Functional Trait Plasticity but Not Coordination Differs in Absorptive and Transport Fine Roots in Response to Soil Depth

Abstract

Absorptive and transport fine roots (diameter ≤ 2 mm) differ greatly in anatomy, morphology, and physiology, as well as their responses to environmental changes. However, it is still not well understood how their functional traits and biomass repartition respond to resource variability associated with increasing soil depth. Herein, we sampled the first five order roots of three hardwoods, i.e., Juglans mandshurica Maxim., Fraxinus mandshurica Rupr., and Phellodendron amurense Rupr. at surface (0–10 cm) and subsurface (20–30 cm) soil layers, respectively, and measured root biomass, anatomy, morphology, chemistry, and physiology at the branch-order level. Based on the anatomical characteristics, absorptive and transport fine roots were identified within each order, and their amounts and functional trait plasticity to soil depth were examined. The results showed that across soil layers, the first three order roots were mainly absorptive roots, while the fourth- and fifth-order roots were transport ones. From surface to subsurface soil layers, both the number and biomass proportion of absorptive fine roots decreased but those of transport fine roots increased. Transport fine root traits were more plastic to soil depth than absorptive ones, especially for the conduit-related traits. Absorptive fine roots in surface soil generally had stronger potential for resource acquisition than those in deeper soil, as indicated by their longer specific root length and greater root branching density. In comparison, transport fine roots in deeper soil were generally enhanced in their transportation function, with wider stele and higher hydraulic conductivity. Our findings suggest that functional specialization via multi-trait plasticity and coordination in both absorptive and transport fine roots along the soil depth would benefit the efficient soil resource exploitation of trees in forest ecosystems.

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

“…Works from Chen et al (2013) and Kong et al (2014) measured branchiness as the ratio between first and second root orders, rather than the more conventional measurement based on entire root systems of 3-4 root orders. Future work in more controlled conditions and standardized methodologies could help elucidate intraspecific variation in the architecture and anatomy of absorptive roots and the integration between root morphology and function in woody plants (Wang et al, 2020).…”

The Hierarchy of Protoxylem Groupings in Primary Root and Their Plasticity to Nitrogen Addition in Three Tree Species

“…Here, three hardwood species, Juglans mandshurica, Fraxinus mandshurica and Phellodendron amurense, were chosen in Northeast China, which were markedly different in root branching architecture and root functional traits (Wang et al, 2016(Wang et al, , 2020. After root pruning and following fertilization performed in the field, newly formed root branches were harvested, the functional traits of different root developmental order were also compared.…”

Do root modules still exist after they die?

“…The findings in this study have the following important implications. Firstly, studies have proved that ephemeral live root module generally exists in the distal branches (i.e., the first three-branch orders) through investigating the life-span, respiration, anatomy, and N concentration of each individual order of fine-root system in some forest tree species (e.g., Guo et al 2008b;Liu et al 2016;Wang et al 2019a;Wang et al 2019b;Yan et al 2019a). In recent years, a growing number of studies have examined the heterogeneity in root decomposition among root order groups in some given tree species (e.g., Goebel et al 2011;Sun et al 2013aSun et al , 2013bSun et al 2016b;Beidler and Pritchard 2017;Yang et al 2019).…”

“…Current evidence has indicated that distal branch orders are a relatively specific subgroup with characteristic traits, e.g. regarding morphology, physiology, anatomy and lifespan, compared to their higher-order mother roots (Guo et al 2008b;Liu et al 2016;Wang et al 2019a;Wang et al 2019b;Yan et al 2019a). Xia et al (2010) had proposed a new approach with root modules separated into groups with different functions instead of traditional classifications based on root diameter.…”