The dendroanalysis of oak trees as a method of biomonitoring past and recent contamination in an area influenced by uranium mining

Trace elements distribution in tropical tree rings through high-resolution imaging using LA-ICP-MS analysis

BACKGROUND

The distribution of trace elements in tree rings although poorly known may be useful to better understand environmental changes, pollution trends, long-term droughts, forest dieback processes, and biology of trees.

METHOD

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used for imaging micronutrients and potentially toxic elements distribution, allowing the investigation of trace elements at high spatial resolution within the tree rings. To ensure a more efficient determination of micronutrients and potentially toxic elements, LA-ICP-MS instrumental conditions were optimized and carbon, a major element in wood, is used as an internal standard during analysis to correct for random fluctuations.

RESULTS

Spatial distributions maps of Ba, Cu, Fe, Mn, Ni, and Pb in growth layers of six tropical tree species were built-up using the LA-iMageS software, namely: Amburana cearensis (Fabaceae), Cedrela fissilis (Meliaceae), Hymenaea courbaril (Fabaceae), Maclura tinctoria (Moraceae), Parapiptadenia zehntneri (Fabaceae), Peltogyne paniculata (Fabaceae). A correlation between the trace element composition and different cell types (parenchyma, fiber, and vessel) was distinctly observed. It was observed a general pattern of Ba, Cu, Ni, Mn, and Pb accumulation mainly in the axial parenchyma and vessels. But the elemental composition of xylem cells is strongly species dependent. The multivariate analysis also points to a distinct accumulation of minerals between heartwood and sapwood in the same species.

CONCLUSIONS

Imaging both essential and deleterious element distributions in the tree rings may improve visualization and can effectively contribute to understanding the lifetime metabolism of trees and evaluating the effects of environmental changes related to climatic seasonality, pollution, and future paleoclimate reconstructions.

An integrated approach combining soil profile, records and tree ring analysis to identify the origin of environmental contamination in a former uranium mine (Rophin, France)

Uranium mining and milling activities raise environmental concerns due to the release of radioactive and other toxic elements. Their long-term management thus requires a knowledge of past events coupled with a good understanding of the geochemical mechanisms regulating the mobility of residual radionuclides. This article presents the results on the traces of anthropic activity linked to previous uranium (U) mining activities in the vicinity of the Rophin tailings storage site (Puy de Dôme, France). Several complementary approaches were developed based on a study of the site's history and records, as well as on a radiological and chemical characterization of soil cores and a dendrochronology. Gamma survey measurements of the wetland downstream of the Rophin site revealed a level of 1050 nSv.h^-1. Soil cores extracted in the wetland showed U concentrations of up to 1855 mg.kg^-1, which appears to be associated with the presence of a whitish silt loam (WSL) soil layer located below an organic topsoil layer. Records, corroborated by prior aerial photographs and analyses of ¹³⁷Cs and ¹⁴C activities, suggest the discharge of U mineral particles while the site was being operated. Moreover, lead isotope ratios indicate that contamination in the WSL layer can be discriminated by a larger contribution of radiogenic lead to total lead. The dendroanalysis correlate U emissions from Rophin with the site's history. Oak tree rings located downstream of the site contain uranium concentrations ten times higher than values measured on unaffected trees. Moreover, the highest U concentrations were recorded not only for the operating period, but more surprisingly for the recent site renovations as well. This integrated approach corroborates that U mineral particles were initially transported as mineral particles in Rophin's watershed and that a majority of the deposited uranium appears to have been trapped in the topsoil layer, with high organic matter content.

Reduced above-ground growth and wood density but increased wood chemical concentrations of Scots pine on relict charcoal hearths

Deciphering the drivers of tree growth is a central aim of dendroecology. In this context, soil conditions may play a crucial role, since they determine the availability of water and nutrients for trees. Yet, effects of systematically differing soil conditions on tree growth render a marginally studied topic. In this context, relict charcoal hearths (RCH) - a widespread legacy of anthropogenic charcoal production - render a valuable 'natural' experiment to study possible effects of artificially altered soil conditions on tree growth. We hypothesize, that the differing physico-chemical properties of RCH result in differing wood properties if compared to trees growing on unmodified soils. To test this hypothesis, we analyzed tree-growth, wood density, and wood elemental concentrations of Scots pine as well as physico-chemical soil properties. We applied a classic control-treatment design to compare RCH with unmodified soils. Our analyses identified significantly lower above-ground wood production but systematically higher wood elemental concentrations in RCH-trees compared to control trees. Since we could not identify treatment-specific growth patterns, we hypothesize the observed lower above-ground productivity of Scots pine to indicate an increased root-shoot ratio to compensate for a potentially lower plant water availability on RCH-sites. The observed higher wood elemental concentrations likely reflect higher soil elemental concentrations of Fe, Ca, K, and Mn in RCH soils. In conclusion, our study highlights diverse effects of RCH on tree growth and wood properties and strengthens the value of dendro-chemistry to use the tree-ring archive as proxy for soil conditions within a dendro-ecological context.

Do tree rings record changes in soil fertility? Results from a Quercus petraea fertilization trial

Through the variations in their dimension, density, anatomy or isotopes composition, tree rings have provided invaluable proxies to evaluate past changes in the environment. Whereas long-term records of changes in soil fertility are particularly desired for forest ecosystem studies, the use of the chemical composition of tree rings as potential marker is still controversial. Dendrochemistry has sometimes been considered as a promising approach to study past changes in soil chemistry, whereas some authors stated that element translocations in the wood preclude any possibility of reliable retrospective monitoring. Here, we aimed at testing whether the wood elemental content of fertilized oaks (Quercus petraea) differed from control trees >30 years after a NPKCaMg fertilization and, if so, if the date of fertilization could be retrieved from the ring analysis. The contents in N, Mg, P, K, Ca and Mn were measured for each of the 43 sampled trees and in every ring of the 58-year long chronology with a non-destructive method coupling a Wavelength Dispersive Spectroscope (WDS) with a Scanning Electron Microscope (SEM). The results showed significantly higher contents in Ca and lower contents in Mn in fertilized compared to control trees. However, there was no difference in elemental content between the rings of the fertilized trees built in the 20 years before and those built after fertilization. Thus, whereas the effect of fertilization on increasing ring width was dramatic, immediate and relatively short-lasting, the elemental composition of the entire ring sequence was impacted, precluding the dating of the event. These results question the possibility to reconstruct long-term changes in soil fertility based on dendrochemistry.

Determination of Metal Concentration in Road-Side Trees from an Industrial Area Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Historical pollution can be elucidated with variations of elements’ concentration in tree rings by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). However, the capacity of chemical elements’ absorption significantly depends on the tree species and element types. Metal concentrations in the rings for five species (Platanus occidentalis, Salix koreensis, Chamaecyparis obtusa, Pinus densiflora, Ginkgo biloba) were investigated in light of metal pollution history in ambient air of D industrial site located in Daejeon, Korea. The calibration for LA-ICP-MS was performed using cellulose-matrix matched standards with 13C normalization. Tree ring series except for Ginkgo sp. showed that the accumulation rates of Pb and Cd were higher between 1992 and 1999. Other elements, such as Fe, Cr, Mn, Cd, Zn, and Sr, showed a variation in the rings, likely due to the different physiological processes of element uptake and radial mobility. Concentrations of Pb and Cd in the annual rings of Pinus sp. corresponded to the metal monitoring data for the ambient air with the correlation coefficients of 0.879 and 0.579, respectively. Moreover, Cd in Platanus sp. and Pb in Salix sp. showed a positive correlation to ambient metal concentration compared to Chamaecyparis sp. and Ginkgo sp. Therefore, caution should be taken to select candidate elements as well as tree species to reconstruct the ambient air metal pollution history by measuring the concentration of metal in the tree ring.

Tree rings reveal the reduction of Cd, Cu, Ni and Pb pollution in the central region of São Paulo, Brazil

The concern about environmental pollution has risen in the last decades because of its effects on human's health. However, evaluation of the exposure to certain pollutants is currently hampered by the availability of past environmental data. Tree rings are an alternative to reconstruct environmental variability of pre-instrumental periods. Nevertheless, this approach has some reported limitations including migration of chemical elements in the tree rings. The aim of this study was to evaluate the distribution of Cd, Cu, Hg, Na, Ni, Pb, Zn in the tree rings of Tipuana tipu (Fabaceae) to aid the reconstruction of past environmental pollution. We sampled trees in the central region of the city of São Paulo, Brazil, and scanned their tree rings using LA-ICP-MS. We used these data to evaluate the temporal trends of chemical elements under investigation. Results show a non-random distribution of these chemical elements within the tree rings, with higher content in the cell-walls of vessels and lower content in the fibers. Sodium was the only element intimately related to the axial parenchyma cells. Due to differences in elemental composition of xylem cells, temporal trends where evaluated using distinct quartiles of data distribution in each tree ring. The first quartile represents the lower content found in fibers and parenchyma, while the third quartile corresponds to the higher content found in vessels. Data from vessels better represent the decreasing trend of Cd, Cu, Pb, and Ni in the last three decades. This reduction is less significant for Na and Zn. Our results highlight the potential to improve the records of environmental pollution using data from different cells. Pronounced reduction in Pb may be attributed to the lead phase-out in gasoline, while the decreasing trend of Cd, Cu, Ni pollution is probably related to increasing efficiency of vehicles and the deindustrialization of São Paulo. Chemical elements are non-randomly distributed in tree rings. Chemical content of vessels cell-walls is a reliable record of metal pollution, which is decreasing in São Paulo.

Oak tree-rings record spatial-temporal pollution trends from different sources in Terni (Central Italy)

Monitoring atmospheric pollution in industrial areas near urban center is essential to infer past levels of contamination and to evaluate the impact for environmental health and safety. The main aim of this study was to understand if the chemical composition of tree-ring wood can be used for monitoring spatial-temporal variability of pollutants in Terni, Central Italy, one of the most polluted towns in Italy. Tree cores were taken from 32 downy oaks (Quercus pubescens) located at different distances from several pollutant sources, including a large steel factory. Trace element (Cr, Co, Cu, Pb, Hg, Mo, Ni, Tl, W, U, V, and Zn) index in tree-ring wood was determined using high-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We hypothesized that the presence of contaminants detected in tree-rings reflected industrial activities over time. The accumulation of contaminants in tree-rings was affected by anthropogenic activities in the period 1958-2009, though signals varied in intensity with the distance of trees from the industrial plant. A stronger limitation of tree growth was observed in the proximity of the industrial plant in comparison with other pollutant sources. Levels of Cr, Ni, Mo, V, U and W increased in tree-ring profiles of trees close to the steel factory, especially during the 80's and 90's, in correspondence to a peak of pollution in this period, as recorded by air quality monitoring stations. Uranium contents in our tree-rings were difficult to explain, while the higher contents of Cu, Hg, Pb, and Tl could be related to the contaminants released from an incinerator located close to the industrial plant. The accumulation of contaminants in tree-rings reflected the historical variation of environmental pollution in the considered urban context.