The role of fluvial landforms as a driver of vegetation variation on a floodplain in southern Thailand

Abstract

Floodplain vegetation is rare in southern Thailand due to the generally steep gradient through which major rivers flow and high pressure of land use. This study aims to increase the understanding of factors and processes that govern vegetation variation and diversity in a floodplain to provide information for conservation and management of this threatened and rare ecosystem. A total of 20 plots, each 625 m2, were laid on three broad categories of fluvial landforms to collect data on the number of tree species (woody plants with diameter at breast height (DBH) ≥ 5 cm), soil variables, flooding, history of land use and distance to the inner curve of the channel in Nong thung thong Non-Hunting Area, Thailand. Three 4 m2 subplots were nested within each 625 m2 plot to estimate shrub composition and coverage. The alpha and beta diversity of the tree species in each fluvial landform were calculated. The relationships between vegetation data and environmental factors were explored through transformation-based Redundancy Analysis (tb-RDA) and univariate analyses. Beta diversity between abandoned channel and ridge plots, and between ridge and shallow depression plots was higher than the dissimilarities within each fluvial landform, and the balanced variation in abundance contributed to the major part of the dissimilarities. These results indicate different tolerance ranges or competitive ability of species between these fluvial landforms. Results from the tb-RDA showed that tree species composition varied with fluvial landforms and soil calcium content. Median of flooding was associated with fluvial landforms. In addition, soil calcium content could be controlled by fluvial landform because clay and calcium were correlated, and both were associated with fluvial landforms. These results suggest that variation in fluvial landforms causes variation in flooding duration, edaphic factors, vegetation variation, and ultimately, the biodiversity in this floodplain.