Positioning absorptive root respiration in the root economics space across woody and herbaceous species

Root respiration is essential for nutrient acquisition. The respiration rate of absorptive roots theoretically relates to the economics of carbon-nutrient exchange, but its empirical role remains largely unexplored in the trait space defining nutrient uptake strategies. Here, we measured the respiration rates of the distal, non-woody, absorptive roots of 252 woody and herbaceous species from subtropical and temperate climate zones, including both arbuscular mycorrhizal and ectomycorrhizal fungal hosts. We found a consistent and positive correlation between root respiration rate and specific root length (root length per dry weight), irrespective of growth form, mycorrhizal type, and climate zone. Root respiration rate was also positively, but less strongly and less frequently correlated with root nitrogen concentration. Root morphology strongly explained the fast-slow gradient of root respiration in the root economics space. By quantifying the ratio of arbuscular mycorrhizal fungal DNA copy number and root tissue DNA copy number using qPCR, we found that the morphology-driven gradient did not explain the full variation in fungal collaboration; thick roots were consistently well colonized, but medium and thin roots displayed a wide range of colonization intensity. Synthesis: These results advance our understanding of the fundamental trait relationships that underpin the root economics space. Our study also provides a physiological linkage to the frequently-measured root morphological traits and relates the root economics space to root-derived carbon-nutrient cycling processes.