Temperate forest floors: Ecosystem hub in transition?

Friederike Lang, Jörg Prietzel, Frank Hagedorn, Klaus Kaiser, Mathias Mayer, Stefanie Schulz, Lars Vesterdal, Jörg Niederberger

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 to current changes in environmental conditions and forest 
management is understudied. Temperate forests are among the best studied ecosystems globally, providing the necessary ecological 
and biogeochemical background information to assess FF changes. Here, we focus on identifying existing knowledge and gaps in our 
understanding of the functioning of the FF. Interactions between FF biota and abiotic FF components show multifactorial depend-
encies with environmental conditions and drive FF turnover. Vice versa, the turnover of FF regulates carbon, nutrient, and water cy-
cling. With slow litter decomposition and limited bioturbation, organic matter accumulates, nutrients cycle tightly within the FF, and 
water passes through this layer partly along preferential pathways. With rapid litter decomposition and intense bioturbation, FF accu-
mulation is little, the mineral soil is the main nexus for plant nutrient uptake and organic matter transformation, and water infiltrates 
the mineral soil more homogeneously. The interconnectedness with the adjacent ecosystem compartments is a crucial feature of the 
FF, feeding back to its functioning and making it a central hub of forest processes. The FF morphology reflects these processes and 
therefore has untapped potential as an indicator of soil and ecosystem health. Under forest change, the FF might lose its functionality, with negative impacts on nutrient provision, water storage, and carbon sequestration. Consequences for forest growth could be strong 
and even detrimental. Hence, improved knowledge of FF characteristics and their linkages to mineral soils and aboveground ecosys-
tem compartments is crucial for assessing forest resilience to progressing environmental changes.