Cities are home to more than half of the global population. As urbanisation increases at an unprecedented rate, by 2050, about two thirds of the worldwide population are anticipated to live in cities (UN, 2018). But let’s look a bit closer at who else is calling cities their home: Urban areas also provide shelter to a considerable range of growing, flying and crawling diversity, living in the soil, on street trees, in urban parks, woodlots, urban forests and their surrounding natural habitat. Read on to learn more about the nexus between urban forests and biodiversity and why a healthy, biodiverse city is critical for the future of human societies.

The impact of humanity on our planet is becoming increasingly visible through climate disasters and the global biodiversity crisis. It is a complex relationship of living things and systems and a growing damage towards our planet’s biodiversity that result in significant risks to humankind. Facing this development, it becomes ever more vital to conserve, protect, restore and sustainably manage those places that offer shelter for different species.

The role of urban biodiversity

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Efforts to preserve biodiversity often focus on large natural habitats. However, the urban environment, with a landscape that can harbour relatively high amounts of biodiversity (Alvey, 2016), represents a significant potential for the preservation and promotion of species, genetic and ecosystem diversity. Humanity relies on a range of ecosystem services provided by the natural environment, including regulating, cultural, supporting and provisioning services – and a diverse urban forestry landscape is necessary to ensure the long-term provision of those ecosystem services in cities. Safeguarding biodiversity must therefore become an integral part of sustainable urban development. To effectively halt and reverse the loss of nature, the adoption of a transformative post-2020 global biodiversity framework needs to be high on the political agenda. COP 15 is on its way to tackle this challenge.

COP 15 – ‘Ecological Civilization – Building a Shared Future for All Life on Earth’

The UN Biodiversity Conference and the Fifteenth meeting of the Conference of the Parties to the Convention on Biological Diversity (COP 15) kicked off online on 11 October 2021. While the first phase of the summit is being held virtually from 11 to 15 October 2021, the second phase is inviting for an in-person meeting from 25 April to 8 May 2022 in the Chinese city of Kunming. The two-part conference will further host the Tenth meeting of the Conference of the Parties serving as the meeting of the Parties to the Cartagena Protocol on Biosafety (CP-MOP 10), and Fourth meeting of the Conference of the Parties serving as the meeting of the Parties to the Nagoya Protocol on Access to Genetic Resources and the Fair and Equitable Sharing of Benefits Arising from their Utilisation (NP-MOP 4).

COP 15 will shed light on the delivery and achievements of the CBD’s Strategic Plan for Biodiversity 2011-2020. The summit aims to finalise the post-2020 global biodiversity framework and establish mid-century long-term goals and short-term targets for 2030 to fight biodiversity loss. Looking back to the previous global targets set, objectives were largely missed. More than ever, it is of utmost importance to agree on clear, measurable targets to boost nature preservation and ensure those targets are anchored within national policies. Read more about the conference and registration here.

The importance of SDG 15 – Life on Land – Urban green matters

Forests provide shelter to more than 80% of all terrestrial animal, plant and insect species. With plant life itself providing 80% of the human diet, the intact functioning of a diverse natural environment is crucial to support food and water security while helping to adapt to and mitigate climate change and fostering peace and security (UNDP). In this context, food security is closely tied to the role of insect pollinators – and those also live in urban areas. Initiatives to promote bee-friendly urban green area management exist beyond Europe. By choosing the right plants, applying appropriate mowing and pruning practices, promoting the creation of planted road margins, flowering green roofs, vertical gardens or by harnessing the potential of urban agriculture initiatives, cities can contribute a significant part to supporting human livelihood based on nature – with potential for upscaling (Van der Slujis & Vaage, 2016).

Urban Forests & Biodiversity

About healthy trees, planting trees in the right places and planting the right trees

Fortunately, urban forests provide critical ecosystem services to sustain human health and well-being and support environmental quality in and around urban areas. As a vital part of the city infrastructure, urban forests also face numerous threats due to climatic changes, conflict over land use, strong anthropogenic influence as well as pests and diseases. To ensure that people can rely on benefits provided by urban forests in the future, it is crucial to understand those threats and foster a healthy and resilient urban forestry landscape.

Biodiversity of Urban Forests

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The long-term provision of ecosystem services provided by healthy urban forests counts on species diversity, diversity within species, age and structural diversity. The many different regulating, cultural, supporting or provisioning services we obtain from urban forests are related to the tree’s characteristics – with some species providing a single ecosystem service better than others due to their individual characteristics.

“To optimise one ecosystem service, diversity is unnecessary. But in order to optimise multiple ecosystem services, high urban tree diversity is essential.”

Urban Tree Diversity for Sustainable Cities’, Nordic Forest Research

In many European cities, urban areas harbour high species richness but are often dominated by few species, especially on stressed sites. To work towards a resilient future city, efforts to decrease the dominance of a small number of species are needed. But which species are best suited for a changing urban environment? And what is the role of non-native species? The wide range of site-adapted species growing (and dying) in urban environments comprises both native and introduced species. While the potentials and risks of non-native species are passionately discussed among scientists, the suitability of long-term resilient tree species appears to be a topic far from black and white. In this context, the role of non-native species remains an understudied topic (Schwarz et al., 2017).

Biodiversity of Fauna supported by Urban Forests

Urban forests also provide shelter for adapted species, including insects and birds. Trees and forests in cities can be regarded as extreme habitats due to their high anthropogenic influences. Looking at urban forests as faunal biodiversity habitats, studies show the important role of native trees for bird species richness and abundance. However, also exotic trees can harbour interesting habitats invertebrates, being an important food resource for insect-eating animals. Newer suburbs are anticipated to provide high-quality habitats with native tree species able to promote faunal biodiversity, albeit non-native trees also form part of cities now and in the future (Livesley et al., 2016).

Since the city lives of being a shared place, the coexistence of nature and people does not come without challenges. Overcrowded urban green areas and recreational overuse can disturb urban biodiversity. The considerable potential of urban forests to increase and hold biodiversity needs to be taken into account when balancing different interests and promoting nature in cities for all beings. In this context, planting trees in the right places provides an important opportunity to create connections between urban biodiversity and surrounding forests and ecosystems.

Improving environmental quality = Improving quality of life?

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Preserving biodiversity in urban areas brings more than the inherent value of biodiversity conservation itself. It also presents significant societal benefits, such as environmental awareness and improvements for mental health and well-being through interacting with a biodiverse natural environment (Morgenroth & Nielsen, 2016). Recent studies suggest that daily contact to nature through street trees close to residents’ homes may reduce the risk of depression (Marselle et al., 2020), while older studies also hint at the fact that improved health may derive from an area perceived as highly biodiverse by residents, without necessarily being highly biodiverse (Qiu et al. 2013). Read more about the nexus between functional biodiversity and a healthy society here.

It remains – diversity is key. Strategic planning, decision-making and management will be crucial in an increasingly urbanising world, with urban forests as a promising stage for the fight against biodiversity loss.

Learn more about Urban Forests and their potential for rehabilitating, reconnecting and restoring urban ecosystems in the Sino-European H2020 CLEARING HOUSE project.