Browsing by Subject "Urbanization"
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(2023)Climate change severely threatens ecosystem services and human well-being: vegetation and soils underneath it can be particularly vulnerable to climate warming. Soils hold the largest carbon stock in terrestrial ecosystems, and urban park soils, especially in cool climates, can hold remarkable carbon stocks and may be able to offset some atmospheric CO2 emissions. Land use changes, such as urbanization, influence soil organic carbon formation and soil carbon storages. In this study, I was interested whether three vegetation types (deciduous trees, conifers and lawn) differ in their capacity to store C in their rhizosphere, and whether this is affected by park size. I measured the proportion of tree canopy layer in class A park areas of the city of Helsinki, to estimate soil C storages of these areas and to examine C density (kg C m-2). Proportions of tree canopy layers in different park size groups were measured using QGIS and ortographs. Soil C storages were calculated using existing soil C data and average proportions of conifer and deciduous trees in parks of the city of Helsinki. Park size had a significant effect on proportion of the tree canopy coverage: canopy cover decreases with an increase in park size. Especially large parks are dominated by lawn. The average soil C densities in small, medium and large parks were 23.98 kg C m-2, 23.47 kg C m-2 and 23.15 kg C m-2, respectively. However, the overall proportion of conifer canopy in parks of the city of Helsinki is rather small, resulting in small differences in C densities between different park size categories, despite significant differences in tree canopy coverage between the three size groups. Most of the stored soil C in parks of the city of Helsinki are under lawn, even though it is the least efficient of the three studied vegetation types (deciduous trees, conifers, lawn) in soil C sequestration. Within a park size category and at park level, large parks store the highest amount of carbon per park. Even so, at the city level, the total amount of carbon is highest in the small parks due to their high number. Conifer trees associate with improved C sequestration to soils compared to deciduous trees and lawn. Increasing the amount of conifer trees in urban parks thus likely increase the important C storages of these soils. Results of this study highlight the importance of the contribution of urban parks and especially conifer trees in carbon sequestration. Future research related to urban soil C sequestration and the effects of vegetation type and climatic conditions is needed to better understand soil C accumulation and how the C sequestration of urban park soils could be enhanced.
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(2022)As the world urbanizes at a rapid rate, urban encroachment into coastal waters has the potential to reshape global carbon cycles by modifying organic matter decomposition processes. Litter decomposition is regulated by the litter quality, environmental conditions, and the decomposer community. This study aims to investigate if different coastline characteristics (urban/hardened versus natural/vegetated) have localized differences in decomposition rates and litter stabilization. To test this, the Tea Bag Index (TBI) has been applied to aquatic systems by including a “leaching factor” to initial masses. By using uniform litter in aquatic systems, the litter quality and moisture conditions are fixed so that other environmental conditions and the decomposer community can be considered for their impact on the rate of decomposition. Three pairs (urban and natural) of sites were selected around Helsinki, Finland in the brackish coastal water of the Baltic Sea in the summer of 2021. At each site, five green and rooibos tea bags were placed with a temperature logger, and on days 15, 29, 43, 60, and 84 a tea bag of each type was removed, dried, and weighed. Additionally, water quality measurements were collected using a YSI multiparameter sonde. There was no significant difference in the decomposition rate nor stabilization factor between urban and natural sites, although, the rate calculated according to the TBI-methodology differed from the rate determined by fitting a model to the observations. Despite no significant difference, the stabilization factors were higher than average in similar environments and are indicative of efficient litter decomposition in the waters around Helsinki. Water temperature was significantly higher, and turbidity was significantly lower at natural sites. Therefore, the cooler waters and inferred higher hydrodynamic forces at urban sites may have counteracted their individual effects on the rate of litter decomposition. In fact, since the decomposer community at natural sites is suggested to be more diverse, the rooibos tea at natural sites may have begun to stabilize, thus, implying that the study period was too long. Overall, this study found that coastlines currently have no clear localized effects on litter decomposition, but in the future, this may begin to change.
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