Community Level Plant Reproductive Traits Of Tropical Rain Forest In Xishuangbanna, SW China

Unlike the seriate lowland rainforests in SE Asia, the tropical vegetation of Xishuangbanna (SW China) has developed in habitats with comparatively lower temperatures and precipitation. Consequently, although most of the families and genera of the flora are components of tropical ecosystems, many have reached their distribution limits in latitude. Selection pressures on reproductive traits in these environments may be different from that experienced in lowland tropical regions. In this thesis, I studied the sexual systems and reproductive phenology of tropical forests in Xishuangbanna by field observations, surveying sample plots, and checking herbaria specimen. Based on these data, I discussed the selective pressures on the evolution of plant sexual systems and the mechanisms underlie the patterns of reproductive phenology.Sexual systems played key role in influencing many components of the ecology and evolution of plant populations. Here, I examine the sexual systems of 685 species of flowering plants belonging to 109 families and 356 genera based on 42 plots and compare our results with surveys of sexual systems from other tropical ecosystems. Among these species, 60.6 percent were hermaphroditic, 14.3 percent were monoecious, and 25.1 percent were dioecious. The percentage of dioecious sexual system among tree species (26.1%) was similar or higher than that of other tropical tree floras. Monoecy was significantly associated with the tree growth form and was relatively common in seasonal forest vegetation. Sexual systems involving unisexual flowers (dicliny) are particularly well represented in the tropical forests of Xishuangbanna accounting for 39.4 percent of all species surveyed. This pattern may be associated with the prevalence of relatively small inconspicuous flowers in the tropical forests of SE Asia and their correlation with diclinous sexual systems generally.Reproductive phenology is determined by the interactions between biotic and abiotic factors and is most diverse in tropical forests. The diversity of reproductive phenology is shaped by variation in timing, frequency, and duration of flowering in tropical plants, yet we have little understanding of the significance of this variation in phenological parameters. In this study, reproductive phenology of 86 species was studied to examine the relationships between phenological characters and ecological traits in a tropical seasonal rain forest. Flowers were available in the community year-round, but 53.2% species flowered in Mar-May corresponding to the late dry and early wet season of the year. At the community level, flowering was temporally associated with rainfall with flowering peak about 2 mo lagged behind rainfall peak. The highest number of species with developing fruits was overlapped with rainfall peak and mature fruits occurred mainly in the late wet season. During the study, the temporal change of rainfall caused the change of flowering and fruiting time. Rainfall may be the key climatic factor in influencing the reproductive phenology of seasonal rain forest in Xishuangbanna.Flowering time (season) was significantly associated with pollinator systems with proportionately more species pollinated by diverse insects flowering in dry season and proportionately more species pollinated by Lepidoptera flowering in wet season. A considerable proportion of species (22.1%) flowered supra-annually although the majority of species (74%) flowered annually. A much lower proportion of species flowered subannually compared with lowland forest in Costa Rica and a lower proportion of species flowered supra-annually compared with dipterocarp forest in SE Asia. Flowering duration of most species was rather brief, usually less than six weeks. Non-dioecious species displayed flowers for a longer period than dioecious species, but this association may be spuriously generated by a suite of ecological correlates with dioecy. Further identification of the exclusive effects of dioecious sexual system on flowering duration should be examined by comparing related species with contrasting sexual systems.Until now, little evidence showed that forest fragmentation and human disturbance caused serious consequences on pollinator systems. However, population decline and extinction of many large birds and animals may impose difficulties on seed dispersal and forest recruitment. The changes in land utilization and deforestation of primary rain forests may bring regional climatic changes especially temporal pattern of rainfall, which may in turn change the reproductive phenology of tropical plants even the distribution and species composition of the tropical forests.