Changes in the anatomy, morphology and mycorrhizal infection of fine root systems ofCryptomeria japonicain relation to stand ageing

Abstract

Biomass allocation to fine roots often increases under soil nutrient deficiency, but the fine root biomass does not often increase in old stands, even under nutrient limitation. Therefore, in old stands, the morphology, anatomy, branching architecture and mycorrhization of fine roots may compensate efficiently for nutrient acquisition by the low fine root biomass. In this study, changes in the morphology, anatomy and arbuscular mycorrhizal infection at each branching position of fine root clusters were evaluated in relation to stand age. A chronosequence (6–90 years of age) of stands in a Cryptomeria japonica D. Don plantation was used for these analyses. The fine root size parameters, such as length, weight and tip numbers of fine root clusters, increased with stand age. The specific root tip length (SRTL) decreased with increasing stand age, suggesting that the allocation to root active portions decreased with stand age. From the anatomical observation, the ephemeral root tips increased with stand age, suggesting that root tip turnover within a root cluster was high in old stands. The proportions of proto-xylem groups among branching positions indicated that the life cycles in branching hierarchy should be clearer in old stands than that in younger stands. The increasing in the mycorrhizal infection of root tips in old stands should enhance the root tip absorptive functions. The SRTL was correlated with the wood/needle ratio, suggesting that carbon limitation as the stand ages may result in decline of carbon allocation to maintain active root tips. However, increasing of the ephemeral tips and mycorrhizal infection rates may compensate the declines of tip allocation in old stands.

Future direction of searching for root economics spectrum: focusing on the fibrous roots “absorptive unit”

“…Experiment of examining fine roots of twenty-three Chinese temperate tree species has demonstrated that one species of similar tree age within a certain climatic region display relatively consistent order constitution of absorptive roots even though researchers did not separate the pioneer roots from fibrous roots (Guo et al 2008). This result is consistent with the studies of Zadworny et al (2016) and Hishi et al (2016).…”

“…Although studies of fine roots in Pinus sylvestris and Cryptomeria japonica have demonstrated that order constitution of absorptive roots adjust along a climatic gradient (Zadworny et al 2016) or may change with stand age (Hishi et al 2016) and Zadworny et al distinguished the first-order pioneer roots from first-order fibrous roots, both of the studies did not exclude all the pioneer roots and the roots in their samples were actually in various ages of different root types. Pioneer roots in their samples could influence the measurements of order constitution of absorptive roots.…”

“…) or may change with stand age (Hishi et al. ) and Zadworny et al. distinguished the first‐order pioneer roots from first‐order fibrous roots, both of the studies did not exclude all the pioneer roots and the roots in their samples were actually in various ages of different root types.…”

“…() and Hishi et al. () have shown that order constitution of absorptive roots adjust along a climatic gradient or change with stand age.…”

“…However, it is difficult to separate all higher-order pioneer roots from fibrous roots within the same order (e.g., second-order roots may contain transport pioneer roots and absorptive fibrous roots; Fig. 3) and this crucial step is often neglected by researchers (e.g., Hishi et al 2016, Zadworny et al 2016. It should be noted that the frequency of pioneer roots formation is influenced by their environment Eissenstat 2011, Bagniewska-Zadworna et al 2012) and tree ages (Zadworny and Eissenstat 2011), this is probably why studies of Zadworny et al (2016) and Hishi et al (2016) have shown that order constitution of absorptive roots adjust along a climatic gradient or change with stand age.…”