Changes in strontium levels in bark and over the past 40 years in the wood of trees exposed to high levels of air pollution

Erdem, R. (2023). “Changes in strontium levels in bark and over the past 40 years in the wood of trees exposed to high levels of air pollution,” BioResources 18(4), 8020-8036.

Abstract

The present study aims to identify the most suitable tree species for monitoring and reducing strontium (Sr) pollution. Strontium is a heavy metal that is extremely harmful to human and environmental health even at low concentrations and is listed as a priority pollutant by the Agency for Toxic Substances and Disease Registry due to its potential harm. Samples were taken from Pinus pinaster, Cupressus arizonica, Picea orientalis, Cedrus atlantica, and Pseudotsuga menziesii species grown in Düzce, a location reported as one of the top 5 cities having the most polluted air in Europe by the World Air Pollution Report. The changes in Sr concentration over the last 40 years were evaluated by species, organ, direction, and age range. The results indicate that Sr pollution significantly increased due to traffic sources. This study also showed that the transfer of Sr within the wood is limited in all the species under consideration; hence, all these species can be used in monitoring the changes in Sr pollution. The most suitable species for reducing Sr pollution were Cupressus arizonica and Picea orientalis, which have the highest capacity to accumulate the most Sr in their wood.

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Changes in Strontium Levels in Bark and Over the Past 40 Years in the Wood of Trees Exposed to High Levels of Air Pollution

Ramazan Erdem *

DOI: 10.15376/biores.18.4.8020-8036

Keywords: Biomonitor; Strontium; Düzce; Heavy metal

Contact information: Department of Forestry, Kastamonu University, Araç Rafet Vergili Vocational School, Kastamonu, Türkiye; *Corresponding author: rerdem@kastamonu.edu.tr

INTRODUCTION

The industrial revolution over the past century has caused irreversible problems, such as climate change and urbanization, on a global scale (Tekin et al. 2022; Cetin et al. 2023). During this period, the increase in environmental pollution is considered to be one of the most important threats to human health worldwide (Elsunousi et al. 2021). Today, almost the entire global population (99%) breathes air that exceeds WHO air quality limits and threatens their health (WHO 2022). Air pollution is estimated to contribute to 6 million preterm births each year and is stated to result in the death of approximately 7 million people annually (Jo et al. 2020; WHO 2023). Moreover, this change in the atmosphere’s composition also contributes to global climate change, which is considered the most critical issue worldwide (Cobanoglu et al. 2023a; Isinkaralar et al. 2023a,b).

The most harmful and deadly components of air pollution, which is important in terms of causing human death, are heavy metals. Nowadays, air concentrations of heavy metals have increased significantly, especially in urban areas, due to industrial activities and traffic (Istanbullu et al. 2023). These metals can stay undegraded in nature for a long period and bioaccumulate in living organisms; some are toxic and carcinogenic even at low concentrations and their concentrations in the air are constantly increasing (Sulhan et al. 2022; Yayla et al. 2022). Studies revealed that the concentrations of many heavy metals in the air, such as Mn, Cr, Ni, Cu, Zn, Al, Cd, and Fe, have increased significantly in recent years (Isinkaralar et al. 2023b,c). Some of these elements are much more harmful than others regarding human and environmental health. Therefore, they are defined as priority pollutants by international organizations such as ATSDR and EPA (Isinkaralar et al. 2022). One of the most toxic heavy metals in terms of environment and human health is strontium (Sr). Compounds of strontium (Sr), even small amounts of which can be harmful to human health, can cause lung cancer and accumulate in the body over a lifetime, leading to significant problems that can even result in sudden death (Cobanoglu et al. 2022). Moreover, studies show that Sr may be radioactive (Burger and Lichtscheidl 2019; Ivanets et al. 2020; Kwon et al. 2021). For this reason, monitoring and reducing the concentrations of these elements in the air is critical for human health.

In the present study, by reviewing the previous studies on the use of annual rings as biomonitors, gaps in the literature were determined. In the literature to date, many species such as Cupressus arizonica, Platanus orientalis, Robinia pseudoacacia, Corylus colurna, and Cedrus atlantica have been used to monitor the change of more known elements such as Pb, Cr, Ni, Co, Mn (Koc et al. 2023). However, the number of studies on elements such as Tl, V, As, and Sr, which have been proven extremely dangerous for human and environmental health, is much more limited (Canturk 2023). In this study, considering the gaps identified, the changes in the concentrations of Sr, one of the most harmful heavy metals for human and environmental health, were determined by species, year range, organ, and direction. Within the scope of the study, it is aimed to compare the Sr accumulation potential of the species examined here and determine how the accumulation in the trunk organs (outer bark, inner bark, and wood) of the tree varies with the concentrations in the air, how the concentrations in the annual rings change by the year range, the level of transition between organs after being taken into the plant body, and which sources are responsible for the accumulation in the trees. Thus, it was aimed to gather information about both the pollutants with Sr origin and the most suitable species that can be used for monitoring and reducing this pollution.

MATERIALS AND METHODS

This study was carried out in Düzce, a province in the Western Black Sea region of Türkiye, where the level of air pollution is very high. According to the 2021 World Air Pollution Report, Düzce is one of the top five cities with the highest level of air pollution in Europe (IQAir 2021). Within the scope of this study, the trunks of Pinus pinaster (Pp), Cupressus arizonica (Cpa), Picea orientalis (Po), Cedrus atlantica (Cda), and Pseudotsuga menziesii (Pm) species marked on their north sides were cut at approximately 10 cm thickness and approximately 50 cm above the ground. These log samples were then brought to the laboratory, and their surfaces were planed to clearly reveal the annual rings. As a result of the count and investigation, it was determined that the trees were approximately 40 years old. Considering the widths of the annual rings, they were grouped into five-year groups, and, using a steel drill, samples were taken from the outer bark, inner bark, and wood of each age group. The process was carried out in triplicate. The collected samples, in the form of shaving, were placed in glass Petri dishes and left with their lids open for 15 days to air dry. Then, the samples were dried in an oven at 45 °C for a week. From the dried samples, 0.5 g was taken, added with 6 mL of 65% HNO₃ and 2 mL of 30% H₂O₂, and then placed in a microwave oven designed for such analyses. The solution samples were transferred to volumetric flasks and diluted with ultrapure water to 50 mL. The samples prepared were analyzed using an ICP-OES device, and the values obtained were multiplied by the dilution factor to calculate Sr concentrations. The method used in the study has been commonly used in recent studies on this subject (Sevik et al. 2020).

Data were analyzed using the SPSS software package. A variance analysis was conducted for the data, and the Duncan test was applied for factors that had statistically significant differences at a confidence level of at least 95% (P < 0.05). Data were interpreted by simplifying and tabularizing them. Thus, the changes in Sr concentration were separately determined and evaluated by species, direction, organ, and year range.

FINDINGS

The changes in Sr concentration by species and organ are presented in Table 1.

Table 1. Changes in Sr (ppb) Concentration by Species and Organ

According to Duncan’s test results, numbers followed by the same letters (a, b, c, or d) are not statistically different at p > 0.05. Lowercase letters illustrate vertical directions; ***P ≤ 0.001

Table 2. Changes in Sr (ppb) Concentration by Directions and Species

According to Duncan’s test results, numbers followed by the same letters (a, b) are not statistically different at p > 0.05. Lowercase letters illustrate vertical directions, ns = not significant; ***P ≤ 0.001; **P ≤ 0.01

Results of the variance analysis revealed that the Sr concentration was significant in all organs by species. The highest values in the outer and inner bark were found in the Cpa and those in wood were observed in the Po. The lowest average value was found in wood and the highest in the outer bark. Therefore, the results can be ranked as wood < inner bark < outer bark. The changes in Sr concentration by species and direction is shown in Table 2.

Given the results presented in Table 2, it was determined that the changes in Sr concentration were significant in all directions, except for the south. The highest concentrations in the north, south, and east were found in the Cpa, whereas the highest concentration in the west was observed in Cpa and Po. The lowest average values were obtained in the Pp, Cda, and Pm species. The changes in Sr concentration by period and direction are given in Table 3.

Table 3. Changes in Sr (ppb) Concentration by Period and Direction

According to Duncan’s test results, numbers followed by the same letters (A, B, or a, b) are not statistically different at p > 0.05. Lowercase letters illustrate vertical directions, while capital letters indicate horizontal directions; ns = not significant; ***P ≤ 0.001; **P ≤ 0.01; *P ≤ 0.01

Given the values presented in Table 3, the changes in Sr concentration by directions were significant in all periods, except for the period 2018 to 2022. The Sr concentration by period was insignificant in directions other than the north and east. The lowest concentration was found in the period 1998 to 2002, whereas the highest concentration was found in the period 1983 to 1987. The lowest concentration in the north was obtained in the periods 1988 to 1992 and 1998 to 2002, while the highest in the east was obtained in the period 2018 to 2022. The changes in Sr concentration by organ and direction are presented in Table 4.

Table 4. Changes in Sr (ppb) Concentration by Organ and Direction

According to Duncan’s test results, numbers followed by the same letters (A, B, or a, b) are not statistically different at p > 0.05. Lowercase letters illustrate vertical directions, while capital letters indicate horizontal directions. Ns = not significant; ***P ≤ 0.001; *P ≤ 0.01

When examining the results shown in Table 4, it was determined that the changes in Sr concentration in all organs, except the outer bark, by direction were significant. The changes in Sr concentration by organ were significant in all directions. The lowest value in the north was found in wood, whereas the highest value was seen in the outer bark. In the east, the highest value was found in the inner and outer bark and the lowest in the wood. The highest concentration in the south was obtained in the outer bark, whereas the highest concentration in the west was obtained in the outer and inner barks. The changes in Sr concentration in Pp by organ and direction are presented in Table 5.

Table 5. Changes in Sr (ppb) Concentration in Pp by Organ and Direction

Table 6. Changes in Sr (ppb) Concentrations in Pp by Period and Direction

Examining the results of the variance analysis, it was found that the change in Sr concentration in Pp was significant in all organs by direction and in all directions, except the east, by organ. The highest values in the north, south, and west were found in the outer bark. The lowest average value was found in the east and the highest value was in the north, south, and west. The changes in Sr concentration in Pp by period and direction are given in Table 6.

Given the results presented above, the changes in Sr concentration in all woods were significant in all periods by direction and in all directions by period. The highest average values were found in the periods 1983 to 1987 and 1993 to 1997. Considering the average values by direction, the lowest value was obtained in the east and the highest value in the south and west. The changes in Sr concentration in the north for the period 2013 to 2017 and in the south for the period 1988 to 1992 were below the detectable limits. In the east, the changes in Sr concentration in the 1983 to 1987, 1993 to 1997, 1998 to 2002, and 2018 to 2022 periods were also lower than the detectable limits. The changes in Sr concentration in Cpa by organ and direction are presented in Table 7.

Table 7. Changes in Sr (ppb) Concentration in Cpa by Organ and Direction

Table 8. Changes in Sr (ppb) Concentration in Cpa Woods by Period and Direction

Examining the results presented in Table 7, the changes in Sr concentration at T5 were significant in all directions based on the organ and in all organs based on direction. The lowest value in the north, east, and west is seen in wood, while the lowest value in the south is seen in wood and inner bark. The highest values were obtained in the outer bark in the north, east, and south, while the highest value in the west was obtained from the inner bark. Therefore, the average results can be ranked as wood < inner bark < outer bark. The changes in Sr concentration in Cpa by period and direction are shown in Table 8.

Considering the results, it was determined that the changes in Sr concentration in Cpa wood were significant in all periods by directions and in all directions by periods. The highest concentration was observed in the north in the period 2008 to 2012, while the highest value in the east was observed during the 1998 to 2002 period. The highest value in the south was found in the period 2018 to 2022 and the one in the west was found in the period 2003 to 2007. Comparing the average values, the lowest one was found in the east and the highest one in the west. There was no significant difference between the periods by average values. The changes in Sr concentration at Po by organ and direction are presented in Table 9.

Table 9. Changes in Sr (ppb) Concentration in Po by Organ and Direction

Given the results of variance analysis, it was determined that the changes in Sr concentration in Po were significant in all organs by direction and in all directions by organ. The highest concentration in the north and east was found in the outer bark and those in the south and west were found in the inner bark. The lowest average value was observed in the north, east, and south, whereas the highest one was found in the west. The average concentration results for organs can be ranked as wood < inner bark < outer bark. The changes in Sr concentration at T6 by period and direction are presented in Table 10.

Table 10 shows that changes in Sr concentration in Po wood were significant in all periods and all directions. The highest value in the north was in the period 1983 to 1987, whereas the highest value in the east was in the period 2018 to 2022. The highest value in the south was obtained in the period 1983 to 1987 and the one in the west was found in the period 1993 to 1997. Comparing the average values, the lowest value was found in the east and the highest in the west. The changes in Sr concentration in Cda by organ and direction are shown in Table 11.

Table 10. Changes in Sr (ppb) Concentration in Po Woods by Period and Direction

Table 11. Changes in Sr (ppb) Concentration in Cda by Organ and Direction

Considering the results presented in Table 11, it was determined that the changes in Sr concentration were significant in all organs by direction and in all directions by organ. The highest concentration in the outer bark was in the west and the lowest concentration in the east. The highest value in the inner bark was obtained in the west and north, whereas the highest value in wood was obtained in the west. Comparing the average values, the lowest average value was found in the east and the highest value in the west. The changes in Sr concentration in Cda woods by period and direction are presented in Table 12.

Table 12. Changes in Sr (ppb) Concentration in Cda Wood by Period and Direction