Kazuki Nanko

The total amounts of radioactively contaminated materials in forests in Fukushima, Japan

There has been leakage of radioactive materials from the Fukushima Daiichi Nuclear Power Plant. A heavily contaminated area (≥ 134, 137Cs 1000 kBq m−2) has been identified in the area northwest of the plant. The majority of the land in the contaminated area is forest. Here we report the amounts of biomass, litter (small organic matter on the surface of the soil), coarse woody litter, and soil in the contaminated forest area. The estimated overall volume and weight were 33 Mm3 (branches, leaves, litter, and coarse woody litter are not included) and 21 Tg (dry matter), respectively. Our results suggest that removing litter is an efficient method of decontamination. However, litter is being continuously decomposed, and contaminated leaves will continue to fall on the soil surface for several years; hence, the litter should be removed promptly but continuously before more radioactive elements are transferred into the soil.

Assessing raindrop impact energy at the forest floor in a mature Japanese cypress plantation using continuous raindrop-sizing instruments

The raindrop size distribution of throughfall and open rainfall was monitored continuously during a rainfall event using laser raindrop-sizing instruments (LD gauges), in order to calculate the raindrop impact energy in a plantation of mature Japanese cypress (Chamaecyparis obtusa), where surface erosion at the forest floor had been a problem. Data from two rainfall events were analyzed. The LD gauges recorded qualitative raindrop size distribution, and the capture rate during each rainfall event was used to manipulate raindrop data quantitatively. Throughfall and open rainfall comparisons revealed several important differences. First, throughfall raindrops were fewer in number and larger in size than open rainfall drops. In one rainfall event, for example, throughfall raindrops were less than one-fifth as frequent as open rainfall raindrops; in addition, the maximum throughfall raindrop diameter was 6.35 mm compared to 3.31 mm for open rainfall raindrops. Second, throughfall raindrops that were larger than the largest open rainfall raindrops comprised 63.8% of the throughfall precipitation by volume. Third, total raindrop impact energy from throughfall was over twice that of open rainfall. Moreover, comparison of throughfall events implied that throughfall raindrops did not always have a uniform distribution between different events or among different periods of time in one rainfall event, in contrast to findings in previous studies which showed that throughfall raindrops had a uniform size distribution independent of rainfall intensity. It is possible that an abrupt transition of throughfall intensity from low to high changes the distribution of throughfall raindrops.

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.

Throughfall under a teak plantation in Thailand: a multifactorial analysis on the effects of canopy phenology and meteorological conditions

Valuable teak (Tectona grandis Linn. f.) plantations cover vast areas throughout Southeast Asia. This study sought to increase our understanding of throughfall inputs under teak by analyzing the abiotic and biotic factors governing throughfall amounts and ratios in relation to three canopy phenophases (leafless, leafing, and leafed). There was no rain during the brief leaf senescence phenophase in our study. Leveraging detailed field observations, we employed boosted regression tree (BRT) analysis to identify the primary controls on throughfall amount and ratio during each canopy phenophase. Whereas throughfall amounts were always dominated by rainfall magnitude (as expected), throughfall ratios were governed by a suite of predictor variables during each phenophase. The BRT analysis demonstrated that throughfall ratio in the leafless phase was most influenced (in descending order of importance) by air temperature, rainfall amount, maximum wind speed, and rainfall intensity. Throughfall ratio in the leafed phenophase was dominated by rainfall amount. The leafing phenophase was an intermediate case where rainfall amount, air temperature, and vapor pressure deficit were most important. Our results highlight the fact that throughfall ratios are differentially influenced by a suite of meteorological variables during each canopy phenophase. Abiotic variables, such as rainfall amount and air temperature, trumped leaf area index and stand density in their effect on throughfall ratio. The leafing phenophase, while transitional in nature and short in duration, has a detectable and unique impact on water inputs to teak plantations. Further work is needed to better understand the biogeochemistry of leaf emergence in teak plantations.

Differences in throughfall drop size distributions in the presence and absence of foliage

ABSTRACT Throughfall drop size distributions (DSDs) are important for plant–soil interactions. This is the first known study to quantify differences in throughfall DSDs with the presence and absence of foliage. Employing a disdrometer, three parameters solely representing throughfall drip were measured and calculated: maximum drop diameter (DMAX), median volume diameter of drops (D50DR) and relative volume percentage of drops (pDR). Beneath Liriodendron tulipifera L. in Maryland (USA), DMAX, D50DR and pDR were substantially larger when the canopy was unfoliated. In fact, the presence or absence of foliage was one of the primary factors affecting all three throughfall DSDs along with air temperature, according to the boosted regression tree analysis. Experimental results were attributed to differing physical properties of intercepted water between foliated and unfoliated periods and differential water behavior on leaves and bark. Future work should examine the effects of concentrated drip points on the development of throughfall-induced hot spots. Editor M.C. Acreman; Associate editor F. Hattermann

Effects of plant roots on the soil erosion rate under simulated rainfall with high kinetic energy

ABSTRACT This study examined the effects of herbaceous plant roots on interrill erosion using two herbaceous species: clover (Trifolium repens) and oats (Avena sativa). We developed a simple rainfall simulator with relatively high normalized kinetic energy (KE; 23.2 J m−2 mm−1). Under simulated rainfall, we measured eroded soil for 42 boxes with various amounts of aboveground and belowground biomass. Aboveground vegetation had a significant effect on the soil erosion rate (SER). We found a clear negative relationship between the percent vegetation cover (c) and the SER. In contrast, plant roots showed no effects on the SER. The SER was not significantly different between the boxes with and without plant roots under similar c conditions. Thus, plant roots could have less of an effect on the SER under higher KE conditions. Editor M.C. Acreman Associate editor N. Verhoest

Stemflow-induced spatial heterogeneity of radiocesium concentrations and stocks in the soil of a broadleaved deciduous forest

The transport of radiocesium from the canopy and quantification of the spatial distribution of radiocesium in the soil of konara oak forests are important to better understand the variability of 137Cs stocks in the soil between proximal and distal stem areas as well as fine-scale variations around the tree trunk. Moreover, a better understanding of fine-scale spatial variabilities of 137Cs concentrations and stocks will provide insights for optimizing soil sampling strategies to provide a more robust estimation of contamination at the stand scale. This study aims to elucidate the transport of 137Cs by stemflow in a radioactively contaminated konara oak forest in Tsukuba, Japan by describing and quantifying the fine-scale spatial distribution of 137Cs in the soil and preferential flowpaths of stemflow on the tree stem by a dye tracing experiment. 137Cs concentrations and stocks were higher in the soils of the proximal stem area than distal stem area when they corresponded with the preferential flowpaths of stemflow. There was a significant relationship between canopy projection area of individual trees and average soil 137Cs concentrations and stocks, even though canopies of the trees overlapped. Our results demonstrate that the spatiality of 137Cs concentrations and stocks in the soil of the proximal stem area are governed (at least partially) by the preferential flowpaths of stemflow along the tree trunk. In addition, higher 137Cs concentrations and stocks in the near-trunk soils of trees with larger crown areas might be caused by an enhanced ability to capture dry deposition.

Assessment of soil group, site and climatic effects on soil organic carbon stocks of topsoil in Japanese forests

Summary The aim of this study was to assess the factors that account for the geographical variation in soil organic carbon stocks at the 0–30‐cm depth (SOC30) of forests in J apan. Boosted regression tree analysis was applied to 2157 points throughout J apan and to four regional geographical subdivisions with 16 environmental variables. The rank of predictor variables was different for J apan as a whole and among the regions. For J apan as a whole, soil group, air temperature, slope inclination, altitude and organic carbon stocks of litter were the most important factors that affected SOC30 stocks. Overall, SOC30 stocks decreased with air temperature, which was attributed to the decomposability of organic carbon. In addition, SOC30 stocks decreased with slope inclination because of instability of the topsoil on slopes, which, in turn, is related to the increase in rock fragment content and decrease in soil bulk density. The distribution of volcanic soil resulted in larger SOC30 stocks than was expected from climatic conditions. Precipitation was not important because of conflicting effects between the increase in soil organic carbon content with increasing net primary production and the decrease in mineral soil mass by the loss in topsoil. The regional analyses provide insight into the factors that cause variation in SOC30 stocks, which were obscured by the macroscale analysis of Japan as a whole, thereby illustrating the power of regional geographical analyses. Our results provide an improved basis for soil, forestry and biogeochemical models that require accurate estimates of SOC30 stocks. HighlightsWe assessed factors that account for geographic variation in soil organic carbon (SOC) stocks.Effect and dependence of factors were estimated by a machine learning approach.SOC stocks were affected by soil type, climate, site‐specific location and organic matter input.Volcanic soil distribution, climate, slope steepness and historical overuse of forest affected SOC stocks.

A phenomenological model for throughfall rendering in real-time

This paper aims at rendering interactive visual effects inherent to complex interactions between trees and rain in real‐time in order to increase the realism of natural rainy scenes. Such a complex phenomenon involves a great number of physical processes influenced by various interlinked factors and its rendering represents a thorough challenge in Computer Graphics. We approach this problem by introducing an original method to render drops dripping from leaves after interception of raindrops by foliage. Our method introduces a new hydrological model representing interactions between rain and foliage through a phenomenological approach. Our model reduces the complexity of the phenomenon by representing multiple dripping drops with a new fully functional form evaluated per‐pixel on‐the‐fly and providing improved control over density and physical properties. Furthermore, an efficient real‐time rendering scheme, taking full advantage of latest GPU hardware capabilities, allows the rendering of a large number of dripping drops even for complex scenes.