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Summary ●Fine roots and mycorrhizal fungi may either stimulate leaf litter decomposition by providing free‐living decomposers with root‐derived carbon, or may slow decomposition through nutrient competition between mycorrhizal and saprotrophic fungi. ●We reduced the presence of fine roots and their associated mycorrhizal fungi in a northern hardwood forest in New Hampshire, USA by soil trenching. Plots spanned a mycorrhizal gradient from 96% arbuscular mycorrhizal (AM) associations to 100% ectomycorrhizal (ECM)‐associated tree basal area. We incubated four species of leaf litter within these plots in areas with reduced access to roots and mycorrhizal fungi and in adjacent areas with intact roots and mycorrhizal fungi. ●Over a period of 608 d, we found that litter decayed more rapidly in the presence of fine roots and mycorrhizal hyphae regardless of the dominant tree mycorrhizal association. Root and mycorrhizal exclusion reduced the activity of acid phosphatase on decomposing litter. ●Our results indicate that both AM‐ and ECM‐associated fine roots stimulate litter decomposition in this system. These findings suggest that the effect of fine roots and mycorrhizal fungi on litter decay in a particular ecosystem likely depends on whether interactions between mycorrhizal roots and saprotrophic fungi are antagonistic or facilitative.

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Feedbacks 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.

Carbon allocated to roots accounts for a large portion of net primary productivity, but the fate of that carbon is poorly understood. Absorptive fine roots are the primary way in which plants acquire nutrients. Previous studies have evaluated relationships among root morphological traits, including specific root length, root tissue density, and mycorrhizal colonization, across broad functional and taxonomic groups to test for the existence of a root economics spectrum (RES). Fine roots also release carbon dioxide through respiration, and other studies have found relationships between root morphological traits and root respiration within individual tree species. The objective of this study was to measure a suite of root traits in six co-occurring temperate tree species that represent a diverse set of aboveground traits to determine whether and how root characteristics influenced root respiration both within and among species. At the Harvard Forest in Petersham, Massachusetts, USA, we measured fine root respiration, root morphology, percent colonization for ectomycorrhizal species, and carbon and nitrogen concentrations on 292 roots from six tree species in June and July 2018. We found that most fine root morphological characteristics varied nearly as much within each tree species as they did among the six species. Root traits were dynamic over time during the two months of our study, where the magnitude of weekly mean trait values varied 32-95% across the study period. Strong correlations among traits suggested trade-offs on a spectrum from resource acquisition (long, thin, high-nitrogen roots) to resource conservation (thick, dense, low-nitrogen roots), and traits were not clustered by tree species within this spectrum. Along with temperature and weekly temporal variation, the resource acquisition strategy (long and thin roots that were high in nitrogen) was associated with higher root respiration, and this relationship was consistent among the six species. This study supported a strong link between the RES and respiration independent of species identity, which provides insight into functional axes for scaling root respiration from individual trees to the forest stand to better quantify belowground carbon flux.