Contributions of urban green spaces for climate change mitigation and biodiversity conservation in Dessie city, Northeastern Ethiopia

 

ABSTRACT Scientific studies about the role of urban green spaces (UGSs) in biodiversity conservation and climate change mitigation are limited in developing countries. Therefore, the aim of this study was to assess the role of UGSs in biodiversity conservation and climate change mitigation. Using a systematic sampling method a total of 137 sample plots of various sizes were laid per cluster. The analysis of variance (ANOVA) was undertaken among clusters using the Honestly Significant Difference (HSD) test at a 5% level of significance. A total of 127 woody species belonging to 58 families and 99 genera were recorded. Of the total species, 7 (5.51%), 34 species (26.77%), and 86 (67.72%) were endemic, exotic, and indigenous to Ethiopia, respectively. The presence of threatened or endangered trees in Ethiopia, such as Hagenia abyssinica and Juniperus procera in the UGSs of Dessie, indicates that UGSs could be another potential site for biodiversity conservation. The overall mean carbon stock (ton ha− 1 ) and its total CO2 equivalent of UGSs of Dessie are 745.17 ± 170.86 and 2734.79 ± 627.06, respectively. This indicated a significant potential of UGSs to mitigate a changing climate. A statistically significant difference (p < 0.05) was revealed for the carbon stock and diversity. The highest mean biomass carbon was recorded in the urban forests (1448.14 ± 736.32 tons C ha− 1 ) and the lowest in urban Homegarden (61.28 ± 0.231 ton C ha− 1 ). Correlation analysis revealed a weak positive relationship (r = 0.26) between biomass carbon stock and Shannon’s index. It is concluded that UGSs have an important role in mitigating climate change through carbon sequestration and biodiversity conservation. Therefore, urban plans should consider the roles of urban green space at every stage of planning.

 

1. Introduction The recent rapid expansion of urbanization has forced most people on earth to relocate to cities (Kong and Nakagoshi, 2006). By the year 2050, the world population is expected to reach 10 billion people (Rudd et al., 2002). In addition, >50% of the world’s population now lives in urban areas (Attwell, 2000). Similarly, by the year 2025, about 65% of the world’s population is expected to live in urban areas (Schell and Ulijaszek, 1999). The urban population in Ethiopia has increased from 3.7% in 1984 to 7.1% in 1994 and from 14.2% in 2007 to 19% in 2014 (Terfa et al., 2017). Furthermore, it was predicted to rise to 38% by 2050 (Seto et al., 2012). Similarly, the urban population in Amhara National Regional State grew from 7.5% in 2007 to 16% in 2015 (Schmidt and Mekamu, 2009). Consequently, the rapid expansion of cities in neighborhood areas led to the alteration of other land uses (agricultural, forest, and grazing lands) to urban settlements or built-up areas (Dearborn and Kark, 2010; Helen et al., 2019; Mengistu et al., 2017; Sun et

 

Recently, urbanization and agricultural expansion have resulted in habitat loss, fragmentation, and biodiversity extinction (Rudd et al., 2002). Besides, urban environments have been patchy, disturbed, and polluted due to rapid urban expansion (Delphin et al., 2016). Scientific studies examined and indicated that anthropogenic events have released up to 71% of carbon dioxide (CO2) and other GHGs emissions (Grimm et al., 2008). In this regard, UGSs maintain biodiversity in the face of rapidly increasing land degradation, habitat loss, anthropogenic disturbance, and species extinctions in natural ecosystems (Jaman et al., 2020). Thus, concerns about urban vegetation and UGSs are growing globally due to their ecosystem services, biodiversity conservation, and other multidimensional roles (Miller et al., 2015). Hence, urban areas and cities should be involved in reducing climate change impacts and biodiversity loss both locally and regionally (Faso and Francisco, 2016; Gharibi and Shayesteh, 2021; Helen et al., 2019; Koricho et al., 2020; Reis and Lopes, 2019). Besides, UGSs preserve and maintain biodiversity via gene flows, sources of dispersal, and species migration (Swanwick et al., 2003). Therefore, though human damage to natural ecosystems has caused biodiversity loss and extinction, UGSs and vegetation in human-dominated urban areas are more vital for biodiversity conservation and climate change mitigation (Bernholt et al., 2009; Faso and Francisco, 2016; Rudd et al., 2002; Zhou and Rana, 2012). As a result, well-vegetated urban land improves the worth, corporeal, and expressive healthiness of city dwellers (Beyene and Borishe, 2021; Chivian, 2002; Coutts and Hahn, 2015; Davern et al., 2017; Douglas, 2008; Haines et al., 2007; Tzoulas and James, 2005). Therefore, it is at UGSs where people interact with nature and biodiversity (Tzoulas and James, 2005). Beyond their biodiversity conservation roles, UGSs improve air quality, eradicate pollutants, offset noise and equable temperatures (Reis and Lopes, 2019), sink CO2, insinuate water, and mitigate climate change and pollution (Kroeger, 2011; Reis and Lopes, 2019).

 

2. Materials and methods

2.1. Study area descriptions 

 

                 This study has been conducted in Dessie City, Amhara National Regional State, Northeastern Ethiopia (Fig. 1). Dessie is a historical and older-aged city that has existed for more than a hundred and ten years (Amhara Design and Supervision Works Enterprise, 2013). Geographically, Dessie is located 401 km Northeast of the country’s capital (Addis Ababa) and 480 km from the regional capital (BahirDar). It is the capital city of the South Wollo Administrative Zone in Amhara National Regional State in Ethiopia. Dessie city is characterized by gentle sloping topography and an undulating landscape (Amhara Design and Supervision Works Enterprise, 2013). The city is found at an elevation of 2470 and 2550 m above sea level (m.a.s.l) (Abegaz, 2020). The central statistical agency of Ethiopia (CSA, 2007) reported that the population of Dessie city was 181,658 (CSA, 2007). Dessie is located geographically at a latitude and longitude of 11◦ 8′ 0” N and 39◦ 38′ 0′′ E, respectively. According to CSA, Dessie occupied an estimated total area of 15.08 km2 and had a population density of 11,213.79 people per square kilometer (CSA, 2007). Dessie is one of the Ethiopia’s most representative cities in terms of rapid urbanization, rising energy consumption, and carbon emissions (Amar et al., 2000; Little et al., 2006)

          
3. Results

3.1. Plant diversity in urban green spaces

A total of 127 woody species belonging to 58 families and 99 genera were recorded in the UGSs of Dessie City (Appendix 1). Trees, shrubs, and lianas constitute 71 (55.91%), 44 (34.65%), and 12 (9.45%), respectively. Of the total species, 7 (5.51%) and 86 (67.72%) were endemic and indigenous to Ethiopia, respectively, while 34 species (26.77%) were introduced (exotic) to Ethiopia. The highest and the lowest number of species were recorded at educational sites (71) and streets (14), respectively. Similarly, the number of genera and families was high at the educational sites, while street sites resembled the lowest number of genera and families (Fig. 4). Fabaceae (17 species), Myrtaceae (9) species, Euphorbiaceae (6 species), Moraceae (6 species), Asteraceae (5 species), Rosaceae (5 species), Lamiaceae (4 species), and Malvaceae (4 species) are the families with the highest number of species, respectively. However, the remaining 6, 9, and 35 families were equally       represented by three, two, and single species, respectively (Appendix 1).

 

4. Discussion

4.1. Biodiversity conservation in UGSs

The variations in the number of species recorded among UGS clusters might be due to differences in the management practices, planting preferences, stand characteristics, size, and nature of UGSs. The variations of plant species composition across UGSs have also been reported in previously studied UGSs (Aronson et al., 2017). Accordingly, UGS clusters of educational sites, Homegardens, and churches have the highest number of species as the number of species depends on the holder’s species needs and choice of planting, species maintenance, and management practices (Jaman et al., 2020). For instance, streets were dominated by exotic species because of the choice of fast-growing plants used for ornamental and beautification purposes. Similar results were reported for the urban green infrastructure of Hawassa city (Feyisa et al., 2022). Likewise, studies from the southwestern parts of Ethiopia confirmed that plant management practices determined the appearance of stands and individuals (Vanderhaegen et al., 2015). Therefore, as the humandominated urban ecosystem (environment) is not equally suitable for all species, there is variation in the number of species among clusters. Besides, as urban ecosystems are usually small in size, patchy, and scarce, there might be variations in the number of species among clusters (Jaman et al., 2020). The presence of the highest number of species, including endemic and indigenous, at educational sites could be because of the awareness of the educated communities on vegetation management, planting, species selection, and site matching. Similarly, as expected, churches have the highest number of endemic and indigenous species next to educational sites because of the longtime experience and practices of the Ethiopian Orthodox Tewahedo Church in biodiversity conservation (Gobena, 2018; Goodin et al., 2019; Wassie et al., 2005). Moreover, Helen et al. (2019) reported the highest plant diversity in monasteries, showing the significant contribution of those areas for biodiversity conservation. Though urban forest clusters of UGSs are reported to have the highest number of species (Helen et al., 2019), it was not confirmed in this study. Likewise, a significant proportion of edible and medicinal plants and other multipurpose plants used for fences, lumber, construction, and fuel were recorded in urban Homegardens. Similar results were reported in Kombolcha Town, eastern Ethiopia, where urban Homegardens are dominated by medicinal, edible, and other multipurpose plants (Semu, 2018). As a result, Homegardens have a great opportunity to provide food, store carbon, and conserve biodiversity (Vanderhaegen et al., 2015). Furthermore, human-dominated UGSs have a unique opportunity to conserve biodiversity outside the natural ecosystems (Butt et al., 2018; Edmondson et al., 2013; Helen et al., 2019; Nero et al., 2016).

The difference in the mean number of understory vegetation (seedlings and saplings) was observed among the UGS clusters because of the difference in understory vegetation management practices and site conditions. Accordingly, the lowest number of seedlings and saplings in the streets might be because of the size of the streets, disturbance, lack of management practices, and less protection of seedlings. A previous study in Dhaka city, Bangladesh confirmed that vegetation dynamics, site characteristics, and proper management practices influence understory vegetation in streets and urban trees (Jaman et al., 2020). Besides, relatively, the highest number of both seedlings and saplings in the Homegarden might be because of enrichment planting and application of different management practices. Similar, results were reported by Semu (2018). Moreover, Homegardens are comparatively fertile because of the frequent application of manure and mulching activities. This enhances the regeneration of understory plants (Vanderhaegen et al., 2015). Churches have also the highest number of seedlings next to Homegardens as church forests have better protection, supervision, and management and thus they enhance seedling regeneration. Besides, church forests have a significant amount of seed banks stored in the soil and support regeneration (Wassie and Teketay, 2006). The number of woody species recorded (including indigenous and endemic) in the UGSs of Dessie is higher than the number of species reported in Addis Ababa (90) and Hawassa (58) cities (Feyisa et al., 2022; Yilma and Derero, 2020). It was also higher than the number of species reported in Pyin Oo city, Myanmar; Zaria Metropolis, North-Western Nigeria; and Dhaka city, Bangladesh, respectively (Dangulla et al., 2021; Helen et al., 2019; Jaman et al., 2020). However, it was lower than reported in Kumasi, Ghana (Nero et al., 2016). This difference in the number of species between UGS of Dessie and another study site could be due to difference in agro-ecologies, management practices, site conditions, environmental factors, and climatic variations (Jogan et al., 2021; Vanderhaegen et al., 2015). Moreover, the variation in the number of species in the forest, street, and recreational sites from other UGS clusters of Dessie could be because of restrictions on enrichment planting new plants on those sites as consent is given by the local authorities (Jaman et al., 2020). Though streets are the least valuable in terms of biodiversity, apparently they still provide more benefits than built form without UGS because they provide some continuity of greenery as linear green space. In this regards, urban areas with green spaces provided multidimensional benefits than urban areas with no green spaces (Butt et al., 2018; Molla and Mekonnen, 2019; Sun et al., 2019). In reality, planned and regular management operations including enrichment planting may improve the quality of UGSs (Sutoyo et al., 2021). So that UGSs that have poor quality might provide better benefits through management activities and enrichment planting (Dangulla et al., 2021).         (oleh:muhammad atha faisal dzaky)