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.
Highlights
IntroductionFine roots can be categorized into two components, that is, absorptive and transport fine roots [3]
Within the fine root system, the individual roots vary greatly in anatomy, morphology, physiology, and mycorrhizal colonization, as well as in their inherent functions [1,2].Functionally, fine roots can be categorized into two components, that is, absorptive and transport fine roots [3]
Most of the first- and second-order roots were absorptive, having an intact cortex and mycorrhizal colonization, while the fourth- and fifth-order roots were transportive, with a disrupted cortex, well-developed secondary xylem, and continuous cork layer, which was consistent at the two soil layers (Figure 1, Table 1)
Summary
Fine roots can be categorized into two components, that is, absorptive and transport fine roots [3]. Any potential shift driven by environmental changes in root abundance and functional traits in both fine root types would result in a switch in dominant function (i.e., absorption or transportation) of a fine root system, and the related ecophysiological processes [4]. A previous study has reported that root tips (i.e., the first-order roots), the typical absorptive roots, show apparent changes in anatomy, morphology, and respiration associated with soil depths across three hardwood species [9]. Some studies have suggested that absorptive and transport fine roots respond differently to changed environmental conditions. Significant changes in root hydraulic conductance and root length were found for the low-order roots of Pinus tabulaeformis
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