Soil microbial effects on tree seedling performance during forest secondary succession

Soil microbiota influence seedling survival and abundance in mature forests. However, their role in tree species turnover during secondary forest succession remains unclear, especially in species-rich tropical forests. We conducted a shade house experiment to assess variation in the direction and strength of net soil microbial effects on seedling emergence, biomass growth, and mortality of seven tree species. We tested for effects of soil successional stage (0, 15, 25, and 115 years of forest recovery), tree species' association with successional stages (home stages where species peak in abundance vs. away stages), and light level (5% vs. 40% of daylight). We quantified net microbial effects as the ratio of seedling performance in live soil (including microbiota) versus sterilized and fungicide-treated soil. We found net positive or net negative microbial effects on seedling performance in six of the seven tree species, highlighting the prevalence of microbial effects. Overall, fewer seedlings emerged and more died in live than sterilized soils, and under high than low-light levels, although these effects varied greatly among tree species. We found no effect of soil successional stage on seedling performance. Tree species' association with successional stages, however, strongly affected seedling performance: Soil microbiota from a species' home successional stage increased the likelihood of seedling emergence by approximately 16% and reduced seedling mortality by approximately 36%. Our results suggest that soil microbiota exert tree species-specific effects on seedling performance during secondary succession of tropical forest. Positive microbial effects on seedling emergence and survival in tree species' home successional stages may promote tree species' establishment and persistence at home stages and could thereby slow tree species turnover during forest recovery. While the generality of our results needs to be tested across forest ecosystems, they highlight the relatively unexplored role of soil microbiota in determining the pace and direction of successional plant community assembly and promote the development of microbial inoculation as a tool for restoration of functionality and biodiversity of forests globally.