Takuo Hishi

Heterogeneity of individual roots within the fine root architecture: causal links between physiological and ecosystem functions

This review covers the heterogeneity in functions within the fine root architecture in order to clarify the multiple functions of fine roots. Many fine root characteristics, such as anatomy, physiology, morphology, and their consequences for the ecosystem, differ among root ages and ontogenetic branching hierarchies. Individual root age can be characterized by tissue development, with the main tissues developing from primary to secondary tissues. The physiological characteristics of individual roots, such as absorptivity and respiration rates, decrease with increasing branching order, mainly because of aging and tissue development. The C/N ratio and lignin and suberin contents also increase with branching order because of root aging. Morphological characteristics, such as diameter and specific root length, differ among root orders because of both aging and ontogenetic differences. The mortality of individual roots differs among branching orders and root diameters. The life cycles of roots in the fine root architecture, that is, ephemeral and perennial, indicate ontogenetic differences in functions and demographic traits, similar to those for leaves and branches in shoots. In addition, differences in individual root life cycles may affect the root chemical composition, in turn, affecting the decomposition rate. Future studies should seek to identify heterorhizic units in mortality related to anatomical, physiological, and morphological differences for various species. The decomposition processes of each mortality unit within the fine root architecture are also important in understanding the link between physiological and ecosystem functions.

Life cycles of individual roots in fine root system of Chamaecyparis obtusa Sieb. et Zucc.

Recent studies have remarked on differences in the life cycles of individual fine roots. However, the dynamics of individual roots with different life cycles, such as ephemeral and perennial, during root system development are still unknown. We examined individual roots during fine root system development in a mature stand of Chamaecyparis obtusa Sieb. et Zucc. (Cupressaceae) using the sequential ingrowth core method and an anatomical method. The visual classification, i.e., orange, red, brown, intact dead, and fragmented dead, of fine roots corresponded well with the anatomical classification. Orange and red roots contained passage cells, and brown roots contained cork cambium. The proportions of protoxylem groups differed among visual classes. Brown secondary roots were mainly triarch (43%) and tetrarch (40%) and rarely diarch (12%), whereas fragmented dead roots, which constituted more than 95% of the dead roots, were mainly diarch (67%). These results imply that triarch and tetrarch roots tend to form secondary roots, whereas diarch roots tend to become dead roots without secondary growth. Using the numbers of root tips and clusters, root system development could be classified into three stages: colonization, branching within the root system, and maintenance. During the colonization stage, mainly triarch and tetrarch roots, which tend to be secondary growth, invaded ingrowth cores. During the branching stage, primarily diarch roots, which tend to be ephemeral, emerged. Fine root system development involved the recruitment of different individual roots during the life cycle depending on the growth stage.

Seasonality of factors controlling N mineralization rates among slope positions and aspects in cool-temperate deciduous natural forests and larch plantations

This study aimed to evaluate the spatial patterns of soil nitrogen (N) transformations in relation to slope aspect and position, and to investigate the main factors controlling N transformation patterns during both the growing and dormant seasons in cool-temperate deciduous natural forests and larch plantations in eastern Hokkaido, northern Japan. Net rates of N mineralization (NRminN) and of nitrification (NRnit) in surface soils on north-facing and lower slopes were higher than those on south-facing and upper slopes, whereas the net rate of ammonium-N production (NRamm) on south-facing and upper slopes was higher than that on north-facing slopes in both the natural forests and larch plantations. Both NRminN and NRnit were higher in the growing than in the dormant season, whereas NRamm was higher in the dormant season. The soil C/N ratio, water content, soil pH and frequency of freeze–thaw cycles were important variables affecting N transformation patterns in any season. In relation to seasonality, the solar radiation index, daily temperature range and earthworm biomass were important controlling factors only during the growing season, and watershed area and soil N concentration only during the dormant season, suggesting that biological control accompanied with wet–dry events were important factors affecting N transformations during the growing season, but that run-off water and chemical controls were important determinants of spatial variation in N transformations during the dormant season.

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

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.

Effects of soil origin and current microclimate conditions on nitrogen mineralization in forest soil on different slope aspects in Hokkaido, Japan

Climate change may alter the rate of soil N transformation. Therefore, it is important to investigate how climate conditions and soil properties affect soil N transformation. In the present study, soil transplantation experiments were performed using soils on a xeric south-facing slope and a mesic north-facing slope in cool-temperate broad-leaved natural forests. Soil N transformation rates and leaching between slopes were compared using the resin-core method to clarify whether soil history (soil origins) or current environmental condition (locations) is the most important factor affecting soil N dynamics. The annual N mineralization did not differ significantly among soil from different origins and locations. In both locations, the annual net ammonification in south-facing soils was higher than that in north-facing soils, whereas the annual nitrification of north-facing soil was higher than that of south-facing soil. N mineralization and nitrification in north-facing soil were significantly higher during the growing season. N mineralization in south-facing soil was not significantly different between seasons. The interaction effect among seasons, soil origin, and location on net ammonification was significant. Net ammonification was higher in south-facing than in north-facing soils, and on south-facing than on north-facing slopes during the dormant season, suggesting that environmental change during winter affected the ammonification of south-facing soil. During the dormant season, N mineralization and leaching were not enhanced in soil of either origin at the transplanted locations, compared with the original locations, suggesting that, in this region, snow regime changes might not enhance the risk of N loss from forest ecosystems.

Nitrate leaching from Japanese temperate forest ecosystems in response to elevated atmospheric N deposition

ABSTRACT The effects of elevated atmospheric nitrogen (N) deposition on N cycling in Japanese forest ecosystems are reviewed here to assess the sensitivity of nitrate (NO3−) leaching from forests to streams in response to this deposition. Long-term monitoring of atmospheric N deposition throughout Japan suggests that long-range transport of air pollutants from East Asia accounts for recent increases in atmospheric N deposition in Japan. A new conceptual model of N saturation proposes that kinetic N saturation can be related to unavoidable hydrological nitrate (NO3−) loss from Japanese forests having an Asian monsoon climate with warm and wet summers. Soil microbes, including fungi and ammonia-oxidizing archaea, are important players in these forest ecosystems, affecting nutrient cycling in response to increased atmospheric N deposition. Similarly, a new framework based on a mycorrhizal-associated nutrient economy provides an understanding of NO3− leaching in soils from temperate forests, suggesting that arbuscular mycorrhizal (AM)-dominated forests leach more NO3− in response to N deposition than ectomycorrhizal fungi-dominated forests. Because Japanese cedar and cypress are AM-associated trees, they are likely susceptible to NO3− leaching. Maturation and poor management of such plantations may accelerate NO3− leaching. Grazing by increasing deer populations also enhances NO3− leaching from the soil system. We argue that several factors, including long-range transport of air pollutants, an Asian monsoon climate setting, as well as maturation and poor management of AM-associated plantations, will make Japanese temperate forests more sensitive to increased atmospheric N deposition in East Asia over the next decades.

A new genus and species of myrmecophilous brentid beetle (Coleoptera: Brentidae) inhabiting the myrmecophytic epiphytes in the Bornean rainforest canopy.

Pycnotarsobrentus inuiae Maruyama & Bartolozzi, gen. nov. and sp. nov. (Brentinae: Eremoxenini) is described from the Lambir Hills National Park, Borneo (Sarawak, Malaysia) based on specimens collected from Crematogaster difformis F. Smith, 1857 ant nests in the myrmecophytic epiphytic ferns Platycerium crustacea Copel. and Lecanopteris ridleyi H. Christ. A second species of Pycnotarsobrentus is known from Malaysia but is represented by only one female and consequently not yet described pending discovery of a male. Pycnotarsobrentus belongs to the tribe Eremoxenini and shares some character states with the African genus Pericordus Kolbe, 1883. No species of Eremoxenini with similar morphological modifications are known from the Oriental region.