Waterlogging and soil but not seedling competition structure tree communities in a catchment containing a tropical freshwater swamp forest

Questions What are the relative influences of soil and hydrology on tree community structure, seedling survival and seedling growth? Do soil and hydrology moderate seedling responses to competition? Location Nee Soon catchment, Singapore. Methods We measured and identified trees >= 5 cm diameter in 40 plots measuring 20 m x 20 m each, set up in the wet and dry areas of the catchment, and collected topsoil for analysis of soil texture, pH, and nutrients. We also conducted randomised experiments on seedlings of two dry specialists, two wet specialists, and two generalists and compared survival and growth traits in different soil and waterlogging treatments. We repeated the experiments with one dry specialist and one wet specialist with additional paired seedling intraspecific and interspecific competition treatments. Results Tree communities were structured along both soil and hydrological gradients, and a distinct freshwater swamp subcommunity was identified. The six species selected for the experiments had differing survival responses to the different soil and waterlogging treatments: dry specialists survived best in the dry soil type and/or well-drained conditions and wet specialists survived best in the wet soil type or waterlogged conditions, with generalist responses in-between. However, both the dry specialist Aporosa frutescens and the wet specialist Baccaurea bracteata grew slower in waterlogged conditions. These results did not differ when seedlings of the two species were grown together, suggesting that dry-specialist seedlings do not particularly outcompete wet-specialist seedlings in the absence of waterlogging stress. Conclusions Soil characteristics, hydrological conditions, and tree community structure are all intertwined in the field. Manipulative experiments show that soil type and waterlogging have separate effects on seedling survival of different species that are consistent with performance trade-offs corresponding to their respective specialisations, but waterlogging reduces seedling growth in both wet and dry specialists. Competition between seedlings is comparatively unimportant.