The Economics Spectrum Drives Root Trait Strategies in Mediterranean Vegetation

Abstract

Extensive research efforts are devoted to understand fine root trait variation and to confirm the existence of a belowground root economics spectrum (RES) from acquisitive to conservative root strategies that is analogous to the leaf economics spectrum (LES). The economics spectrum implies a trade-off between maximizing resource acquisition and productivity or maximizing resource conservation and longevity; however, this theoretical framework still remains controversial for roots. We compiled a database of 320 Mediterranean woody and herbaceous species to critically assess if the classic economics spectrum theory can be broadly extended to roots. Fine roots displayed a wide diversity of forms and properties in Mediterranean vegetation, resulting in a multidimensional trait space. The main trend of variation in this multidimensional root space is analogous to the main axis of LES, while the second trend of variation is partially determined by an anatomical trade-off between tissue density and diameter. Specific root area (SRA) is the main trait explaining species distribution along the RES, regardless of the selected traits. We advocate for the need to unify and standardize the criteria and approaches used within the economics framework between leaves and roots, for the sake of theoretical consistency.

Keywords: ecophysiology; fine root strategies; non-mass normalized traits; root anatomy; root economic spectrum; specific root area; specific root length.

Figure 1

Plot of the first and second axes of the principal component analysis (PCA) for the total dataset (532 observations from 320 species) performed with: root morphological traits (A); the analogous leaf morphological traits (B); root morphological traits only for woody (C); and herbaceous plants (D). SRL, specific root length; SRA, specific root area; RDMC, root dry matter content; RD, root tissue density; SLA, specific leaf area; LDMC, leaf dry matter content; LD, leaf tissue density; Rdi, root diameter; Lthick, leaf thickness.

Figure 2

Plot of the first and second axes of the principal component analysis (PCA) demonstrating the trade-off between the specific area per unit mass (SRA/SLA) with tissue density (RD/LD) and diameter/thickness (Rdi/Lthick) at root (A) and leaf levels (B) for 561 observations from 310 Mediterranean plant species. Abbreviations are as in Figure 1.

Figure 3

Plot of the first and second axes of the PCA from 80 observations of 64 Mediterranean woody species (A) with morphological traits used in Bergmann et al. (2020); (B) same PCA as in A but with SRA instead of SRL; and (C) PCA with analogous root and leaf morphological traits. Trait abbreviations are as in Figure 1 and Table 1. Data from de la Riva et al. (2021).