Root hairs play an important role in the acquisition of soil resources by increasing absorptive surface area. Yet, key factors driving the variation of root hair traits across different species along biogeographic gradients are still poorly quantified, limiting our understanding of the functional relevance of root hairs. We measured root hair length, diameter and areal density of 75 xerophytic species across a 1000-km, latitudinal gradient in a dry valley system. The influences of phylogeny, environment, and fine-root morpho-anatomical traits on root hairs were quantified. We found that 95% of sampled species had root hairs, total absorptive root area increased by roughly 0.3% for species with very few roots hairs, and up to a 180% for species with large and dense root hairs. Phylogeny did not appear to be a significant factor influencing root hair traits. Root hair diameter and length were positively related to root diameter and cortex thickness, while root hair densities were negatively related with cortex thickness and root diameter. Across latitudes, mean annual precipitation (MAP) was the main factor driving the variations in root hairs as hair density generally increased with higher MAP.

Synthesis: Root hairs were prevalent in native species in these arid ecosystems. Variation in root hair traits was more directly dependent on root traits rather than phylogeny across species. Climate was main driver of biogeographic patterns of root hair density. These results provide a more comprehensive understanding of the fine-root foraging strategies and the role of root hairs within the root economics space.