C. Yang, Machilus ichangensis Rehder & E. H. Wilson, and Actinodaphne omeiensis (H. Liu) C. K. Allen], and Theaceae [Schima superba Gardner & Champ., Schima argentea, Gordonia sichuanensis H. T. Chang, and Gordonia chrysandra Cowan], and Magnoliaceae [e.g. Michelia martini (H. Lév.) Finet & Gagnep. ex H. Lév. and Manglietia fordiana Oliv.] as well as Elaeocarpaceae [e.g. Elaeocarpus japonicus Siebold & Zucc. and Elaeocarpus sylvestris (Lour.) Poir.].

To summarize: in SW China, the EBLFs are mainly distributed at higher altitudes [1000‒(2400)2800 m] in the western areas of Yunnan and Sichuan, while occurring at lower altitudes [600‒1500(2000) m] toward the southern and the eastern areas (Figs. 7 and 8). Communities dominated by Castanopsis are widely distributed over the region of SW China. In Yunnan, many plant communities dominated by Lithocarpus and/or Cyclobalanopsis are conspicuous compared with those in the Sichuan Basin and eastern Guizhou.

Eastern China

In Figure 9, representing 23 mountains of eastern China, we find that the Sichuan Basin, eastern Guizhou, and eastern China share some dominant canopy tree species of Fagaceae [e.g. Castanopsis carlesii, Castanopsis fargesii, Castanopsis eyrei, Castanopsis tibetana Hance, Lithocarpus glaber (Thunb.) Nakai, Cyclobalanopsis glauca (Thunb.) Oerst., Cyclobalanopsis myrsinifolia], Lauraceae (e.g. Phoebe neurantha), Theaceae (e.g. Schima superba) and Elaeocarpaceae (e.g. Elaeocarpus sylvestris). In the understory, they often share some genera including Ilex L. (Aquifoliaceae), Lindera Thunb., Litsea Lam. (Lauraceae), Camellia L., Eurya Thunb. (Theaceae), Symplocos Jacq. (Symplocaceae), and Vaccinium L. (Ericaceae). However, these forests lack the flourishing epiphytes and lianas on tree trunks and branches, as compared to the mid-montane moist EBLF of Yunnan. Additionally, in the Yunnan area there are more types of plant communities dominated by Lithocarpus or Cyclobalanopsis or Castanopsis than in eastern China, where the chief dominant genus is Castanopsis. The upper limits of the EBLFs are mainly 600‒1500 m in eastern China (but 2500 m in Taiwan), much lower than in western Yunnan and western Sichuan (2400‒2800 m) (Fig. 10).

As an evidence of environmental effects on species diversity of the EBLF ecosystems, Tang et al. (2013) provided species diversity models comparing the EBLF ecosystems of Yunnan and Taiwan (Fig. 11A, B). In Yunnan, the monsoon EBLF in the mesic-humid sites tends to have higher species richness and diversity (Fig. 11). Along the altitudinal gradients in Taiwan, higher diversity is found in the Machilus–Castanopsis zone at the middle altitudes (500–1500 m) and in the lower Cyclobalanopsis zone at the mid-high altitude (1500–2000 m) (Fig. 11B) (see Fig. 9 for the altitudinal distribution of dominant evergreen broad-leaved tree communities in Taiwan). Species diversity, as influenced by temperature and precipitation, varies with altitude. Thus, altitude in the EBLFs of Taiwan may be a proxy for temperature, because the annual rainfall is abundant (1800–3000 mm) throughout the EBLF zone. In short, site moisture is the best predictor of species composition and diversity in the EBLFs of Yunnan, while temperature along altitudes plays a decisive role in Taiwan. For forest ecosystems, various temporal trends in diversity have been observed during succession, including increases, decreases, and one or more peaks with passing time (e.g. Auclair & Goff, 1971; Peet, 1978; Halpern & Spies, 1995; Turner et al., 1997). Relationships between diversity and succession are complex and should be seen as system-specific. Secondary forests under varying degrees of human disturbance, such as coppice woods or those growing on abandoned farmland, exhibit a range of stages of plant succession in the EBLF zones of both the Yunnan and Taiwan areas. The peak in diversity in the stands at the middle successional stage of each study site in Yunnan and Taiwan (Fig. 11A, B) reflects the persistence of species with varying attributes, including fast growth, slow growth, light demands, limited or greater shade-tolerance, contributing to significantly higher species diversities as compared with the early successional stage. In general, as a forest develops, diversity in the late stage tends to decrease with the disappearance of short-lived light-demanding pioneer species (e.g. Pinus, Alnus Mill., Populus L., Carpinus L., Acacia Mill., Mallotus Lour., and Diospyros L.); however, in both Yunnan and Taiwan the differences in species diversity between the middle and late successional stages are not significant, which reflects the recruitment of shade-tolerant evergreen trees (e.g. Cyclobalanopsis, Castanopsis, Machilus, Ficus L., Manglietia, and Michelia) into the overstory.

This brief comparison clearly shows the effects of moisture, altitude, and disturbance/succession on plant species diversity patterns in the two EBLF ecosystems of Yunnan and Taiwan.