Mt. Hayachine in Iwate prefecture is characterized by serpentine site, which is known to have high concentrations of Ni. In general, few plants that can tolerate high concentrations of Ni can grow in serpentine soil. Thujopsis dolabrata var. hondai occurs naturally on Mt. Hayachine. In this study, we hypothesized that T. dolabrata var. hondai shows Ni tolerance due to detoxification by phenolics. We collected T. dolabrata var. hondai seedlings and root-zone soil from the Kadoma National Forest on Mt. Hayachine to analyze the concentrations of Ni, other heavy metals, and macronutrients. The seedling roots had high concentrations of Ni. Further, we conducted a pot experiment by using 1-month-old sterile seedlings grown in three types of sterilized soils-Kadoma soil (obtained from the Kadoma National Forest), Tsugaru forest soil, and nursery soil-and compared the concentrations of Ni, nutrients, and catechin, as well as the growth of seedlings in the different soils. The pot experiment indicated that the roots of seedlings grown in Kadoma forest soil contained high concentrations of Ni and catechin, which could act as an antioxidant and a possible Ni-chelating compound that detoxified Ni in the plant cells. We concluded that T. dolabrata var. hondai seedlings growing in the serpentine site of Mt. Hayachine accumulated Ni and could detoxify it by producing high concentrations of catechin.
Fine roots of Cryptomeria japonica were separated into two functional groups: primary roots that serve as the principal agent for water and nutrient absorption and secondary roots that have transport capacity and protect the plant from environmental stress. Individual roots can also be categorized by three characteristics: diameter, branching order, and number of protoxylem groups. We investigated the relationships of these two functional groups with the three categories and evaluated which category was a better index for distinguishing primary from secondary roots by using the Pianka overlap index. Primary and secondary roots showed no exact correspondence to any of the three categories and had overlap in each category. Therefore neither was a useful indicator to distinguish primary from secondary roots. However, in the case of Cryptomeria japonica, we can roughly distinguish primary from secondary roots on the basis of whether root diameter is less than or greater than 0.6 mm.
The importance of fine roots in forest ecosystem processes is well known. However, the contribution of understory vegetation to underground ecosystem processes is not well understood. We tested the hypothesis that fine-root biomass (FRB) and performance of the overstory and understory independently decrease with increasing soil N availability in cool-temperate deciduous broad-leaved natural forests and larch plantations in Japan. The mean contribution of understory FRB to total FRB (tree + understory) ranged from 4% to 78% (mean 37%). Tree FRB was negatively correlated with understory FRB, and understory FRB was dominant to tree FRB in infertile soil. Understory and total FRB were negatively correlated with soil net N mineralization rate, whereas tree FRB showed a quadratic relationship with soil N mineralization rate with the peak observed at mineralization of 58.4 kg N ha-1 y-1. The low tree FRB at infertile sites may be due to a belowground competitive effect of understory fine roots on tree FRB. Understory fine-root nitrogen concentration (FRN) and leaf to fine-root (L/FR) ratio were positively correlated with N mineralization rate. However, tree L/FR was not significantly correlated, whereas tree FRN was positively correlated, with soil N mineralization rate, suggesting that the leaf production efficiency of trees might not increase even on infertile soil. We suggest that belowground processes of overstory trees might change depending on understory vegetation, and that understory vegetation might affect the fine roots of overstory trees, which did not increase mass allocation but increased N use efficiency under low FRN.
Anthropogenically increased nitrogen (N) deposition may affect the nutrient dynamics of forested ecosystems. To investigate the potential effects of excessive N deposition on Japanese forests, we treated the soil in a 20-year-old Japanese cedar (Cryptomeria japonica) stand with 10 l m-2 of 10 mM HNO3 solution, 10 mM NH4NO3 solution, or tap water (as a control), monthly for 7 years. A total of 168 and 336 kg N ha-1 year-1 was added in the HNO3 and NH4NO3 plots, respectively. Tree growth, the amount of nutrients and the carbon concentration of both current shoots and fine roots (<2 mm in diameter) in the surface soil (0-5 cm) were measured. The foliar N concentration increased in both N-fertilized plots during the first 3 years, particularly in the NH4NO3 plots. Similarly, the fine-root N concentration was greater in the N-fertilized plots than in the control plots. However, growth in both height and diameter at breast height of Japanese cedar trees were not significantly affected by N fertilization. The foliar K and P concentrations tended to decrease in treatment plots over time when compared with the control plots. Our study suggests that 7 years of excessive N fertilization had no positive or negative effect on the growth of young Japanese cedar trees, although the nutrient status of current shoots and fine roots was altered.