Abstract
AbstractFeedbacks between plants and their soil microbial communities often drive negative density dependence in rare, tropical tree species, but their importance to common, temperate trees remains unclear. Additionally, whether negative density dependence is driven by natural enemies (e.g., soil pathogens) or by high densities of seedlings has rarely been assessed. Density dependence may also depend on seedling size, as smaller and/or younger seedlings may be more susceptible to mortality agents. We monitored seedlings of Quercus rubra, a common, canopy‐dominant temperate tree, to investigate how the density of neighboring adults and seedlings influenced their survival over two years. We assessed how the soil microbial community influenced seedling survival by growing seedlings in a glasshouse inoculated with soil collected from beneath conspecific and heterospecific mature trees. In the field, seedling survival was lower in areas with high densities of mature conspecifics but was unrelated to either conspecific or heterospecific seedling density. Smaller seedlings were also more sensitive than larger seedlings to neighboring adult conspecifics. In the glasshouse, seedlings grown with soil from beneath a conspecific adult had a higher mortality rate than seedlings grown with soil from beneath heterospecific adults or sterilized soil, suggesting that soil microbial communities drive the patterns of mortality in the field. These results illustrate the importance of negative density‐dependent feedbacks resulting from the soil microbial community in a common and ecologically important temperate tree species.
Highlights
The Janzen–Connell hypothesis is one wellsupported mechanism for the maintenance of tree species richness in tropical forests
We found an additional 22 Q. rubra seedlings in 2018 and an additional 12 Q. rubra seedlings in 2019 that were not included in subsequent analyses
Our findings that support the existence of adult conspecific-negative density dependence are in agreement with previous work showing that temperate tree species in the eastern USA typically exhibit negative density dependence in seedling survival (Johnson et al 2012)
Summary
The Janzen–Connell hypothesis is one wellsupported mechanism for the maintenance of tree species richness in tropical forests This negative feedback between trees and their offspring results in the greatest recruitment of seedlings at an intermediate distance from their parent tree, due to the trade-off between dispersal distance and density-dependent pathogens and seed predators (Janzen 1970, Connell 1971). The seedlings of common species may be more likely to encounter conspecifics and be targets of natural enemies, resulting in lower recruitment, while rare species, because of their rarity, may have fewer specialist natural enemies, and greater recruitment This would present a stabilizing pressure on both common and rare populations, and promote diversity (Chesson 2000). There is increasing evidence that in some systems, rare species exhibit stronger CNDD, perhaps as a result of negative density-dependent interactions keeping their density on the landscape low (Comita et al 2010, Mangan et al 2010, Johnson et al 2012)
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