Temperate forest floors: Ecosystem hub in transition?

The forest floor (FF) plays a key role in carbon, nutrient, and water cycling. It is the biologically most active compartment of forest soils, highly responsive to environmental conditions. Yet, its response on currently changing forests is understudied. Here we (1) compile existing knowledge on provision of ecosystem services by the FF, (2) evaluate its vulnerability to environmental change, and (3) identify crucial knowledge gaps about future functioning of FFs. Reinforcing interactions between FF biota and abiotic FF components show multifactorial dependencies with environmental conditions and control FF turnover. Vice versa, the turnover of the FF regulates its role in carbon, nutrient, and water cycling. With slow litter decomposition and limited bioturbation, organic matter accumulates, nutrients are tightly cycled within the FF, and water is channelled through preferential flowpaths. In contrast, with rapid litter decomposition and intense bioturbation, FF accumulation is low, the mineral soil is the main nexus for plant nutrient uptake and organic matter transformation, and water infiltrates more homogeneously into the mineral soil. A characteristic feature of the FF is its interconnectedness with the adjacent ecosystem compartments making it a central hub of forest processes. FF morphology reflects these processes and therefore has untapped potential as an indicator of soil and ecosystem health. Under forest change FFs might lose their functionality with negative impacts on nutrient provision, water storage, and carbon sequestration. Consequences for sustained forest growth might be strong and even catastrophic. Therefore, improved knowledge of FF characteristics and their linkages to mineral soils and aboveground ecosystem compartments is crucial for assessing forest resilience to ongoing environmental changes.