Summary 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 ( 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.
Pietig et al. (2026) studied this question.