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.

Evidence of Differences in Covariation Among Root Traits Across Plant Growth Forms, Mycorrhizal Types, and Biomes

“…Such discrepancy between them may be related to two possible reasons. First, RD was negatively related to RTD for woody species, which is in line with previous studies reporting a trade-off between RD and RTD in Mediterranean woody plants (de la Riva et al, 2016b(de la Riva et al, , 2021a. As mentioned above, such a negative relationship can be explained by the anatomical tradeoffs between RD and RTD.…”

“…For example, RD was negatively associated with RTD in the tropical and subtropical forests, which is consistent with the study of Valverde-Barrantes et al (2021). Such a negative relationship can be explained by the anatomical relationships that is commonly observed in leaves (Laughlin, 2014;de la Riva et al, 2016a), plants can have similar specific leaf area (or SRL) values with different proportional investments in leaf tissue density (or RTD) and leaf thickness (or RD) (John et al, 2017;de la Riva et al, 2021a), which would depend on the species identity and their environmental conditions (Olmo et al, 2014).We found that RTD was negatively related to SRL in the temperate forest, suggesting the tradeoff between resource acquisition and construction costs of roots (Eissenstat et al, 2000). From the cost-benefit theory, roots with higher SRL and lower RTD would have lower construction costs, higher metabolic rates and faster return of investments (de la Riva et al, 2021b).…”

“…However, roots with a larger mycorrhizal colonization may increase the resource uptake capacity under resource-limited conditions without necessarily implying an acquisitive strategy (Navarro-Fernández et al, 2016). Therefore, it is needed to uncover how this root-mycorrhizal collaboration gradient links to the differences in resource uptake capacity for species with fast-and slow-traits (de la Riva et al, 2021a). Furthermore, we found that RCC formed an additional axis that was decoupled from the first two dimensions.…”

“…The notion of onedimensional RES has been supported by some regional studies (Roumet et al, 2016;de la Riva et al, 2018). For example, de la Riva et al (2021a) found that SRL and specific root area (SRA) were negatively related to root tissue density (RTD) and root dry matter content in Mediterranean vegetation.…”

“…For the same set of root traits -namely RD, SRL, SRA, RTD, RNC, root C content (RCC), RCN, stele diameter, and cortex thickness reported two primary dimensions of trait covariation in the subtropical forests of China, whereas Zhou et al (2018) demonstrated three main dimensions in the temperate steppes of China. These inconsistencies in results among the regional studies may be related to multiple factors, including the differences in the selection of root traits defining the trait coordination and tradeoffs (de la Riva et al, 2021a), species composition (e.g., plant growth form and phylogenetic group) (Wang et al, 2018a;Weigelt et al, 2021), mycorrhizal association type (Comas et al, 2014;Akatsuki and Makita, 2020), and biome type (Wang et al, 2018b;Valverde-Barrantes et al, 2021).…”