Root anatomy helps to reconcile observed root trait syndromes in tropical tree species

Studying the organization of functional traits in plant leaves and stems
has revealed notable patterns linking function and form; however, evidence
of similarly robust organization in root tissues remains controversial. We
posit that anatomical traits in roots can provide insight on the overall
organization of the root system. We hypothesize that A) size variation in
the tissue outside the stele relates in a non-linear fashion with
functional traits associated with direct resource uptake, including a
negative relationship with root architectural traits; and B) similar
relationships detected in tropical areas also hold true in other biomes.
We address our hypotheses using empirical data from 24 tropical tree
species in French Guiana, including anatomical measurements in first order
roots, and functional trait description for the entire fine root system.
In addition, we compiled a global meta-analysis of root trait with 500+
forest species across tropical, subtropical and temperate forests. Our
results supported the expected non-linear relationships between cortical
size and morphological traits, and a negative linear trend with
architectural traits. We confirmed a global negative relationship between
SRL, diameter and tissue density, suggesting similar anatomical
constraints in root systems across woody plants. However the importance of
factors vary across biomes, possibly related to the unequal phylogenetic
representation of families across latitudes. Our findings imply that the
rhizocentric hypothesis can be a valuable approach to understand fine root
trait syndromes and the evolution of absorptive roots in vascular plants.