Functional traits and ecological niches as correlates of the interspecific growth-mortality trade-off among seedlings of 14 tropical tree species

1. The interspecific trade-off between growth versus mortality rates of tree species is thought to be driven by functional biology and to contribute to species ecological niche differentiation. Yet, functional trait variation is often not strongly correlated with growth and mortality, and few studies have investigated the relationships of both traits and niches, specifically encompassing above and belowground resources, to the trade-off itself. These relationships are particularly relevant for seedlings, which must often survive resource limitation to reach larger size classes. 2. We investigated the functional basis of the interspecific growth-mortality trade-off and its relationship with ecological niches for seedlings of 14 tree species in a tropical forest in southwest China. 3. We found evidence for an interspecific growth-mortality trade-off at the seedling stage using 15 functional traits and 15 ecological niche variables. None of the organ-level traits correlated with growth, mortality, nor the trade-off, whereas specific stem length (SSL), a biomass allocation trait, was the only trait to have a significant correlation (positive). Moreover, light-defined niches were not correlated with growth, mortality or the trade-off, but soil-defined niches did. Species at the faster growth/higher mortality end of the trade-off were associated with higher fertility defined by lower soil bulk density and slope, and higher soil organic matter concentration and soil total nitrogen. 4. Our findings indicate the importance of stem elongation and soil fertility for growth, mortality and their trade-off at the seedling stage in this Asian tropical forest. Our findings contrast with analogous studies in neotropical forests showing the importance of photosynthesis-related leaf traits related to insolation. Therefore, the functional drivers of demographic rates and trade-offs, as well as their consequences for ecological niches, can vary among forests, likely owing to differences in biogeography, canopy disturbance rates, topography and soil properties. Moreover, the effects of functional trait variation on demographic rates and trade-offs may be better revealed when biomass allocation is accounted for in a whole-plant context.