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

Abstract

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 × 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.

Leaf and twig traits predict habitat adaptation and demographic strategies in tropical freshwater swamp forest trees

“…Among these, 21 plots occurred within swamp conditions, that is with open water bodies and permanently waterlogged soils with a shallow water table of < 1 m belowground level (hereafter 'swamp plots'). The remaining 19 plots (hereafter 'nonswamp plots') occurred in well-drained areas, with a water table of > 1 m belowground level (Chong et al, 2016(Chong et al, , 2018(Chong et al, , 2021. All trees with a diameter at breast height (DBH) ≥ 5 cm were surveyed in the 20 × 20 m plots.…”

“…Seedlings with heights ≥ 20 cm and DBH < 1 cm were surveyed in four randomly placed 1 × 1 m quadrats in each of 37 out of the 40 plots. Plots were surveyed in 2013, 2018, 2019 and 2020. Using the tree data from the 2013 surveys, Chong et al (2021) found that the swamp (i.e. waterlogged) plots had a clearly different tree community from the nonswamp plots, although among species there was a gradient of association where some tree species could be found in both types of conditions.…”

“…Prolonged waterlogging depletes oxygen supply to roots, reduces the availability of some nutrients, increases the susceptibility of roots to pathogens, and thus reduces water and nutrient uptake and increases the chance of tree mortality (Kozlowski, 1997). Waterlogging can develop through seasonal flooding or occur in permanently waterlogged habitats such as peat or freshwater swamp or flooded forests (Page et al ., 2006; Junk et al ., 2011; Chong et al ., 2021). Hydrology is a master variable controlling species diversity and abundance in wetland habitats, leaving only inundation‐tolerant species to thrive (Mitsch & Gosselink, 2015; Chong et al ., 2021).…”

“…Waterlogging can develop through seasonal flooding or occur in permanently waterlogged habitats such as peat or freshwater swamp or flooded forests (Page et al ., 2006; Junk et al ., 2011; Chong et al ., 2021). Hydrology is a master variable controlling species diversity and abundance in wetland habitats, leaving only inundation‐tolerant species to thrive (Mitsch & Gosselink, 2015; Chong et al ., 2021). Swamp‐associated tree species often possess specialized traits that function to cope with abiotic stressors associated with existing in inundated soils (e.g.…”

Leaf decomposition and flammability are largely decoupled across species in a tropical swamp forest despite sharing some predictive leaf functional traits

“…Leaf litter for decomposition experiments, flammability experiments and litter trait measurements was collected between July 2020 and January 2021 from 10 replicate 400 m 2 long-term vegetation plots (from a total of 40 plots) established in 2013 (described in Chong et al, 2021;Lam et al, 2021Lam et al, , 2022. Freshly fallen leaf litter was collected from 1 m long × 1 m wide litter traps suspended at least 1 m above the ground on a fortnightly basis (Lam et al, 2021).…”