Effects of tree diversity and mycorrhizal type on the spatio-temporal variability of leaf litter production and quality in temperate forests

Forest ecosystem productivity is affected by tree species richness and may be further modulated by different mycorrhizal fungal types, for example, arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi, which strongly affect soil processes. Productivity and soil processes are linked through leaf litterfall, a major pathway for carbon and nutrient input to soils. Higher tree diversity and contrasting mycorrhizal strategies may enhance the variability and heterogeneity of litterfall quantity and quality, yet the combined effects of tree species richness and mycorrhizal type mixtures on the temporal dynamics and spatial distribution of litterfall remain underexplored. We investigated leaf litterfall dynamics over 1 year in the 8-year-old MyDiv tree diversity experiment, spanning a gradient of tree species richness (one, two, four species) and mycorrhizal treatments (AM, EM, AM+EM). Monthly litterfall production and nutrient concentrations were measured, and annual wood production (i.e. wood biomass increment) was assessed to quantify aboveground biomass allocation to wood and litter. Tree species richness did not affect annual production of wood or leaf litter. In contrast, annual wood production varied with mycorrhizal types, being highest in EM tree communities, whereas mycorrhizal type did not affect annual leaf litterfall. However, litterfall dynamics were influenced by both tree species richness and mycorrhizal type. Specifically, higher tree diversity extended the litterfall period and increased spatial variability (CV) of litterfall at the meter scale, while temporal variability remained unaffected. Phosphorus concentration in leaf litter differed among mycorrhizal types, with the highest values observed in AM communities. Species-rich stands exhibited lower C:P and N:P ratios in leaf litter and greater phosphorus fluxes. Contrary to expectations, mixed mycorrhizal communities did not produce more leaf litter than monotypic combinations (AM or EM tree species). Synthesis: Our findings show that leaf litterfall dynamics are modified by tree species richness and mycorrhizal type. The increased spatial variability of litterfall at the meter scale and the extended litterfall period associated with higher tree species richness could create a wider range of resource niches for soil organisms in time and space. This may promote the diversity and activity of the decomposer community, ultimately strengthening overall forest nutrient cycling.