Sikang-Yunnan Tropical Seasonal Rain Forest
Concentrated on the lower slopes and in the wet valleys these forests occur on both laterite and limestone soils with only minor changes in floristic composition. They usually have three or four indistinct tree layers with a top canopy of emergent trees reaching heights of up to 45 m. Below this the main canopy layer reaches about 30 m and can have almost complete crown coverage and is rarely less than about 80%. The third layer (5-18 m tall), usually has a crown coverage of less than 40%. Buttresses and cauliflory are common and both lianas and epiphytes are abundant. The suite of dominant trees vary according to changes in local conditions but usually include various combinations of the following: Alphonea monogyna, Pometia tomentosa, and the endemic taxa Celtis philippensis var. wightii (Ulmaceae) and Lasiococca comberi var. pseudoverticillata (Euphorbiaceae). Typical sub canopy species include Canarium pimela Cleidion spiciflorum, Debregeasia squamata, Diospyros hasselly, Ficus altissima, Glycosmis ferruginea, Pseudovaria indochinensis and a variety of endemic species like Aglaia testicularis, Amoora tetrapetala, Dysoxylum lenticellatum (Meliaceae), Aporosa yunnanensis (Phyllanthaceae), Beilschmeidia yunnanensis (Lauraceae), Bombax insignis (Bomacaceae), Cryptocarya acutifolia, Litsea dilleniaefolia (Lauraceae), Ehretia tsangii (Boraginaceae), Elaeocarpus austroyunnanensis (Elaeocarpaceae), Garcinia bracteata (Hypericaceae), Horsfieldia pandurifolia, Knema furfuracea (Myristicaeae), Mitrephora maingayi (Annonaceae), Murraya tetramera (Rutaceae), Sumbaviopsis albicans (Euphorbiaceae), Tapiscia yunnanensis (Staphyleaceae), Tetrameles nudiflora (Tetramelaceae), Walsura yunnanensis (Meliaceae) and the endemic palm Caryota urens (Arecaceae). Loeseneriella lenticellata and the endemic Ventilago calyculata var. trichoclada (Rhamnaceae) are two of the common lianas, and eiphytes are typified by Pothos chinensis and the endemic Rhaphidophora hongkongensis (Araceae). Shrubs are usually sparse on the ground but among the most common are Miliusa tenuistipitata, Sauropus macranthus and the endemic Psychotria siamica (Rubiaceae). The herb layer is not particularly well developed and mainly consists of seedlings and creeping lianas such as the endemic Derris caudatilinba (Fabaceae) and Loeseneriella yunnanensis (Celastraceae).
Sikang-Yunnan Tropical Seasonal Moist Forest
These forests are largely confined to middle and upper slopes from about 650 – 1300 m altitude on limestone as seen, for example, in Xishunangbanna in southern Yunnan. Two distinct tree layers are usually distinguished with the upper canopy reaching heights of 20-25 m. Buttresses and cauliflory are relatively scarce, but woody lianas and epiphytes are common. Many of the latter have small, thick leaves. However, these forests display a somewhat diverse physiognomy and floristic composition due to the great diversity of microhabitats. Additionally, the upper slope forests are largely evergreen whereas on the much dryer lower slopes they tend to be semi-evergreen. In the evergreen zone the dominant tree species are either Dracaena cochinchinensis and Osmanthus polyneurus or the two endemic taxa Cleistanthus sumatranus and Lasiococcus comberi var. pseudoverticillata (both of the Euphorbiaceae). Associated trees include Aglaia parviridus, Garruga floribunda var. gamblei, Mallotus paniculata, Salacia polysperma and several endemic taxa like Amoora calcicola (Meliaceae), Beilschmeidia yunnanensis (Lauraceae), Calona floribunda (Tiliaceae), Diospyros yunnanensis (Ebenaceae), Ficus orthoneura (Moraceae), Laportea urentissima (Urticaceae), Mitrephora thorellii (Annonaceae), Myrsine semiserrata (Myrsinaceae) and Stercula villosa (Malvaceae). The under storey consists of many saplings and creeping lianas such as Hiptage benhalensis and the endemic Loeseneriella yunnanensis (Celastraceae), and many herbaceous elements particularly members of the family Urticaceae such as species of Elatostema and Pilea, and the endemic Procris crenata (Urticaceae). In the semi-evergreen zones the upper canopy is usually deciduous and typically dominated by endemic taxa like Bombax insignis (Bombacaceae), Calona floribunda (Tiliaceae), Garcinia bracteata (Hypericaceae) and Tetrameles nudiflora (Tetramelaceae), while the second layer is largely evergreen. Other associated trees include Eriolaena kwangsiensis, Erythrina lithosperma, Hymenodictyon excelsum, Kydia calycina, Lagerstroemia venusta, Radermachera microcalyx, Spondias pinnata and several endemic taxa such as Millettia tetraptera (Fabaceae) and Pistacia weinmannifolia (Anacardiaceae). The under storey consists of shrubs such as the endemic Colebrookea oppositifolia (Lamiaceae) and Murraya koenigii (Rutaceae), and creeping lianas like the endemic Amalocalyx yunnanensis (Apocynaceae), but epiphytes are infrequent.
Sikang-Yunnan Montane Dwarf Forest
Occurring on limestone hill and mountaintops of Xishuangbanna in southern Yunnan between 900-1600 m these stunted forests grow to no more than about 15 m in height. They usually have just one tree layer but may form both evergreen and semi-evergreen stands depending on moisture levels. The dominant or co-dominant trees include Dracaena cochichinensis and Ficus neriifolia in the semi-evergreen zones and the endemic Myrsine semiserrata (Myrsinaceae) and Pistacia weinmannifolia (Anacardiaceae) in the evergreen zones. Other characteristic tree species include the endemic Pterospermum proteus (Malvaceae). Epiphytes are very common with many orchids such as Bulbophyllum nigrescesns and Eria hainanensis, and a variety of bryophytes and lichens.Among the typical under storey species is the strange endemic Agapetes burmanica (Ericaceae) with its distinctive swollen, water storage roots. Less common species are Clematis kerrii, Fagopyrum tataricum, Kalanchoe laciniata and several endemics such as Bauhinia carcinophylla (Fabaceae), Peperomia heyneana (Piperaceae), Pilea platanifolia (Urticaceae) and Tetrastigma delavayi (Vitaceae).
Sikang-Yunnan Montane Evergreen Oak Forest
Forest dominated by the evergreen oak Quercus longispica typically occur below the Abies forest of Yunnan and range in altitude from about 2400 – 3200 m. The canopy is usually quite low (about 8 m) but very dense with crowns of tree closely touching. Associated trees may include Acer wardii, Cyclobalanus glauca, Magnolia globosa, Pasania leucostachya, Quercus griffithii, Q. serrata, Q. variabilis and Sloana forrestii. The trees are often crooked and festooned with beard lichens such as Usnea longissima. Other epiphytes are common especially the epiphytic fern Drynaria delavayi that adorns virtually every tree, and climbers such as Akebia trifoliate and Ampelopsis heterophylla may also be encountered. The rich shrub layer is often dominated by Pittosporum heterophyllum var. ledoides, but many other species like Euptelea franchetii, Lonicera koehneana, Prunus pilosiuscula, Ribes tenue and Stachyurus chinensis occur. Despite the fact that the forest floors are often covered in a thick layer of fallen leaves the ground layer flora is very rich. Other endemic taxa associated with these forests include Cercis yunnanensis (Fabaceae), Machilus yunnanensis var. duclouxiana (Lauraceae) and Michelia yunnanensis (Magnoliaceae).
Sikang-Yunnan Montane Coniferous Forest
Montane forests dominated by conifers can be found in eastern Sikang and northwestern Yunnan. In the latter they are chiefly dominated by the endemic Abies delavayi (Pinaceae), whereas in Sikang Abies squamata and the endemic Picea likiangensis (Pinaceae) usually dominate. In other respects they are similar and among the less common conifers are various endemic species such as Abies forrestii (Pinaceae), Pinus yunnanensis (Pinaceae) and Tsuga yunnanensis (Pinaceae). Of considerable geobotanical interest is the presence of the ancient relict conifers Metasequoia glyptostroboides (Cupressaceae) and Taiwania cryptomerioides (Cupressaceae). Originally Taiwania was thought to be confined to Taiwan, but relict stands were discovered along the sheltered sides of valleys above 2000 m in the upper Chiu-Kiang, Nu-Kiang and other places. It is a forest giant towering above the forest canopy. Trees of Metasequoia (dawn redwood) are thought to be the remnants of an extensive ancient forest of giant conifers contemporaneous with the giant redwoods of the New World. Many of these often isolated conifer forests are in a completely natural state stocked with many old and mature trees. The tree are often covered in a thick layer of bryophytes and draped with beard lichen (Usnea longissima) giving the canopy a grayish appearance. At ground level the vegetation is varied and maybe completely absent in some of the denser stands where there are thick layers of fallen needles. In other places scattered shrubs of Cotoneaster, Rubus or Spiraea may be present or herbaceous species like Beesia calthaefolia, Bergenia purpuracens, Berneuzia thibetica, Cardamine macrophy, Circaeaster agrestis, Clintonia udensislla, Corydalis cheirifolia, Hemiphragma heterophyllum and Tiarella polyphylla.
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