Root quantity traits: a leading dimension in root trait space

Plant resource uptake depends on the interplay between the quantity and quality of roots, yet their coordination at the community level remains poorly understood. Using standardized root cores across 20 diverse grassland sites on the Inner Mongolian Plateau and the Tibetan Plateau, we quantified community-level root quantity traits (mass, length, and nitrogen density per soil volume) and quality traits (specific root length (SRL), nitrogen concentration, and root tissue density (RTD)). The two regions differ markedly in soil, climate, and species composition. Community-level root traits clustered into three orthogonal dimensions. The root quantity dimension, reflecting carbon investment in roots for soil exploration, was correlated with soil nitrate concentration. Two root quality dimensions captured contrasting strategies: a foraging-efficiency dimension represented by a negative correlation between RTD and SRL, and an uptake-efficiency dimension related to root nitrogen concentration. Different from the quantity dimension, the efficiency dimensions were regulated by distinct environmental factors within each region, highlighting the context-dependency of root trait-environment interactions. Collectively, our findings show that community-level root strategies can be captured by a tri-dimensional root quantity-quality trait matrix, which represents a novel understanding of the community-level plant assemblies and their responses and adaptation to climate change.