Fine-root trait variation in temperate trees follows arc-shape pattern along deep soil profiles

Roots are plants' interface with the soil, controlling access to water and nutrients. Yet, fine-root trait variation along deep soil profiles and its functional implications remain poorly understood. We quantified vertical fine-root trait variation in four temperate tree species with contrasting rooting types - beech, oak, pine and Douglas fir. Roots were excavated to 380 cm depth in German Pleistocene sandy soils. Fine-root morphological and functional traits were related to soil properties, nutrient and water availability. Across species, fine-root traits followed a consistent arc-shaped pattern with depth. Roots in the topsoil (< 50 cm) and deep subsoil (> 300 cm) exhibited acquisitive traits - high specific root length (SRL), root N content and specific root tip abundance (SRTA) indicating high metabolic activity and resource uptake potential. By contrast, roots in intermediate subsoil layers (50-300 cm) displayed more conservative traits, with thicker diameters and denser tissues. Ectomycorrhizal associations occurred throughout the soil profile. Our findings provide rare empirical evidence of fine-root functional differentiation across deep soil layers, as root economic traits vary nonlinearly with depth. Thus, trees express vertically contrasting resource-use strategies. Incorporating vertical root trait plasticity into vegetation models can improve predictions of resource uptake and carbon turnover in deep-rooted species.