Temperate forest floors: Ecosystem hub in transition?

The forest floor (FF) is the central hub in forests where organic matter, nutrients, and water are stored, transformed, and transferred. The FF is dominated by plant litter and its decomposition products, and thus differs fundamentally from the mineral soil. Although the FF is the most dynamic compartment of forest soils, it is often neglected, and inconsistent terminology complicates the synthesis of information from existing studies. The FF is expected to be the most responsive to changing climate, management, eutrophication, and acidification. Here, we (1) compile existing knowledge on the role of FFs for ecosystem service provision, (2) assess the vulnerability of FFs to forest change, and (3) identify important knowledge gaps for temperate forests. The role of FFs in nutrient, carbon, and water cycles depends on the turnover rates and accumulated mass of the FF, which reflects strong interdependences with the abundance, activity and composition of soil microbial and faunal communities. These mutually reinforcing interactions determine how much the FF or mineral soil dominates biogeochemical cycles and energy fluxes. With slow FF turnover, large proportions of nutrients are tightly cycled within the FF, organic matter accumulates due to limited decomposition and impaired bioturbation, and water is channelled through preferential flow-paths. With rapid FF turnover, the mineral soil is the main place for plant nutrient uptake and organic matter transformation, and water percolates more homogeneously. Under forest change in particular ecosystems with slow FF turnover could turn from carbon sinks into sources, losing their adaptability to nutrient-poor conditions and a large part of their water storage capacity. We conclude, that combined analyses of the key organisms and feedbacks with decomposing FF litter are required to understand the resilience of temperate forests against future changes.