| Summary: | 博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 102 === Why tree diversity is so high and what drives coexistence of species in a tropical forest is the central question that community ecologists concern. Niche- and dispersal-based processes are two main processes which affect distribution pattern and coexistence of trees in a forest. Recently, forest dynamics plots (FDP) established by the Smithsonian Institution Center for Tropical Forest Science (CTFS). However, none of the above is the same as in Taiwan, a subtropical and mountainous island with rough terrain and frequent disturbance. Therefore, in 2008, I used the standard protocol as the Smithsonian CTFS’s to establish a 25-ha FDP in the Lienhuachih Experimental Forest (23o54’49”N, 120o52’43”E) in Central Taiwan. Expect precise stem mapping and topography measurement, I also collected soil and disturbance data to assess how niche- and dispersal-based processes affect coexistence of species and diversity of a subtropical forest and whether these were the same as the tropical FDPs with relative flat terrains and little disturbance.
My results show the Lienhuachih FDP is dominated by Fagaceae and Lauraceae and which is characteristic of the Machilus-Castanopsis forest zone of Taiwan. The size-class structure of trees show most tree species have plenty saplings, which display good recruitment in the forest. Fisher’s alpha diversity of the Lienhuachih FDP is the highest among low-elevation FDPs in Taiwan, but lower than which of other FDPs in tropical plots.
On the other hand, the Lienhuachih forest could be divided to four types, these are Pasania nantoensis - Randia cochinchinensis type locates on the ridge and the highest elevation was with the highest stem density, Mallotus paniculatus - Engelhardtia roxburghiana type locates on the upper slope was with the middle stem density and basal area, Diospyros morrisiana - Cryptocarya chinensis type locates on the lower slope and stream side was with lower stem density and Machilus japonica var. kusanoi - Helicia formosana type locates on west stream side was with the lowest stem density and basal area. All four plant communities and species composition varied across micro-topographic gradients.
In addition, both niche process and dispersal process work together to assemble the Lienhuachih tree communities. When comparing above two processes affect community assembly, if only topography is included, dispersal-based processes prevail. But if including soil variables along with topography variables into variation partitioning, the result reverses. Furthermore, including disturbance variables with topography and soil variables, niche-based processes still prevail for both pioneer and non-pioneer saplings. However, dispersal limitation is also an important process influencing the spatial distribution of species diversity for both functional saplings, especially for the non-pioneer saplings.
My study also demonstrates that topography affects distribution of non-pioneer saplings, whereas disturbance affects distribution of pioneer saplings. Although soil is an important factor to explain distribution of both pioneer and non-pioneer saplings, it is also evident that plant species, geomorphic processes, and disturbance have different effects on the physical and chemical properties of soils. Soil and disturbance effects contribute to meso- and fine-scale spatial variations for distribution of species, whereas topographic effects contribute to broad-scale spatial variations for distribution of species. To conclude, in a rough terrain and highly disturbance area, niche-based process is the main ecological process to maintain the coexistence and diversity, and which also affect vegetation classification in the Lienhuachih subtropical forest.
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