Forest Structure,Dynamics And Population Persistence Of Tertiary Relict Taiwania Cryptomerioides On The Gaoligong Mountains,Yunnan,SW China

Taiwania cryptomerioides is the one of China’s Tertiary relict tree species. It grows on steep slopes in remote mountains. Little research has been done toward understanding the structure and regeneration dynamics of Taiwania forests and their habitat preferences. An extensive primeval Taiwania forest in the eastern slope of the northern Gaoligong Mountains has recently been found. For this paper, forests containing Taiwania cryptomerioides of various ages and diverse habitats on the eastern slope of the Gaoligong Mountains were studied in terms of forest structure and composition, population structure (size, age), regeneration patterns and population persistence. Taiwania forests and population structure in various regions of East Asia were compared, Our results can provide a scientific base for the persistence and conservation of Taiwania and other Tertiary relict tree species with similar ecological characteristics. The main results are as follows:(1) We grouped46plots into five forest groups based on floristic similarity cluster analysis. They are Taiwania forest (Group1), Alnus-Taiwania-Betula forest (Group2), Pinus-Taiwania forest (Group3), Lithocarpus-Tsuga-Cyclobalanopsis forest (Group4) and Cyclobalanopsis-Manglietia-Taiwania forest (Group5).(2) In the Taiwania forest (Group1), Taiwania overwhelmingly dominated the forest, becoming emergent at40-70m and rising above a forest canopy comprised of evergreen broad-leaved trees, where competition from other plants is limited. In the other three forest groups(Alnus-Taiwania-Betula forest, Pinus-Taiwania forest and Lithocarpus-Tsuga-Cyclobalanopsis forest), Taiwania along with other species dominated the canopy and subcanopy layers (20-40m).(3) The maximum age of Taiwania was calculated to be c.1872years, with358cm DBH (diameter at a height of1.3m) and70m high. Relationship between diameter at a height of1.3m and age at stem base for Taiwania cryptomerioides is:x=0.0103y2+1.5414y, where n=57, x is Age, y is DBH. The overall age-class of Taiwania were of the reverse-J type. But in the Taiwania forest (Group1), the size and age classes of Taiwania were multimodal, indicating that the regeneration varied by chance, depending on episodic natural disturbances. In the Alnus-Taiwania-Betula forest (Group2) and Pinus-Taiwania forest (Group3), Age classes were also of the reverse-J type, which showed that regeneration was good. This also indicates that Taiwania depends on certain degrees of disturbances, both natural and by humans. In the Lithocarpus-Tsuga-Cyclobalanopsis and the Cyclobalanopsis-Manglietia-Taiwania forests where there were no signs of disturbance, Taiwania was of the unimodal type, showing regeneration to be very poor.(4) Taiwania thrives in unstable habitats on riverbanks in deep valleys, on steep slopes, on cliffs, on roadsides, or by mountain paths, on the eastern slope of the Gaoligong Mountains at1700-2500m asl. All these locations were subject to frequent landslides. Taiwania seedlings/saplings were only found in disturbed and unstable microhabitats, as in canopy gaps, on roadsides and on cliffs. This suggests regeneration maintained by various occasional disturbances with erosive forces keeping the habitats relatively open. Its seeds are minute and can take advantage of wind-dispersal to fall on the forest floor, giving the shade-intolerant and long-lived coniferous Taiwania the chance to regenerate in unstable habitats keep away from competition with othe plants. So Taiwania depends on occasional disturbance for regeneration.(5) In the Gaoligong Mountains, Taiwania is mainly associated with various evergreen broad-leaved tree species, typically including Manglietia insignis, Cyclobalanopsis gambleana, and Lithocarpus variolosus. In Guizhou and Hubei, Taiwania mixes with pioneer coniferous Pinus massoniana and Cunninghamia lanceolata. In Taiwan, Taiwania grows along with coniferous Chamaecyparis formosensis. Taiwania forests are old-growth forests in the Gaoligong Mountains and in Taiwan, while in Guizhou and Hubei they grow in degraded habitats.(6) Regeneration of Taiwania in the Gaoligong Mountains is stronger than in Guizhou, Hubei and Taiwan. This is probably because of frequent natural disturbances, mainly landslides, in the Gaoligong Mountains. The ecological traints of Taiwania are similar to those of other Tertiary relict tree species growing in unstable habitats. In order to protect Taiwania, We need to protect the natural habitat. It may be possible to create some secondary terrain for the regeneration of Taiwania.