Radiocaesium accumulation in stemwood: integrated approach at the scale of forest stands for contaminated Scots pine in Belarus

Scots pine stands biomass assessment using 3D data from unmanned aerial vehicle imagery in the Chernobyl Exclusion Zone

Thirty-five years after the accident, large forest areas in the Chernobyl Exclusion Zone still contain huge amounts of radionuclides released from the Chernobyl Nuclear Power Plant Unit 4 in April 1986. An assessment of the radiological and radioecological consequences of persistent radioactive contamination and development of remediation strategies for Chernobyl forests imply acquiring comprehensive data on their contamination levels and dynamics of biomass inventories. The most accurate forest inventory data can be obtained in ground timber cruises. However, such cruises in radioactive contaminated forest ecosystems in the Chernobyl Exclusion Zone result in radiation exposures of the personnel involved, which means the need for development of the remote sensing methods. The purpose of this study is to analyze the applicability and limitations of the photogrammetric method for the remote large-scale monitoring of aboveground biomass inventories. Based on field measurements, we estimated the biomass inventories in 31 Scots pine stands including both artificial plantations and natural populations. The stands differed significantly in age (from a few years in natural populations to 115 years in the oldest plantation), productivity (from 0.4 to 19.8 kg m^-2), mean height (from 4.1 to 36 m), and other parameters. Photogrammetric data were obtained from the same stands using unmanned aerial vehicle (UAV). These data were then processed using two approaches to derive the canopy height model (CHM) parameters which were tested for correlation with the aboveground biomass inventories. In the first approach, we found that the inventories correlated well with the mean value of CHM of the site (R² = 0.79). In the second approach, the total aboveground biomass was approximated by a function of the average height of trees detected at the site and the total crown projection area (R² = 0.78). Among other local parameters, the total crown projection area was identified as the major factor impacting the accuracy of the aboveground biomass inventory estimates from the UAV survey data in both approaches. In the dense stands with the high total crown projections areas (more than 0.90), the average relative deviations of the UAV-based aboveground biomass estimates from the results of the field measurements were close to 0, which means the adequate accuracy of the UAV surveys data for radioecological monitoring purposes. The relative deviations of the UAV-based estimates in both approaches increased in the stands consisting of separated groups of trees, which indicates potential limitation of the approaches and need for their further development.

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F_l/t), the migration center (X_c) and the relaxation depth (α) using 99 soil inventory data sets. F_l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F_l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X_c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X_c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high ¹³⁷Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of ¹³⁷Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of ¹³⁷Cs from organic to surface mineral soil layer after the second year. These results indicate that ¹³⁷Cs retention capacity of the organic layer can affect the apparent vertical migration of ¹³⁷Cs in the underlying mineral soil layer.

TRIPS 2.0: Toward more comprehensive modeling of radiocaesium cycling in forest

Because internal transfers can play a key role in radiocaesium persistence in trees, a reliable representation of radiocaesium recycling between tree organs in forest models is important for long-term simulations after radioactive fallout in Chernobyl and Fukushima. We developed an upgraded 2.0 version of the initial TRIPS ("Transfer of Radionuclides In Perennial vegetation System") model involving explicit differentiation between tree organs (i.e., foliage, branches, stemwood and bark). The quality of TRIPS 2.0 was evaluated by testing model outputs against independent datasets for pine stands in Belarus and Ukraine. Scenarios involving "hot particle" deposits in forest remained challenging, but in all other scenarios generally positive verification results for soil and tree compartments indicated that the TRIPS 2.0 model adequately combines the major relevant processes. Interestingly, the response of stemwood contamination to changes in radiocaesium availability in soil, as determined by soil conditions, was shown to be more sensitive than for other tree compartments. We recommend the conceptual tree discretization of TRIPS 2.0 for generic forest modeling for two reasons: 1) regardless of different soil conditions, there was concurrent good agreement between simulations and data for individual tree compartments (foliage, branches, stemwood and bark), and 2) the measurements necessary to estimate internal tree transfers are easily accessible to usual field monitoring in forest biogeochemistry (for details, see Goor, F. & Thiry, Y., 2004. Science of the total environment, 325(1-3), 163-180).

Development and assessment of a simple ecological model (TRIPS) for forests contaminated by radiocesium fallout

The management of vast forested zones contaminated by radiocesium (rCs) following the Chernobyl and Fukushima fallout is of great social and economic concern in affected areas and requires appropriate dynamic models as predictive or questioning tools. Generally, the existing radio-ecological models need less fragmented data and more ecological realism in their quantitative description of the rCs cycling processes. The model TRIPS ("Transfer of Radionuclide In Perennial vegetation Systems") developed in this study privileged an integrated approach which makes the best use of mass balance studies and available explicit experimental data for Scots pine stands. A main challenge was the differentiation and calibration of foliar absorption as well as root uptake in order to well represent the rCs biocycling. The general dynamics of rCs partitioning was simulated with a relatively good precision against an independent series of observed values. In our scenario the rCs biological cycling enters a steady-state about 15 years after the atmospheric deposits. At that time, the simulations showed an equivalent contribution of foliage and root uptake to the tree contamination. But the root uptake seems not sufficient to compensate the activity decline in the tree. The initial foliar uptake and subsequent internal transfers were confirmed to have a great possible impact on the phasing of tree contamination. An extra finding concerns the roots system acting as a buffer in the early period. The TRIPS model is particularly useful in cases where site-specific integrated datasets are available, but it could also be used with adequate caution to generic sites. This development paves the way for simplification or integration of new modules, as well as for a larger number of other applications for the Chernobyl or Fukushima forests once the appropriate data become available. According to the sensitivity analysis that involves in particular reliable estimates of net foliar uptake as well as root uptake not disconnected from rCs exchange reactions in soil.

The impact of radiocesium input forms on its extractability in Fukushima forest soils

The effects of ¹³⁷Cs deposit forms on its ageing in soil have not yet been reported. Soluble and Solid ¹³⁷Cs input forms were mixed with the mineral soils collected under Fukushima's coniferous and broadleaf forests, incubated under controlled laboratory, and examined the evolution of ¹³⁷Cs availability over time. Results show that the extracted ¹³⁷Cs fraction with water was less than 1% for the soluble input form and below detection limit for the solid input forms. Likewise, with an acetate reagent, the extracted ¹³⁷Cs fraction ranged from 46 to 56% for the soluble input and from 2 to 15% for the solid input, implying that the nature of the ¹³⁷Cs contamination strongly influences its extractability and mobility in soil. Although the degradation of organic materials was apparent, its impact on the ¹³⁷Cs extractability was found to be weak. Nevertheless, more Ac-available ¹³⁷Cs was obtained from broadleaf organic material mixes than the coniferous counterparts, suggesting that the lignified nature of latter tend to retain more ¹³⁷Cs. When extrapolated to a field context, more available ¹³⁷Cs fraction may be expected from wet-derived contaminated forest soils than contaminated via solid-derived inputs. Such information could be helpful for radioecological management schemes in contaminated forest environments.

Radiocaesium partitioning in Japanese cedar forests following the "early" phase of Fukushima fallout redistribution

Our study focused on radiocaesium (¹³⁷Cs) partitioning in forests, three vegetation periods after the Fukushima Daiichi nuclear power plant accident. ¹³⁷Cs distribution in forest components (organic and mineral soil layers as well as tree compartments: stem, bark, needles, branches and roots) was measured for two Japanese cedar stand ages (17 and 33 years old). The results showed that around 85% of the initial deposit was found in the forest floor and topsoil. For the youngest stand almost 70% of the deposit is present in the forest floor, whereas for the oldest stand 50% is present in the 0-3 cm mineral soil layer. For trees, old and perennial organs (including dead and living needles and branches, litter fall and outer bark) directly exposed to the fallout remained the most contaminated. The crown concentrated 61-69% of the total tree contamination. Surprisingly the dead organs concentrated 25 ± 9% (young cedars) to 36 ± 20% (mature cedar) of the trees' residual activity, highlighting the importance of that specific compartment in the early post-accident phase for Japanese cedar forests. Although the stem (including bark) represents the highest biomass pool, it only concentrates 3.3% and 4.6% of the initial ¹³⁷Cs deposit for mature and young cedars, respectively.