Spatial variation of floodplain vegetation in Nong thung thong Non-Hunting Area, peninsular Thailand

Abstract

This thesis aims to gain increased understanding of the processes that govern plant species composition and structure of floodplain vegetation in Nong thung thong Non-Hunting Area, one of the last few patches of floodplain vegetation remaining in southern Thailand, to provide ecological information relevant to conservation and management of this Non-Hunting Area and other floodplain vegetation in this region. To achieve this objective, a detailed analysis of vegetation in a range of fluvial landforms and its relationship to environmental factors was conducted. Results showed that the vegetation gradient identified by a non-metric multidimensional scaling using tree species composition varied along flooding and fluvial landform gradients, suggesting that fluvial landforms and their flooding durations are key environmental factors underlying a major part of this vegetation gradient, and the distribution of plant species probably relates to the ability of species to tolerate the seasonal flooding. The results from the vegetation analysis has generated a hypothesis that that differential flood tolerance ability is an important factor that determines the distribution of plant species. To test this hypothesis, an ex situ experiment to compare the flood tolerance of seedlings of some dominant species in Nong thung thong Non-Hunting Area was conducted. Results showed that Hydnocarpus anthelminthicus, a species that mainly occupies ridges which experience about one month of flooding per year, could not survive through the recovery period after eight weeks of complete submergence nor through the recovery period after 24 weeks of partial submergence, while all seedlings of Barringtonia acutangula and Crateva religiosa, species found in longer flooded areas, survived through the flooding and recovery periods. These results indicate that differential flood tolerance among species is an important factor that regulates species distribution along flooding and micro-topographic gradients. Interspecific differences in seeds? responses to flooding may be an important process that controls distribution and abundance of plants in this Non-Hunting Area. It is hypothesized that diaspores of woody species that occupy the longer flooded areas can maintain longer seed viability than species that experience shorter flooding. Results showed that seeds of C. religiosa, Diospyros sp. and H. anthelminthicus sank in water immediately, but B. acutangular fruits and Xanthophyllum lanceatum seeds floated. Seed germination of B. acutangular, C. religiosa and H. anthelminthicus were constant throughout eight weeks in water. Seed germination of X. lanceatum increased with increasing time in water, while seed germination of Diospyros sp. significantly decreased with increasing time in water. The decline in seed germination over time of Diospyros sp. was significantly higher than X. lanceatum, and this may partly explain why X. lanceatum can colonize abandoned channels and swales, but Diospyros sp. cannot. The stronger negative effect of flooding on seed germination of Diospyros sp. compared to X. lanceatum may be attributed to an inability of Diospyros sp. seed coat to maintain long term impermeability. However, there is no evidence to conclude that post-flooding seed germination alone is the limiting factor that restricts H. anthelminthicus to ridges. Knowledge about environmental factors required for seedling recruitment and establishment in natural conditions is essential for understanding niche partitioning, species distribution and vegetation gradients. Thus, a field monitoring investigation of seedling recruitment and establishment of some dominant species in Nong thung thong Non-Hunting Area was conducted to assess the relative contribution of fluvial landform, canopy cover and conspecific mature plants to seedling recruitment and establishment. Results showed that selected species established successfully in different fluvial landforms and so the heterogeneity of landforms appears to play an important role in vegetation gradient and biodiversity of this patch of floodplain vegetation. Finally, a synthesis of key results found in the context of the main objective is presented. This includes outlining the implications for conservation of floodplain ecosystems along with methodological considerations and limitations; some areas for further research are also proposed.