In Northeast temperate forests, individual trees influence soils through phenological effects on microclimate, differences in roots and mycorrhizae, and litter inputs. In experimental grasslands, higher plant diversity increases both aboveground productivity and total soil respiration due to niche partitioning belowground. It remains unclear whether links between diversity and belowground carbon cycling exist in tree species mixtures in unmanaged forests. This study asks 1. whether tree species identity influences soil respiration through microclimate, mycorrhizal association, or litter characteristics; and 2. what the relationship between tree diversity and soil respiration is at multiple spatial scales.
At the Harvard Forest in central Massachusetts, we established 49 plots at the centroid of three trees which were either all one species or a mixture of species. These plots capture a natural gradient of diversity that includes trees with different phenological patterns, mycorrhizal associations, and litter characteristics. Each triangle plot was located within a 10x10m plot, in which tree species and DBH were already mapped as part of the SIGEO project. CO2 efflux was measured in each plot twice monthly, and litterfall was collected, sorted and weighed by species. Data loggers measured hourly soil temperature in a subset of plots to capture differences in soil microclimate.
Soil respiration in plots surrounded with three trees of the same species are significantly different from each other (p = .024), and these differences seem to be due to a combination of pathways. Soil temperatures in deciduous plots are generally higher than in mixed and coniferous plots in spring and fall, but respiration is not correlated with temperature within sampling days. All 10x10m plots have between 50 and 100% ectomycorrhizal trees by basal area, which is negatively correlated with soil respiration (R2 = .14, p < .01). Litter quantity and quality varies across plot types, and soil respiration is positively correlated with total amount of litter fall (R2 = .25, p < 0.01).
Tree diversity operates differently depending on the spatial scale of interest. Plots with species mixtures exhibit soil respiration rates that are intermediate to the single-species rates. However, soil respiration is positively related to the species richness of trees in the 10x10m plots (R2 = .12, p < 0.05). These results suggest that trees species identity may influence soil respiration at a small scale, but that across larger scales tree diversity may contribute to soil respiration rates.