Forest floor vegetation contributes to a reduction in nitrogen fluxes in temperate forest understories

Background and Aims: Forest floor vegetation can both respond to and affect the water and nitrogen (N) availability in forest ecosystems. However, their role for influencing the relationship between throughfall and N fluxes has hardly been studied, leaving a knowledge gap in our mechanistic understanding of forest biogeochemical cycling. Here we investigate the impact of the structural and functional role of the herbaceous and moss layer in linking N fluxes and throughfall patterns in beech, spruce, and pine forests in Central Germany. Methods We monitored herbaceous and moss species cover and diversity, as well as throughfall and N fluxes for 93 plots capturing small-scale microclimate variability. For all co-occurring herbaceous species, we measured intraspecific trait variation for specific leaf area (SLA) and plant height (n = 685). Results Multivariate analyses reveal strong differences in the herb and moss layer composition between forest types. The results of analyses of covariance, and of piecewise structural equation models consistently show that N fluxes decreased most under pine and spruce plots where herb and moss cover was high. Species with a high SLA and plant height positively contributed to overall herb cover. Conclusion Our results suggest that plant growth, particularly moss cover, contributed to overall N retention, while acquisitive and fast-growing species with a high SLA contributed to a fast nutrient return to the system, thereby partly increasing N fluxes. We conclude that taxonomic and functional composition of the forest floor vegetation is an important mediator in the link between throughfall and N fluxes.