Variation in root hair traits in 75 xerophytic species: Constraints of phylogeny, trait trade-offs and environment

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.