Abstract
The objective of this study was to determine suitable tree species to monitor and reduce Sn concentrations in the environment of Düzce province in Türkiye. A further goal was to test the hypothesis that, possibly due to air transport, the uptake of Sn in tree rings would show a significant and consistent dependency on compass direction. The timber samples were from the trunks of Tilia tomentosa (linden), Robinia pseudoacacia (black locust), Cedrus atlantica (cedar), Pseudotsuga menziesii (Douglas fir), and Fraxinus excelsior (European ash), which are commonly used in landscaping in Düzce province. Levels of Sn concentrations in annual rings were determined. Cedrus atlantica and F. excelsior were found to be suitable biomonitors that can be used to monitor changes in annual amounts of Sn contamination. Among the studied tree species, R. pseudoacacia had the highest average values and C. atlantica had the second-highest levels of Sn uptake. However, no consistent dependency on compass direction was found. It follows that rather than depending on the direction of prevailing winds, the uptake of metals to the xylem of trees must be due to direction-independent processes, such as transport via roots and xylem or absorption into leaves and subsequent transport via the phloem.
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Directionality in Tree Ring Accumulation of Tin (Sn) in Three Tree Species
Ayse Ozturk Pulatoglu *
The objective of this study was to determine suitable tree species to monitor and reduce Sn concentrations in the environment of Düzce province in Türkiye. A further goal was to test the hypothesis that, possibly due to air transport, the uptake of Sn in tree rings would show a significant and consistent dependency on compass direction. The timber samples were from the trunks of Tilia tomentosa (linden), Robinia pseudoacacia (black locust), Cedrus atlantica (cedar), Pseudotsuga menziesii (Douglas fir), and Fraxinus excelsior (European ash), which are commonly used in landscaping in Düzce province. Levels of Sn concentrations in annual rings were determined. Cedrus atlantica and F. excelsior were found to be suitable biomonitors that can be used to monitor changes in annual amounts of Sn contamination. Among the studied tree species, R. pseudoacacia had the highest average values and C. atlantica had the second-highest levels of Sn uptake. However, no consistent dependency on compass direction was found. It follows that rather than depending on the direction of prevailing winds, the uptake of metals to the xylem of trees must be due to direction-independent processes, such as transport via roots and xylem or absorption into leaves and subsequent transport via the phloem.
DOI: 10.15376/biores.19.4.8542-8562
Keywords: Air quality; Heavy metal; Biomonitor; Tin pollution; Sn
Contact information: Kastamonu University, Faculty of Forestry, Department of Forest Engineering, 37150, Kastamonu, Türkiye; *Corresponding author: ayseozturk@kastamonu.edu.tr
INTRODUCTION
Air pollution is considered one of the most significant environmental issues. Air pollution and its effects are at a higher level in urban areas because of industrial activities and traffic density. These factors increase the heavy metal concentrations. Elements that are considered as heavy metals have relatively high density, usually higher than 5 g/cm3, and are indestructible or non-degradable; even low levels of heavy metals can be toxic or poisonous (Sulhan et al. 2023). In addition to their ecological effect on natural environments, heavy metals are also a major concern for global public health (Cetin et al. 2022; Isinkaralar et al. 2023). Inhaling high amounts of heavy metal particles in the course of time can increase the metal load in the human body, and it poses a health risk (Gray et al. 2003). Heavy metal pollutants can affect people living near the source through suspended dust or direct contact (Chen et al. 2010). In agricultural lands, these pollutants can also enter the human food chain through edible plants and cause people to be exposed to heavy metals (Sevik et al. 2020). Heavy metals accumulate in the atmosphere and pose harm to ecosystems and organisms. Plant leaves and stems can also absorb heavy metals from atmospheric particles (Karacocuk et al. 2022). Heavy metals can be transported to the soil through atmospheric deposition (Nabuloa et al. 2006). Plant roots uptake heavy metals from soil (Erdem et al. 2023). Some changes are observed in physiological and biochemical processes in plants grown in heavy metal-contaminated soils such as DNA damage, disruptions in biosynthetic pathways, and reduced growth (Taofeek and Tolulope 2012).
Tin (Sn) is one of the most concerning heavy elements. Its tendency to biologically accumulate increases the severity of its toxic effects. It is known that inorganic tin compounds have mutagenic, carcinogenic, and teratogenic potential and that they can damage the cardiovascular system. In addition to symptoms, such as shortness of breath, coughing, and wheezing, inhaling tin can lead to dizziness, balance disorders, headaches, diarrhea, vomiting, abdominal pain, muscle weakness, paralysis, anemia, and severe liver and kidney damage (Cima 2011; Sharma and Kumar 2020). Inhalation, oral intake, or dermal contact with specific Sn compounds was observed to be associated with skin and eye irritation, respiratory distress, gastrointestinal disorders, and neurological problems (Nakanishi 2008). Poisoning related with certain tin compounds can result in permanent neurological problems, and even death (ATSDR 2015).
For a sustainable environment, it is important to assess the risk levels associated with heavy metals that can persist in nature for extended periods without degradation, identify high-risk areas, and monitor the heavy metal levels. Trees in urban areas contribute to the filtration of the surrounding air and the reduction of pollution levels by absorbing heavy metals (Dzierżanowski et al. 2011). By capturing pollutants and reducing the amounts in the air or soil, there is potential to improve urban air or soil quality (Freer-Smith et al. 2005; Tomašević et al. 2005; Chakre 2006; Peachey et al. 2009; Warczyk et al. 2024; Zhao et al. 2024). However, plant capacity for heavy metal translocation and accumulation is highly variable, depending on genotypic and environmental traits (Pietrini et al. 2010; Di Baccio et al. 2014). Popek et al. (2017) investigated the accumulation of particulate matter (PM), including heavy metals and polycyclic aromatic hydrocarbons, on the foliage of small-leaved lime (Tilia cordata Mill.) in five Polish cities. The study showed that there were significantly different PM amounts found in the trees between the cities which related to the different quantities of PM in the atmosphere at these cities. The results of the study suggested that T. cordata improves the air quality in cities. A similar phenomenon was observed in another study, in which the root systems of Salix integra accumulated relatively high concentrations of Zn and Cd in the root and above ground tissues and in Quercus spp. and Salix matsudana, the highest absolute concentrations of Pb, Zn, and Cd were retained in roots. (Shi et al. 2017). Another study states that Lolium multiflorum is suitable for phytoestabilization since it is able to uptake heavy metals such as Pb and Zn and improve the soil properties (Mugica-Alvarez et al. 2015).
The use of urban trees as bioindicators is a sustainable ecological approach to preserve urban living spaces. Therefore, trees can be used as bioindicators to obtain time-dependent information on pollutant levels in cities (Gupta et al. 2011; Ghoma et al. 2023). The use of annual rings as indicators of heavy metal pollution can yield valuable data on the chronology and distribution of elements contributing to pollution (Chen et al. 2021; Savas et al.2021; Key et al. 2023; Cobanoglu et al. 2023). Previous studies documented the usability of tree rings in monitoring heavy metal pollution (Edusei 2021; Isinkaralar 2022, 2024; Cuciurean et al. 2024). It was reported in many studies that there is a relationship between elemental concentrations in annual rings and environmental pollution (Key et al. 2022; Erdem et al. 2024; Ozturk Pulatoglu 2024; Şevik et al. 2024). However, the transfer of elements within wood varies between the plant species. Further studies are necessary to analyze the concentration and long-term level of Sn in the air-soil-plant system for realistic risk assessments. Monitoring urban air quality is important to determine the atmospheric pollution and potential damage. Therefore, it is important to identify tree species suitable for detecting heavy metal pollution separately for each heavy metal. The objective of this study is to determine the most suitable species to monitor and accumulate Sn concentrations, with an assumption that the primary route of contamination is through the air. The main hypothesis of the study is that, because of prevailing winds, the Sn accumulation in the tree rings of the species under study will depend on the compass direction.
MATERIALS AND METHODS
As reported in the 2021 World Air Pollution Report, Düzce province has the fifth-highest pollution level in Europe (IQAir Staff Writers 2021). The topography and meteorological characteristics of Düzce province located in the Western Black Sea region of Türkiye can intensify some air pollution effects. The main pollutants causing air pollution in Düzce province generally originate from industrial facilities, residential fuel use, and vehicular traffic.
The timber samples used in this study were obtained from the trunks of Tilia tomentosa Moench (linden), Robinia pseudoacacia L. (black locust), Cedrus atlantica (Endl.) G. Manetti ex Carrière (Cedar), Pseudotsuga menziesii (Mirb.) Franco (Douglas fir), and Fraxinus excelsior L. (European ash), which are commonly used in landscaping in Düzce province. Trees of similar ages were preferred for this work. The timber samples were collected in the year 2022 and were approximately 10 cm thick, taken from an aboveground height of approximately 50 cm during the non-vegetation season. Because the trees are close to each other, it is thought that they are exposed to similar amounts of soil and airborne pollutants. To date, this method has been used in studies on the accumulation and transmission of elements in wood depending on the pollution source (Sevik et al. 2020; Cesur et al. 2021; Isinkaralar et al. 2022). The area where the trees were taken is on the edge of the city and there is a highway on one side and an agricultural area on the other. To interpret the pollutant source correctly, during the collection of timber samples, directions (east, west, north, and south) were labeled on the logs. Sections taken from the trunk logs were sanded in the laboratory to flatten the upper surface for clearer visibility of annual rings.
Because the annual rings are narrow, samples cannot be taken from the rings formed each year. Rather, the annual rings were grouped by considering their width and the age of the tree. In the studies, it was determined that 20-year-old trees were grouped for two years of annual rings (Turkyilmaz et al. 2019), 55-year-old trees were grouped for 5 years (Ozturk Pulatoglu 2024), 30-year-old trees were grouped for 3 years (Isinkaralar et al. 2022), and 33-year-old trees were grouped for 3 years (Koc 2021; Savas et al. 2021). Annual rings were clustered considering the ring width and the age of the trees. Therefore, trees that were approximately 40 years old were divided into 5-year age groups. Then, the outer bark, inner bark, and wood samples were collected from each age group using stainless steel drills and then placed in glass Petri dishes. These samples were processed into sawdust without using any tools made of the metals examined in this study and they were left in the laboratory, uncovered, for 15 days until completely dry to achieve air-dried specimens. Then, these samples were subjected to one week of drying in an oven set at 45 °C. Following this process, 0.5 g of the dried samples were mixed with 6 mL of 65% HNO3 and 2 mL of 30% H2O2 before placing them in a microwave oven (Key et al. 2023; Erdem et al. 2024; Şevik et al. 2024).
After the combustion, the samples were transferred to measuring bottles, and the final volume was completed to 50 mL by using ultra-pure water. The samples were analyzed by using an ICP-OES (Inductively Coupled Plasma-Optic Emission Spectrometer, GBC Scientific Equipment Pty Ltd., Melbourne, Australia) device, and Sn-concentrations were determined by multiplying the results with the corresponding dilution factor. This method has been commonly used in literature (Işınkaralar et al. 2022; Key et al. 2023).
Variance analysis was conducted by using the SPSS package program. Moreover, the Duncan test was conducted for factors showing statistically significant differences at a minimum of 95% confidence level (P < 0.05). Considering the results achieved from the Duncan test, analyses and interpretations were conducted after tabularizing the results. In each organ (outer bark, inner bark, wood) on a tree basis, in each tree on an organ basis, in each tree’s annual rings on a direction basis, in each tree’s annual rings on an age range basis, and in the process, the changes in heavy metal concentrations in the air were analyzed separately.
RESULTS
The annual variations of Sn concentrations in annual rings were determined in this study. Additionally, changes in Sn concentration by years and directions were calculated by comparing Sn concentrations in the inner bark (IB) and outer bark (OB) to the wood (WD). The statistical analysis results, average values, and the Sn concentration changes by species and directions are shown in Table 1.
Table 1. Sn Concentrations (ppb) by Species and Direction
According to statistical analysis, values followed by the different letters mean they are different at P ≤ 0.05. Lowercase letters (a, b) show vertical directions, while uppercase letters (A, B) show horizontal directions; * = P ≤ 0.05; ** = P ≤ 0.01; *** = P ≤ 0.001; ns = not significant; UL: under limit
Changes in Sn concentration were statistically significant in all directions (Table 1), though the trends were not consistent among different species. The changes by direction were found to be statistically significant in all species other than F. excelsior. Tin concentrations in the wood were found to change and there were differences between different directions and periods. In R. pseudoacacia, the change in Sn concentration in the north remained lower than the measurable limits. The highest concentration found in the north (9490 ppb) was measured in C. atlantica, whereas the highest concentrations in the east (15300 ppb), west (14500 ppb), and south (15900 ppb) were measured in R. pseudoacacia. Considering the average values, R. pseudoacacia was found to yield the highest concentration (15300 ppb).
Table 2. Sn Concentrations (ppb) by Periods and Directions
Given the results obtained from variance analysis, the changes in Sn concentration by periods and directions were not determined to be statistically significant (Table 2). Similarly, no significant changes were determined in the average values. Tin concentrations by organs and directions are presented in Table 3.
Table 3. Sn Concentrations (ppb) by Organs and Directions
OD: Outer bark, IB: Inner bark, WD: Wood
The changes in Sn concentration by directions and organs were not statistically significant (Table 3). Examining the average values, the results also confirmed that there was no significant difference by organ and direction. The changes in Sn concentration by periods and species are presented in Table 4.
Table 4. Sn Concentrations (ppb) by Periods and Species
Variance analysis results showed that the changes in Sn concentrations were statistically significant by period in all species (except for C. atlantica and F. excelsior) and by species in all periods. When the changes in Sn concentration on a period basis were examined, it was determined that it ranged between 336 to 521 ppb in T. tomentosa, 14800 to 15700 ppb in R. pseudoacacia, 6690 to 10400 ppb in C. atlantica, 96.9 to 231.3 ppb in P. menziesii, and 130 to 1570 ppb in F. excelsior. The highest value in C. atlantica was obtained in the periods 2013-2017 and 2018-2022. In F. excelsior, however, the highest concentration was obtained in the period 2018-2022. Moreover, the highest average concentration was found in R. pseudoacacia (15238.3 ppb), whereas the lowest ones were found in T. tomentosa (421.8 ppb), P. menziesii (153.8 ppb), and F. excelsior (338.7 ppb).
Table 5. Sn Concentrations (ppb) by Organs and Species
Changes in Sn concentration were statistically significant by species in all organs (except for R. pseudoacacia) and by organ in all species. The lowest concentration was found in wood, followed by inner bark and outer bark, respectively, in T. tomentosa and P. menziesii. In C. atlantica, however, the ranking is outer bark < inner bark < wood. Given the average values, the highest average concentration was found in R. pseudoacacia (15267.2 ppb), followed by C. atlantica (7645.0 ppb).
Table 6. Sn Concentrations (ppb) in Tilia tomentosa by Organs and Directions
The changes in Sn concentration by organs and directions were determined to be statistically significant in T. tomentosa (Table 6). The lowest level of Sn in the north was found in wood (475 ppb), followed by inner bark (699 ppb) and outer bark (1450 ppb). The highest levels in the south and west were found in the outer and inner bark, whereas the highest value in the east was obtained in the outer bark. Further, the highest average Sn levels were observed in the north (560 ppb), south (494 ppb), and west (516 ppb), whereas the ranking by organs is wood (422 ppb) < inner bark (651 ppb) < outer bark (1020 ppb).
Table 7. Sn Concentrations (ppb) in Tilia tomentosa by Periods and Directions
The variance analysis results revealed that there were significant changes in Sn concentration in T. tomentosa by directions and periods. The highest values in the south were found in the period 2018-2022 (650 ppb), whereas the highest values were found in the period 1983-1987 (624 ppb) in the north and in the periods 1988-1992 (648 ppb) in the west. Examining the average Sn concentrations, the highest average levels were found in the west (479 ppb), north (475 ppb), and south (422 ppb).
Table 8. Sn Concentrations (ppb) in Robinia pseudoacacia by Organs and Directions
As shown in Table 8, the changes in Sn concentrations in R. pseudoacacia were statistically significant by direction were significant in organs other than the inner bark. The change in Sn concentration in the north was determined to be lower than the detectable limits in all organs. However, the changes by organs were not statistically significant in directions other than the east. The highest Sn level in the east was measured in the inner bark (14810.4 ppb) and the lowest one in the outer bark (14250.9 ppb) and wood (14546.4 ppb). Considering the average values, the highest average value was measured in the south (15925.4 ppb).
Table 9. Changes in Sn Concentration (ppb) in Robinia pseudoacacia by Periods and Directions
Given the variance analysis results, the changes in Sn concentration in R. pseudoacacia by directions were found to not be statistically significant in periods other than 1963-1967, 1973-1977, 1988-1992, 1993-1997, and 1988-2002. The concentration changes in the north direction remained lower than detectable limits for all periods. Moreover, the only significant change in concentration was found to be in the west. The highest value in this direction was obtained in the periods 2003-2007 (15400 ppb) and 2013-2017 (15500 ppb).
Table 10. Sn Concentrations (ppb) in Cedrus atlantica by Organs and Directions
The changes in Sn concentration in C. atlantica by direction were found to be statistically significant in all organs (Table 10). However, the changes by organs were not statistically significant in directions other than the north. Considering the average values by organs, the highest level was measured in the inner bark (9390 ppb) and the lowest ones in the wood (7540 ppb) and outer bark (7160 ppb). Similarly, regarding the averages by directions, the highest value was found in the north (9490 ppb).
Table 11. Sn Concentrations (ppb) in Cedrus atlantica by Periods and Directions