Japanese Forest Ecosystems Research Articles

Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident 137Cs fallout

In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with 137Cs. However, 137Cs transfer processes determining tree contamination (particularly for stem wood, a prominent commercial resource) remain insufficiently understood. We propose a model for simulating dynamic behavior of 137Cs in a forest tree–litter–soil system and applied it to contaminated forests of cedar plantation and natural oak stand in Fukushima to elucidate relative impact of distinct 137Cs transfer processes determining the tree contamination. The transfer of 137Cs to the trees occurred mostly (>99%) through surface uptake of 137Cs trapped by needles and bark during the fallout. Root uptake of soil 137Cs was several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, internal contamination of the trees proceeded through an enduring recycling (translocation) of 137Cs absorbed on the tree surface. A significant surface uptake of 137Cs through bark was suggested, contributing to 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining uptake occurred at needles) of the total uptake by the trees, although that pathway still needs to be evaluated by experimental evidence. It was suggested that the activity concentration of 137Cs in stem wood of the trees at these sites are currently (as of 2021) decreasing by ~3% per year, mainly through radioactive decay of 137Cs and partly through dilution effect from tree growth. Although further refinement of the model is recommended, for example by including tree species specific 137Cs transportation in stem, these findings provide vital information for planning of forestry reactivation in Fukushima; e.g., removal of forest floor organic layer will not reduce the tree contamination for a long term because of the 137Cs absorption via the tree surface substantially greater than root uptake of 137Cs deposited to the floor.

Spatial distribution of mercury accumulation in the surface soil of Japanese forests

ABSTRACT Mercury (Hg) is a pollutant that can affect human and ecosystem health. The transport and fate of Hg in the environment are dynamic and complex, but our understanding remains unclear for Japanese forest ecosystems. Here, we report the first country-wide survey of Hg concentrations in litter and surface mineral soil layers in 42 forest stands across Japan. The median concentrations of Hg in the litter layer and 0–10 cm of mineral soils were 99 (range: 56.7–297) and 145 (range: 22.8–294) μg kg−1, respectively, and tended to decrease down to 50 cm. There was a positive relationship between the Hg and total organic carbon concentrations in soil, suggesting that organic carbon strongly binds with Hg in soil. There was a significant positive correlation between the Hg and lead concentrations in the litter layer. Partial least square regression analysis indicated that the soil and litter properties as well as the atmospheric and geological Hg concentrations and some soil characteristics such as soil pH, organic matter properties and soil physical factors were effective explanatory variables of the country-wide spatial patterns of Hg concentrations in litter and mineral soil. In addition, we found the influence of atmospheric Hg is stronger in litter layer, whereas the influences of geological and soil factors are stronger in mineral soil. These results suggest that air pollution partly affects the spatial patterns of Hg concentrations in litter and soil of Japanese forests under the given geological and soil conditions at the country scale.

Organic and inorganic nitrogen deposition in an urban evergreen broad-leaved forest in central Japan

Chronic atmospheric nitrogen (N) deposition could influence the functioning of ecosystems as well as their biodiversity. However, N deposition in urban forest ecosystems, especially natural evergreen broad-leaved forests, is not well known. In this study, the concentrations and fluxes of dissolved inorganic N (DIN) and dissolved organic N (DON) in bulk deposition, throughfall, and stemflow were assessed in an urban evergreen broad-leaved forest site over three years, in order to clarify the characteristics of N deposition. At the study site, bulk DIN deposition was 3.7 kg N ha−1 year−1 (1.5 kg N ha−1 year−1 for NH4–N and 2.2 kg N ha−1 year−1 for NO3 + NO2–N), which is the same level as that found in rural areas. In contrast, 6.5 kg N ha−1 year−1 for bulk DON deposition contributed to 66% of the bulk N deposition, which suggests the importance of bulk DON deposition in Japanese forest ecosystems. Passing through the tree canopy, DIN was enriched by 8.8 kg N ha−1 year−1 (3.7 kg N ha−1 year−1 for NH4–N and 5.1 kg N ha−1 year−1 for NO3 + NO2–N) and DON was enriched by 1.5 kg N ha−1 year−1 as net throughfall in the evergreen broad-leaved forest. This reveals that dry deposition of DIN dominates the total DIN deposition onto the urban forest floor, compared to that found in the rural areas, due to the non-negligible N emissions from outside and possibly because of the evergreen broad-leaved forest's greater ability to capture N.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified 137Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated 137Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the 137Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of 137Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1–2 years after the accident) for Japanese forest ecosystems.

Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with 137Cs. It is important to characterize the behavior of 137Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, 137Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and 137Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of 137Cs could be suppressed rapidly in Japanese forests. At all sites, 137Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous 137Cs supply from the organic layer. The diffusion coefficients of 137Cs in the mineral soil were estimated to be 0.042–0.55 cm2 y−1, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited 137Cs will migrate to the mineral soil but only less than 10% of the total 137Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the 137Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

Low 137Cs retention capability of organic layers in Japanese forest ecosystems affected by the Fukushima nuclear accident

Quantitative understanding of 137Cs retention behavior in organic layers in Japanese forest ecosystems is of primary importance in the assessment of long-term radioecological impacts of the Fukushima nuclear accident. Litter samples were collected at five forest sites in Fukushima in 2011, 2015, and 2017, and separated into eight fractions with different precursor species and degrees of degradation; afterward, 137Cs inventory was determined. All fractions exhibited a significant decrease in 137Cs inventory over the six-year period. The results suggested that the organic layers in Japanese forest ecosystems have a low 137Cs retention capability, although it differs depending on the forest type.

Radioactive and stable cesium isotope distributions and dynamics in Japanese cedar forests

Dynamics of the Fukushima-derived radiocesium and distribution of the natural stable isotope 133Cs in Japanese cedar (Cryptomeria japonica D. Don) forest ecosystems were studied during 2014–2016. For the experimental site in Yamakiya, Fukushima Prefecture, we present the redistribution of radiocesium among ecosystem compartments during the entire observation period, while the results obtained at another two experimental site were used to demonstrate similarity of the main trends in the Japanese forest ecosystems. Our observations at the Yamakiya site revealed significant redistribution of radiocesium between the ecosystem compartments during 2014–2016. During this same period radionuclide inventories in the aboveground tree biomass were relatively stable, however, radiocesium in forest litter decreased from 20 ± 11% of the total deposition in 2014 to 4.6 ± 2.7% in 2016. Radiocesium in the soil profile accumulated in the 5-cm topsoil layers. In 2016, more than 80% of the total radionuclide deposition in the ecosystem resided in the 5-cm topsoil layer.The radiocesium distribution between the aboveground biomass compartments at Yamakiya during 2014–2016 was gradually approaching a quasi-equilibrium distribution with stable cesium. Strong correlations of radioactive and stable cesium isotope concentrations in all compartments of the ecosystem have not been reached yet. However, in some compartments the correlation is already strong. An increase of radiocesium concentrations in young foliage in 2016, compared to 2015, and an increase in 2015–2016 of the 137Cs/133Cs concentration ratio in the biomass compartments with strong correlations indicate an increase in root uptake of radiocesium from the soil profile. Mass balance of the radionuclide inventories, and accounting for radiocesium fluxes in litterfall, throughfall and stemflow, enabled a rough estimate of the annual radiocesium root uptake flux as 2 ± 1% of the total inventory in the ecosystem.

High-resolution data on the impact of warming on soil CO2 efflux from an Asian monsoon forest

This paper describes a project for evaluation of global warming’s impacts on soil carbon dynamics in Japanese forest ecosystems. We started a soil warming experiment in late 2008 in a 55-year-old evergreen broad-leaved forest at the boundary between the subtropical and warm-temperate biomes in southern Japan. We used infrared carbon-filament heat lamps to increase soil temperature by about 2.5 °C at a depth of 5 cm and continuously recorded CO2 emission from the soil surface using a multichannel automated chamber system. Here, we present details of the experimental processes and datasets for the CO2 emission rate, soil temperature, and soil moisture from control, trenched, and warmed trenched plots. The long term of the study and its high resolution make the datasets meaningful for use in or development of coupled climate-ecosystem models to tune their dynamic behaviour as well as to provide mean parameters for decomposition of soil organic carbon to support future predictions of soil carbon sequestration.

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident.

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.

Post-deposition early-phase migration and retention behavior of radiocesium in a litter–mineral soil system in a Japanese deciduous forest affected by the Fukushima nuclear accident

The fate of radiocesium (137Cs) derived from the Fukushima nuclear accident and associated radiation risks are largely dependent on its migration and retention behavior in the litter–soil system of Japanese forest ecosystems. However, this behavior has not been well quantified. We established field lysimeters in a Japanese deciduous broad-leaved forest soon after the Fukushima nuclear accident to continuously monitor the downward transfer of 137Cs at three depths: the litter–mineral soil boundary and depths of 5 cm and 10 cm in the mineral soil. Observations were conducted at two sites within the forest from May 2011 to May 2015. Results revealed similar temporal and depth-wise variations in 137Cs downward fluxes for both sites. The 137Cs downward fluxes generally decreased year by year at all depths, indicating that 137Cs was rapidly leached from the forest-floor litter layer and was then immobilized in the upper (0–5 cm) mineral soil layer through its interaction with clay minerals. The 137Cs fluxes also showed seasonal variation, which was in accordance with variations in the throughfall and soil temperature at the sites. There was no detectable 137Cs flux at a depth of 10 cm in the mineral soil in the third and fourth years after the accident. The decreased inventory of mobile (or bioavailable) 137Cs observed during early stages after deposition indicates that the litter–soil system in the Japanese deciduous forest provides only a temporary source for 137Cs recycling in plants.

Factors contributing to soil nitrogen mineralization and nitrification rates of forest soils in the Japanese archipelago

Nitrogen (N) is the primary limiting nutrient for forest production. Therefore, understanding how environmental factors affect N transformation rates is essential for the provision of sustainable ecosystem services. Because these factors are interlinked, it is important to consider direct and indirect structural relationships to better understand the factors contributing to N transformations. In this study, we analyzed the structural cause–effect relationships surrounding N transformations by structural equation modeling using a database containing net and gross N transformation rates and related soil chemical properties from 38 sites across the Japanese archipelago. The average net N mineralization and nitrification rates in the Japanese forest soils were 0.62±0.68 and 0.59±0.65mgNkg−1d−1, respectively, and gross N mineralization and nitrification rates were 4.22±3.59 and 0.98±0.68mgNkg−1d−1, respectively. Compared with previous large scale studies, net and gross N transformation rates in Japanese forest soils were considerably diverse despite their relatively small land area and were representative of temperate forest ecosystems. Structural equation modeling analysis showed that net N transformations were directly affected by gross N transformations, which in turn were significantly and directly affected by soil organic matter contents. Soil organic matter was significantly affected by organic layer amount, tree species and soil group. The effect of soil group was the greatest among these factors, suggesting that soil organic matter contents in Japanese forest soils were mainly influenced by soil parent materials. This was especially evident for Andosols, which are derived from volcanic sediments and contain large amounts of soil organic matter leading to high N transformation rates in the Japanese forest soils. Among the factors related to organic layers and mineral soil layers, soil organic matter content and organic layer amount, which represent substrate availability, had significant effects on gross and net N transformation rates. However, by refining the scale of the dataset using soil groups/soil parent materials, the influence of substrate quality and soil chemical properties on N transformations was suggested. From the current dataset, it was indicated that soil parent materials were the most important factor controlling the pattern of N transformations in the soil of Japanese forest ecosystems. This conclusion should be repeatedly refined considering the spatial distribution of factors such as climatic conditions and forest types with additional site datasets obtained from future surveys.

Predicted spatio-temporal dynamics of radiocesium deposited onto forests following the Fukushima nuclear accident

The majority of the area contaminated by the Fukushima Dai-ichi nuclear power plant accident is covered by forest. To facilitate effective countermeasure strategies to mitigate forest contamination, we simulated the spatio-temporal dynamics of radiocesium deposited into Japanese forest ecosystems in 2011 using a model that was developed after the Chernobyl accident in 1986. The simulation revealed that the radiocesium inventories in tree and soil surface organic layer components drop rapidly during the first two years after the fallout. Over a period of one to two years, the radiocesium is predicted to move from the tree and surface organic soil to the mineral soil, which eventually becomes the largest radiocesium reservoir within forest ecosystems. Although the uncertainty of our simulations should be considered, the results provide a basis for understanding and anticipating the future dynamics of radiocesium in Japanese forests following the Fukushima accident.

Monotropastrum humile var. humile is associated with diverse ectomycorrhizal Russulaceae fungi in Japanese forests

AbstractMonotropastrum humile is nearly lacking in chlorophyll and obtains its nutrients, including carbon sources, from associated mycorrhizal fungi. We analyzed the mycorrhizal fungal affinity and species diversity of M. humile var. humile mycorrhizae to clarify how the plant population survives in Japanese forest ecosystems. We classified 78 samples of adult M. humile var. humile individuals from Hokkaido, Honshu, and Kyusyu Islands into 37 root mycorrhizal morphotypes. Of these, we identified 24 types as Russula or Lactarius fungal taxa in the Russulaceae, Basidiomycetes, but we could not identify the remaining 13 types as to their genus in the Basidiomycetes. The number of fungal species on M. humile var. humile was the highest in the plant subfamily. The diversity of fungal species revealed its increased trends in natural forests at the stand level, fagaceous vegetation, and cool‐temperate climate. The most frequently observed fungus colonized mainly samples collected from sub‐alpine forests; the second most frequently observed fungus colonized samples collected from sub‐alpine to warm‐temperate forests. These results suggest that Japanese M. humile populations are associated with specific but diverse fungi that are common ectomycorrhizal symbionts of various forest canopy trees, indicating a tripartite mycorrhizal relationship in the forest ecosystem.

Effects of high nitrogen load on growth, photosynthesis and nutrient status of Cryptomeria japonica and Pinus densiflora seedlings

To evaluate the sensitivity of Japanese cedar (Cryptomeria japonica D. Don) and Japanese red pine (Pinus densiflora Sieb. et Zucc.) to high N deposition, 1-year-old seedlings were grown in brown forest soil treated with N as NH4NO3 at 0, 25, 50, 100 and 300 mg l–1 fresh soil volume, equivalent to 0, 28, 57, 113 and 340 kg N ha–1. Net photosynthetic rate and whole-plant dry mass of C. japonica seedlings were increased by the N treatment, whilst those of P. densiflora seedlings were significantly reduced by the highest N treatment. The reduction in the net photosynthesis of P. densiflora seedlings was mainly due to a depression of carboxylation efficiency accompanied by a decrease in concentration and activity of Rubisco in the needles. In P. densiflora seedlings, needle concentrations of P and Mg were decreased, and the concentrations of N and Mn were increased by the highest N treatment. The reductions in needle protein concentration and Rubisco activity were negatively correlated with the ratios of N/P and Mn/Mg in the needles. These results suggest that nutrient imbalances of these elements may be induced in P. densiflora seedlings grown under high N deposition. We conclude that P. densiflora is more sensitive to high N deposition than C. japonica, and that the relatively high atmospheric N deposition to Japanese forest ecosystems may adversely affect the health of N-sensitive tree species such as P. densiflora.

Hydrobiogeochemistry of forest ecosystems in Japan: major themes and research issues

AbstractHydrobiogeochemical information on forested watersheds in the Japanese archipelago is reviewed to qualitatively summarize the major factors affecting the hydrology and biogeochemistry of this region. Major features that distinguish Japanese forest watersheds from those of north central Europe and the northeast United States generally include higher temperature, greater precipitation and steeper topography. There have been three major themes in hydrobiogeochemistry research on Japanese forest ecosystems: (1) investigations of nutrient cycles with particular emphasis on the establishment and maintenance of forest ecosystems; (2) evaluations of streamwater chemistry as an output from the forest ecosystem; and (3) hydrological studies using biogeochemical tracers. High precipitation inputs during the growing season affect the seasonality of the streamwater NO3− concentration, resulting in different temporal patterns than those generally found in north central Europe and the northeast United States. The high alkalinity and pH of Japanese surface waters is due to the rapid weathering of relatively young soils that are generated by steep hillslopes. Warm temperatures and elevated soil moisture enhance high rates of mineral weathering. Hydrological studies using biogeochemical tracers have shown that the steep topography contributes to the highly heterogeneous movement of water within Japanese catchments. Copyright © 2001 John Wiley & Sons, Ltd.

Importance of Internal Proton Production for the Proton Budget in Japanese Forested Ecosystems

Annual biogeochemical fluxes (bulk precipitation, throughfall, stem flow, soil solution and vegetation uptake) of inorganic elements were observed in eight cool temperature forested ecosystems in Hokkaido, northern Japan, in order to determine the mechanisms of acid neutralization in Japanese forest ecosystems. We compared our results with the other biogeochemical studies in Japan, north Europe and US from the literature. In many Japanese forests, the internal proton production (IPS) by base cation accumulation into the vegetation was a major proton source, rather than external acidic deposition, and the IPS also affected the base cation fluxes from the mineral. IPS in Japanese forest tended to be larger than that in north Europe and US. Our results suggested that the high acid neutralizing ability of Japanese forests could be attributed to the strong relationship between the base cation buffering of the soil and the larger contribution of IPS as a proton source. acidic deposition|biogeochemical cycling|forest ecosystem|Japan|proton budget