Root and chemical traits

Many studies have quantified the functional variation of fine root traits
to understand the overarching trade-off between maximizing resource
acquisition or conservation (root economics spectrum -RES-). However, we
know remarkably less on how plant strategies along the RES are actually
constrained by the amount of photosynthates required to construct roots
(i.e. construction costs, CC) or how belowground interactions with
symbiotic organisms modify root trait patterns and their relationships
with CC. Our main aim was to quantify CC of fine roots (<2 mm) and
their underlying components (carbon, minerals and organic nitrogen
concentrations) in 60 Mediterranean woody species with contrasting
symbiotic association types (ectomycorrhizas, arbuscular and ericoid
mycorrhizas and N-fixing bacteria). We examined (1) if the covariation
among fine root traits along the RES was related to the intrinsic cost of
producing roots and if this relationship was dependent on the type of root
symbiosis; (2) if the relationship of each CC component with the RES was
dependent on the type of root symbiosis; and (3) whether soil water and
nutrient availability determined differences in CC across sites. According
to the RES hypothesis, fine root traits showed a main covariation trend
(acquisition vs. conservation), defined by the first PCA axis, which also
segregated species by their two main contrasting symbiotic types
(arbuscular and ectomycorrhizal). We found a positive relationship between
root CC and the RES (i.e. PCA axis 1) and, interestingly, slopes differed
among symbiotic types, in response to the different role of each specific
CC component. In addition, independently of symbiotic type, root CC
decreased linearly with soil nutrient availability and quadratically with
plant water availability. Synthesis. Our study demonstrates that woody
plants display different functional strategies in their root CC, related
with their position on the RES, and that CC differ among symbiotic groups.
The influence of the root CC components across species varied among
symbiotic associations, pointing to a trade-off between structural and
metabolic compounds. Root CC were also strongly modulated by soil resource
availability (nutrients and water). This study highlights that root CC are
fundamental to better understand belowground resource use strategies.