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

Abstract

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 hypothesized that size variation in the tissue outside the stele is related in a nonlinear fashion with functional traits associated with direct resource uptake, including a negative relationship with root architectural traits, and that similar relationships detected in tropical areas also hold true in other biomes. METHODS:We addressed 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 metaanalysis of root traits for 500+ forest species across tropical, subtropical, and temperate forests. RESULTS:Our results supported the expected nonlinear relationships between cortical size and morphological traits and a negative linear trend with architectural traits. We confirmed a global negative relationship among specific root length (SRL), diameter, and tissue density, suggesting similar anatomical constraints in root systems across woody plants. However, the importance of factors varies across biomes, possibly related to the unequal phylogenetic representation across latitudes. CONCLUSIONS: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.

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

“…Differences in environmental condition and phylogenetic group may cause differences in root trait adjustment (Wang et al, 2018b) and mycorrhizal dependency (Valverde-Barrantes et al, 2016), and changes in both factors could strengthen, weaken, or fully decouple correlations among root traits (Ma et al, 2018;Valverde-Barrantes et al, 2020. 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).…”

“…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).…”

“…Another knowledge gap in root ecology involves a comparison of root trait covariation across biomes, as plants adapted to different environmental conditions may exhibit diverse root trait syndromes (Ma et al, 2018;Valverde-Barrantes et al, 2021). According to the RES hypothesis, SRL is expected to be negatively associated with RTD, reflecting the trade-off between root lifespan and construction cost (Roumet et al, 2016;Weemstra et al, 2016).…”

Tradeoffs and Synergies in Tropical Forest Root Traits and Dynamics for Nutrient and Water Acquisition: Field and Modeling Advances

“…We assessed a subset of 240 tropical plant species (Appendix 3) included in a broader study of root trait syndromes (Valverde-Barrantes et al, 2021), to show that among tropical plants, the strongest fine root morphological tradeoff is between SRL and diameter, which create an axis along which woody and non-woody tropical species separate (Figure 3), with a second axis separating along RTD and root N content. These axes separating fine root morphological traits and nutrient content could be explained by different root architectural anatomies, and in particular differences in cortical versus vascular root tissues (Figure 3).…”

Comparison of Leaf and Fine Root Traits Between Annuals and Perennials, Implicating the Mechanism of Species Changes in Desertified Grasslands

“…Literature demonstrated that this coordination varied between growth forms and strategies. For example, Valverde-Barrantes et al (2021) found that among woody plants, high LN is associated with low RTD, whereas among non-woody plants, low SRL is associated with high LN. Fort et al (2012) showed that for the same SLA, drought-tolerant species exhibit higher root mass and lower SRL than drought-sensitive species, to develop and maintain a coarser and deeper root system.…”