Resilient Trees for Urban Environments: The Importance of Intraspecific Variation

Key Benefits of Urban Trees:
- Provide shade
- Mitigate heat and floods
- Reduce noise and pollution
- Enhance pollination
- Contribute to beauty and biodiversity
- Larger and healthier trees enhance ecosystem service delivery.

Biogeochemical Role:
- Trees play a crucial role in Earth's biogeochemical processes:
- Influence on soil production
- Affect hydrological, nutrient and carbon cycles
- Influence global climate.
Carbon Storage:
- Trees store approximately 50% of the world’s terrestrial carbon stocks.
Biodiversity Support:
- Provide habitat for half of Earth’s known terrestrial flora and fauna.
Societal Relevance:
- Critical in urban settings where a growing population seeks environmental balance amid social, economic, and ecological challenges.

Environmental Threats:
- Urban tree populations are challenged by:
- Climate change
- Outbreaks of pests and pathogens
- Increasing urban density and impermeable surfaces.
Significance of Intraspecific Variation:
- Importance of genetic variability within species is essential for resilience against these threats.
Genetic Material and Adaptation:
- Right genetic plant material in urban environments leads to diversity and enhanced ecosystem services.

Threat to Tree Species:
- Approximately 30% of over 70,000 known tree species are under threat of extinction (Cazzolla Gatti et al., 2022).
Main Threats Include:
- Forest clearance for agriculture and urbanization
- Direct exploitation (timber, other products)
- Climate change pressures and biosecurity risks (Newton, 2021).

Global forest area has decreased by about 40% in the past 300 years.
29 countries have lost over 90% of their forest cover (FAO and UNEP, 2020).
Net loss of over 100 million hectares (approx. 2.4%) of tree cover from 2000 to 2020 (FAO and UNEP, 2020).

Forests cover roughly 31% of the world's land surface.
Contain about 50% of terrestrial carbon storage and provide over 75% of the world’s accessible freshwater.
Habitat support for diverse species, playing a role in ecological balance.

Services Provided by Urban Trees:
- Offer regulating, cultural, provisioning, and supporting services crucial for sustainable urban development.
Challenges in Urban Settings:
- Increasing urban density and prevalence of impermeable surfaces hinder successful tree development leading to stressors of drought and flooding.

Climate change elevates risks of:
- Higher tree mortality
- Reduced ecosystem service delivery.
Studies indicate exceeding tolerance capacities for 56-65% of urban trees across 164 cities globally (Esperon-Rodriguez et al., 2022).

Recognizing intraspecific variation is essential for improving resilience against climatic variables and urban stressors.
Drought Adaptation Traits:
- Specific traits in tree species like Acer and Betula vary across environmental gradients, linked to habitat and precipitation.
Critical relationship between local adaptation and successful survival during environmental fluctuations (Temunović et al., 2012; Zohner et al., 2020).

Current Urban Tree Diversity Limitations:
- Urban inventories often consist of a few dominating species, risking outbreaks amidst pathogens and pests.js
Policies Needed:
- Urgent need for urban tree diversity policies focusing on genetic variation and ecological resilience.

Genetic variation lost due to forest clearcutting can take centuries to recover, severely hindering resilience and biodiversity in urban settings.

Climate Change:
- Increased frequency and intensity of drought, heat waves, flooding, storms, and wildfires.
Pests and Pathogens:
- Invasive species and climate adaptiveness of pathogens increase mortality risks.
Urban Development:
- Dense settlements limit above and below-ground spaces for tree growth, contributing to increased tree mortality and diminished ecosystem services.

Evidence suggests a sixth mass extinction, with two in five plant species threatened (Antonelli et al., 2020).
Many conservation programs overlook intraspecific variation—a rapid genetic loss exceeds that of species loss (Hughes et al., 1997).

Promote more significant efforts in conserving both species and genetic variability. Policies should focus on collecting ecotypes suitable for urban resilience.
Strong importance placed on seed bank efficacy and the necessity for maintaining broad genetic representation during conservation efforts.

Houses over 2.4 billion seeds from 40,000 species, focusing on economically important, threatened, or narrowly distributed species.
Questions to Address:
- Sufficiency of current genetic materials for future climate needs.
- Proportions of ecotypes present in seed banks.

Immediate need to understand genetic variation in species for local environments.
Policies to mandate conservation efforts at all scales targeting urban environments.

Genetic Screening:
- Assessment of intraspecific variation and unique ecotypes within existing collections.
Climate Modelling and Habitat Matching:
- Identification of areas with valuable genetic materials resistant to urban ecological challenges.
Utilizing Native Flora:
- Evaluate native tree species for urban use, particularly in ecosystems where exotic species dominate plantings.

Urban trees are essential against climate challenges and biodiversity loss. To maintain their benefits, urgent revisions in tree selection practices are necessary, considering species and genetic diversity appropriate for urban contexts.