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Volume 37, Issue 2 (2022)
GeoRes 2022, 37(2): 213-219 |
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Received: 2022/03/5 | Accepted: 2022/04/22 | Published: 2022/06/18
Received: 2022/03/5 | Accepted: 2022/04/22 | Published: 2022/06/18
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Janjani N, Azizi Z, Dehshiri M, Baikpour S. Effects of aspect and elevation on carbon sequestration process in Tehran Suburbs vegetation. GeoRes 2022; 37 (2) :213-219
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1- Department of Environmental and Forest Sciences, Faculty of Natural Resource and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran
2- Department of Remote Sensing and GIS, Faculty of Natural Resource and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran
3- Department of Biology, Borujerd Branch, Islamic Azad University, Borujerd, Iran
2- Department of Remote Sensing and GIS, Faculty of Natural Resource and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran
3- Department of Biology, Borujerd Branch, Islamic Azad University, Borujerd, Iran
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Introduction
In mountainous regions, topographic complexity leads to considerable variation in slope aspect and the formation of steep gradients in certain areas. These conditions result in increased soil erosion due to gravitational forces [Konz et al., 2010; Azizi & Montazeri, 2018], the development of microclimates [Beniston, 2014], high spatial heterogeneity of soils over short distances [Haring et al., 2012], and consequently, substantial variability in vegetation cover [Fischer et al., 2014]. Overall, such conditions give rise to spatially heterogeneous ecological units, particularly at fine and intermixed scales, each characterized by a distinct set of climatic, physical, chemical, and biological factors [Hansen et al., 2000; Swetnam et al., 2017].
In addition to the influence of spatial variability on the formation of ecological units [Thompson & Kolka, 2005; Taylor et al., 2015], these factors also induce variations in carbon fluxes among different units [Riveros-Iregui et al., 2015; Kang et al., 2003; Stielstra et al., 2015]. Given the high complexity of ecological units, plot-scale assessments are considered an appropriate approach for evaluating landscape-level processes [Kelsey et al., 2012]. Regarding the main driving factors, it is noteworthy that a relatively longer growing season at lower elevations compared to adjacent higher-altitude areas generally leads to increased net primary production (NPP) and enhanced soil CO₂ fluxes [Rodeghiero & Cescatti, 2005; Swetnam et al., 2017]. Conversely, at higher and colder elevations, slower decomposition of soil organic carbon stocks results in an accumulation of soil organic matter [Tashi et al., 2016].
Slope aspect determines the amount of incoming solar radiation and thus influences plant photosynthesis, transpiration, and site-specific microclimatic conditions [Agren & Andersson, 2012; Gomroki et al., 2017]. Increased solar radiation enhances carbon uptake through photosynthesis; however, solar-induced warming also intensifies soil evaporation and plant transpiration rates [Kirchen et al., 2017; Rehschuh et al., 2017]. In shallow soils with low water-holding capacity, increased plant water uptake may lead to soil moisture deficits on warmer slopes, such as south-facing slopes, whereas north-facing slopes may simultaneously maintain sufficient moisture availability. Nevertheless, relatively few studies have assessed the effect of slope aspect on soil carbon sequestration in vegetated regions, most of which have focused on forested ecosystems [Smith et al., 2016; Kobler et al., 2019; Azizi et al., 2018; Azizi, 2018]. Results from a study examining the effects of fire and grazing exclusion in parts of rangelands in East Azerbaijan indicated that slope aspect significantly influenced belowground carbon sequestration in vegetation [Faraji et al., 2019]. Similarly, an investigation of the relationship between carbon sequestration in certain plant species and soil and vegetation characteristics in a rangeland ecosystem in Qazvin revealed a significant association between vegetation cover density, soil condition, and the amount of carbon sequestered by specific plant species [Badehian & Mansouri, 2017].
The present study was conducted to identify spatial patterns of carbon sequestration cycles on the southern slopes of the Central Alborz Mountains along the margins of Tehran. To achieve this objective, the effects of slope aspect and elevation above sea level were evaluated across the existing landscapes in the study area.
Methodology
This study was carried out on the northwestern outskirts of the Tehran metropolitan area, within the geographical coordinates of 35°49′12″ to 35°49′59″ E longitude and 51°17′34″ to 51°31′16″ N latitude. According to various climatic classifications, the study area is characterized by a semi-arid climate and a piedmont mountainous setting. At elevations around 2,000 m above sea level on the southern slopes of the Alborz Mountains north of Tehran, the climate becomes semi-humid and cold, with relatively long winters. Precipitation in the region is generally higher during winter months. The cold season begins in December and lasts for approximately three to four months. The mean annual precipitation is 245 mm, and the mean annual temperature is 18 °C. The average annual minimum and maximum temperatures are 13 °C and 23 °C, respectively.
Based on existing geological maps and the Tehran Atlas, the bedrock of the study area consists primarily of young alluvial fan deposits. The soils are shallow and classified mainly as coarse-textured, with varying proportions of fine clay particles. According to Henry Pabot’s vegetation classification for Tehran Province, the area falls within the Iran-Turanian floristic region. The dominant vegetation comprises herbaceous and shrub species, while shrubs and trees occur at very low densities. The region also exhibits a high diversity of landforms.
To evaluate the effects of elevation and slope aspect on the carbon sequestration cycle, four elevation classes (m above sea level: <1700, 1700–1900, 1900–2100, and >2100) were defined across three slope aspects (south, east, and west) and three slope gradient classes (low, moderate, and steep) along the northern margins of Tehran. Vegetation (biomass) and soil (carbon stocks) data were collected and measured within each plot. The primary focus was on assessing the influence of slope aspect and elevation on carbon uptake by plant biomass. Spearman’s rank correlation was used to examine the relationships between belowground biomass and slope gradient, aspect, and elevation.
A total of 183 field samples were selected based on landform units. Within each plot, plant species composition and density per unit area, canopy cover percentage by species, canopy cover by growth form, percentage of bare soil, and litter cover were recorded. Root sampling was then conducted within designated plots measuring 1×1 m, selected according to shrub size and canopy dimensions. Given the expansion of litterfall zones around plants, vegetation cover was considered when locating plots. Belowground biomass was estimated using a metal corer 30 cm in length and 6 cm in diameter [Han et al., 2008], which was driven into the soil within each plot to extract soil and root samples.
To determine the conversion factor of root biomass to organic carbon, the combustion method was employed. Completely dried plant samples were ground, weighed, and combusted in a furnace at 500 °C for 4 h. The combusted samples were then cooled in a desiccator and reweighed. Based on ash weight, initial dry weight, and the organic carbon–organic matter relationship presented below, plant organic carbon content was calculated. Ultimately, the conversion factor for belowground biomass was determined using the percentage of initial dry weight and organic carbon content.
Soil samples were air-dried to measure selected physical and chemical properties, including bulk density, electrical conductivity, pH, nitrogen, phosphorus, potassium, and particle size fractions (sand, silt, and clay). After crushing soil aggregates and removing roots, stones, and other impurities, samples were sieved through a 2-mm mesh. Soil organic carbon content was determined using the Walkley–Black method.
Prior to variance analysis, data normality was assessed using the Kolmogorov–Smirnov test, and homogeneity of variances was evaluated with Levene’s test. Pearson correlation analysis was applied to compare vegetation characteristics and carbon sequestration in soil and plant biomass across different slope gradients, aspects, and elevation classes within the study area. All statistical analyses were performed using SPSS version 22.
Findings
A strong, positive, and statistically significant correlation was observed between belowground biomass and slope gradient, elevation above sea level, slope aspect, and plant carbon sequestration (p<0.05). The results indicated that variations in topographic and geomorphological factors play a key role in controlling belowground biomass and associated carbon dynamics.
Carbon sequestration exhibited a significant inverse relationship with elevation above sea level, such that an overall decline in carbon sequestration was observed for all investigated species with increasing elevation. Among the studied slope aspects, the western slopes showed the highest levels of carbon sequestration in vegetation. Moderate levels were recorded on southern slopes, whereas the lowest vegetation carbon sequestration was observed on eastern slopes.
Further analysis of conversion coefficients among the plant species identified across the study sites revealed notable interspecific differences. The species Psathyrostachys festuca, Ferula stipa, and Aethionema psathyrostachys exhibited the highest levels of soil organic carbon, while Astragalus p. showed the lowest soil organic carbon content.
The assessment of carbon sequestration by individual plant species indicated that Ferula psathyrostachys had the highest carbon sequestration capacity, whereas Stipa onosma exhibited the lowest.
Analysis of root organic carbon across the studied samples showed that the lowest root organic carbon content was associated with Ferula stipa, while the highest values were recorded for Ferula psathyrostachys.
Finally, soil bulk density varied among sites dominated by different plant species. The lowest and relatively similar bulk density values were observed in areas dominated by Astragalus p., Ferula onosma, Ferula psathyrostachys, and Stipa onosma. In contrast, the highest soil bulk density values were recorded in areas dominated by Aethionema psathyrostachys and Stipa psathyrostachys
Discussion
Soil texture, particularly clay content, is one of the most important factors influencing soil carbon sequestration, as soils with higher clay percentages generally exhibit a greater capacity for carbon storage [Varamesh et al., 2015]. This pattern was also evident in the present study, where the relatively high clay content of the soils contributed to elevated levels of carbon sequestration. Bruce et al. [1999] similarly have reported that soil carbon sequestration and soil organic carbon content are significantly affected by vegetation type and soil depth.
Previous studies have demonstrated the influence of slope aspect [Faraji et al., 2019], elevation above sea level [Tashi et al., 2016], slope gradient [Kobler et al., 2019], and soil condition and vegetation cover density [Badehian & Mansouri, 2017] on vegetation carbon sequestration. Abdi et al. [2008], in their investigation of factors affecting soil organic carbon sequestration in the Haftad Qoleh Protected Area of Arak, have found that soil organic carbon was positively and significantly correlated with soil loam content, electrical conductivity, saturated soil moisture, aboveground biomass, belowground biomass, and litter mass, while exhibiting a significant negative relationship with the proportion of stones and gravel. Gao et al. [2007] also have examined the effect of soil depth on carbon sequestration and have concluded that, in arid and semi-arid regions, soil carbon sequestration is inversely related to soil depth. This finding is consistent with the results of Schuman et al. [2002] and can be attributed to the gradual decomposition of litter and its transformation into humus, a process that primarily initiates in the upper soil layers.
The analysis of physiographic factors indicated that slope gradient, slope aspect, and elevation above sea level were directly associated with carbon sequestration in the study area. Similar conclusions have been reported by Kang et al. [2003] and Kobler et al. [2019]. Overall, the findings of this study suggested that vegetation cover on gentle slopes and at lower elevations plays a critical role in maintaining soil organic carbon stocks. This observation is supported by the results of Bahrami et al. [2014], who have conducted research in rangelands of arid and semi-arid regions. Furthermore, the effect of slope aspect on carbon sequestration varies depending on the geographical position of the study area on the Earth’s surface. In arid and semi-arid regions of the Northern Hemisphere, moister slopes provide more favorable conditions for higher vegetation density and enhanced carbon sequestration.
Conclusion
Elevation above sea level, slope gradient, and slope aspect significantly influence carbon sequestration. In addition, air pollution and climate change, particularly noticeable changes in temperature and precipitation regimes, may adversely affect the carbon sequestration cycle. These negative impacts are expected to be more pronounced on south- and east-facing slopes
Acknowledgements: There are no acknowledgements to report.
Ethical Approval: There are no ethical issues to report.
Conflict of Interest: This article is derived from the PhD dissertation of Nayereh Janjani entitled “Investigating the Effects of Physiographic and Soil Factors on Belowground Biomass of Vegetation (Case Study: Margins of Tehran City)”, conducted under the supervision of Dr. Zahra Azizi and Dr. Mohammad Mehdi Dehshiri, with consultation from Dr. Shahram Beikpour, at the Science and Research Branch of Islamic Azad University.
Author Contributions: Janjani N (First Author), Methodologist/Main Researcher/Discussion Writer/Statistical Analyst (40%); Azizi Z (Second Author), Assistant Researcher/Introduction Writer/Statistical Analyst (35%); Dehshiri MM (Third Author), Introduction Writer/Statistical Analyst (15%); Baikpour Sh (Fourth Author): Discussion Writer (10%)
Funding: All research costs were covered by the student, and no institution or organization provided financial support
In mountainous regions, topographic complexity leads to considerable variation in slope aspect and the formation of steep gradients in certain areas. These conditions result in increased soil erosion due to gravitational forces [Konz et al., 2010; Azizi & Montazeri, 2018], the development of microclimates [Beniston, 2014], high spatial heterogeneity of soils over short distances [Haring et al., 2012], and consequently, substantial variability in vegetation cover [Fischer et al., 2014]. Overall, such conditions give rise to spatially heterogeneous ecological units, particularly at fine and intermixed scales, each characterized by a distinct set of climatic, physical, chemical, and biological factors [Hansen et al., 2000; Swetnam et al., 2017].
In addition to the influence of spatial variability on the formation of ecological units [Thompson & Kolka, 2005; Taylor et al., 2015], these factors also induce variations in carbon fluxes among different units [Riveros-Iregui et al., 2015; Kang et al., 2003; Stielstra et al., 2015]. Given the high complexity of ecological units, plot-scale assessments are considered an appropriate approach for evaluating landscape-level processes [Kelsey et al., 2012]. Regarding the main driving factors, it is noteworthy that a relatively longer growing season at lower elevations compared to adjacent higher-altitude areas generally leads to increased net primary production (NPP) and enhanced soil CO₂ fluxes [Rodeghiero & Cescatti, 2005; Swetnam et al., 2017]. Conversely, at higher and colder elevations, slower decomposition of soil organic carbon stocks results in an accumulation of soil organic matter [Tashi et al., 2016].
Slope aspect determines the amount of incoming solar radiation and thus influences plant photosynthesis, transpiration, and site-specific microclimatic conditions [Agren & Andersson, 2012; Gomroki et al., 2017]. Increased solar radiation enhances carbon uptake through photosynthesis; however, solar-induced warming also intensifies soil evaporation and plant transpiration rates [Kirchen et al., 2017; Rehschuh et al., 2017]. In shallow soils with low water-holding capacity, increased plant water uptake may lead to soil moisture deficits on warmer slopes, such as south-facing slopes, whereas north-facing slopes may simultaneously maintain sufficient moisture availability. Nevertheless, relatively few studies have assessed the effect of slope aspect on soil carbon sequestration in vegetated regions, most of which have focused on forested ecosystems [Smith et al., 2016; Kobler et al., 2019; Azizi et al., 2018; Azizi, 2018]. Results from a study examining the effects of fire and grazing exclusion in parts of rangelands in East Azerbaijan indicated that slope aspect significantly influenced belowground carbon sequestration in vegetation [Faraji et al., 2019]. Similarly, an investigation of the relationship between carbon sequestration in certain plant species and soil and vegetation characteristics in a rangeland ecosystem in Qazvin revealed a significant association between vegetation cover density, soil condition, and the amount of carbon sequestered by specific plant species [Badehian & Mansouri, 2017].
The present study was conducted to identify spatial patterns of carbon sequestration cycles on the southern slopes of the Central Alborz Mountains along the margins of Tehran. To achieve this objective, the effects of slope aspect and elevation above sea level were evaluated across the existing landscapes in the study area.
Methodology
This study was carried out on the northwestern outskirts of the Tehran metropolitan area, within the geographical coordinates of 35°49′12″ to 35°49′59″ E longitude and 51°17′34″ to 51°31′16″ N latitude. According to various climatic classifications, the study area is characterized by a semi-arid climate and a piedmont mountainous setting. At elevations around 2,000 m above sea level on the southern slopes of the Alborz Mountains north of Tehran, the climate becomes semi-humid and cold, with relatively long winters. Precipitation in the region is generally higher during winter months. The cold season begins in December and lasts for approximately three to four months. The mean annual precipitation is 245 mm, and the mean annual temperature is 18 °C. The average annual minimum and maximum temperatures are 13 °C and 23 °C, respectively.
Based on existing geological maps and the Tehran Atlas, the bedrock of the study area consists primarily of young alluvial fan deposits. The soils are shallow and classified mainly as coarse-textured, with varying proportions of fine clay particles. According to Henry Pabot’s vegetation classification for Tehran Province, the area falls within the Iran-Turanian floristic region. The dominant vegetation comprises herbaceous and shrub species, while shrubs and trees occur at very low densities. The region also exhibits a high diversity of landforms.
To evaluate the effects of elevation and slope aspect on the carbon sequestration cycle, four elevation classes (m above sea level: <1700, 1700–1900, 1900–2100, and >2100) were defined across three slope aspects (south, east, and west) and three slope gradient classes (low, moderate, and steep) along the northern margins of Tehran. Vegetation (biomass) and soil (carbon stocks) data were collected and measured within each plot. The primary focus was on assessing the influence of slope aspect and elevation on carbon uptake by plant biomass. Spearman’s rank correlation was used to examine the relationships between belowground biomass and slope gradient, aspect, and elevation.
A total of 183 field samples were selected based on landform units. Within each plot, plant species composition and density per unit area, canopy cover percentage by species, canopy cover by growth form, percentage of bare soil, and litter cover were recorded. Root sampling was then conducted within designated plots measuring 1×1 m, selected according to shrub size and canopy dimensions. Given the expansion of litterfall zones around plants, vegetation cover was considered when locating plots. Belowground biomass was estimated using a metal corer 30 cm in length and 6 cm in diameter [Han et al., 2008], which was driven into the soil within each plot to extract soil and root samples.
To determine the conversion factor of root biomass to organic carbon, the combustion method was employed. Completely dried plant samples were ground, weighed, and combusted in a furnace at 500 °C for 4 h. The combusted samples were then cooled in a desiccator and reweighed. Based on ash weight, initial dry weight, and the organic carbon–organic matter relationship presented below, plant organic carbon content was calculated. Ultimately, the conversion factor for belowground biomass was determined using the percentage of initial dry weight and organic carbon content.
Soil samples were air-dried to measure selected physical and chemical properties, including bulk density, electrical conductivity, pH, nitrogen, phosphorus, potassium, and particle size fractions (sand, silt, and clay). After crushing soil aggregates and removing roots, stones, and other impurities, samples were sieved through a 2-mm mesh. Soil organic carbon content was determined using the Walkley–Black method.
Prior to variance analysis, data normality was assessed using the Kolmogorov–Smirnov test, and homogeneity of variances was evaluated with Levene’s test. Pearson correlation analysis was applied to compare vegetation characteristics and carbon sequestration in soil and plant biomass across different slope gradients, aspects, and elevation classes within the study area. All statistical analyses were performed using SPSS version 22.
Findings
A strong, positive, and statistically significant correlation was observed between belowground biomass and slope gradient, elevation above sea level, slope aspect, and plant carbon sequestration (p<0.05). The results indicated that variations in topographic and geomorphological factors play a key role in controlling belowground biomass and associated carbon dynamics.
Carbon sequestration exhibited a significant inverse relationship with elevation above sea level, such that an overall decline in carbon sequestration was observed for all investigated species with increasing elevation. Among the studied slope aspects, the western slopes showed the highest levels of carbon sequestration in vegetation. Moderate levels were recorded on southern slopes, whereas the lowest vegetation carbon sequestration was observed on eastern slopes.
Further analysis of conversion coefficients among the plant species identified across the study sites revealed notable interspecific differences. The species Psathyrostachys festuca, Ferula stipa, and Aethionema psathyrostachys exhibited the highest levels of soil organic carbon, while Astragalus p. showed the lowest soil organic carbon content.
The assessment of carbon sequestration by individual plant species indicated that Ferula psathyrostachys had the highest carbon sequestration capacity, whereas Stipa onosma exhibited the lowest.
Analysis of root organic carbon across the studied samples showed that the lowest root organic carbon content was associated with Ferula stipa, while the highest values were recorded for Ferula psathyrostachys.
Finally, soil bulk density varied among sites dominated by different plant species. The lowest and relatively similar bulk density values were observed in areas dominated by Astragalus p., Ferula onosma, Ferula psathyrostachys, and Stipa onosma. In contrast, the highest soil bulk density values were recorded in areas dominated by Aethionema psathyrostachys and Stipa psathyrostachys
Discussion
Soil texture, particularly clay content, is one of the most important factors influencing soil carbon sequestration, as soils with higher clay percentages generally exhibit a greater capacity for carbon storage [Varamesh et al., 2015]. This pattern was also evident in the present study, where the relatively high clay content of the soils contributed to elevated levels of carbon sequestration. Bruce et al. [1999] similarly have reported that soil carbon sequestration and soil organic carbon content are significantly affected by vegetation type and soil depth.
Previous studies have demonstrated the influence of slope aspect [Faraji et al., 2019], elevation above sea level [Tashi et al., 2016], slope gradient [Kobler et al., 2019], and soil condition and vegetation cover density [Badehian & Mansouri, 2017] on vegetation carbon sequestration. Abdi et al. [2008], in their investigation of factors affecting soil organic carbon sequestration in the Haftad Qoleh Protected Area of Arak, have found that soil organic carbon was positively and significantly correlated with soil loam content, electrical conductivity, saturated soil moisture, aboveground biomass, belowground biomass, and litter mass, while exhibiting a significant negative relationship with the proportion of stones and gravel. Gao et al. [2007] also have examined the effect of soil depth on carbon sequestration and have concluded that, in arid and semi-arid regions, soil carbon sequestration is inversely related to soil depth. This finding is consistent with the results of Schuman et al. [2002] and can be attributed to the gradual decomposition of litter and its transformation into humus, a process that primarily initiates in the upper soil layers.
The analysis of physiographic factors indicated that slope gradient, slope aspect, and elevation above sea level were directly associated with carbon sequestration in the study area. Similar conclusions have been reported by Kang et al. [2003] and Kobler et al. [2019]. Overall, the findings of this study suggested that vegetation cover on gentle slopes and at lower elevations plays a critical role in maintaining soil organic carbon stocks. This observation is supported by the results of Bahrami et al. [2014], who have conducted research in rangelands of arid and semi-arid regions. Furthermore, the effect of slope aspect on carbon sequestration varies depending on the geographical position of the study area on the Earth’s surface. In arid and semi-arid regions of the Northern Hemisphere, moister slopes provide more favorable conditions for higher vegetation density and enhanced carbon sequestration.
Conclusion
Elevation above sea level, slope gradient, and slope aspect significantly influence carbon sequestration. In addition, air pollution and climate change, particularly noticeable changes in temperature and precipitation regimes, may adversely affect the carbon sequestration cycle. These negative impacts are expected to be more pronounced on south- and east-facing slopes
Acknowledgements: There are no acknowledgements to report.
Ethical Approval: There are no ethical issues to report.
Conflict of Interest: This article is derived from the PhD dissertation of Nayereh Janjani entitled “Investigating the Effects of Physiographic and Soil Factors on Belowground Biomass of Vegetation (Case Study: Margins of Tehran City)”, conducted under the supervision of Dr. Zahra Azizi and Dr. Mohammad Mehdi Dehshiri, with consultation from Dr. Shahram Beikpour, at the Science and Research Branch of Islamic Azad University.
Author Contributions: Janjani N (First Author), Methodologist/Main Researcher/Discussion Writer/Statistical Analyst (40%); Azizi Z (Second Author), Assistant Researcher/Introduction Writer/Statistical Analyst (35%); Dehshiri MM (Third Author), Introduction Writer/Statistical Analyst (15%); Baikpour Sh (Fourth Author): Discussion Writer (10%)
Funding: All research costs were covered by the student, and no institution or organization provided financial support
Keywords:
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