Urban trees age diversity- Missing link

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Toso Bozic

Tree/Forestry Expert

During my article writing on urban prairie provinces tree diversity , using publicly available data, I have realized that there is little or no information about age of trees  in urban centers (at least no publicly available). Urban trees age diversity can be defined as to the distribution of trees of different ages within an urban environment. The importance of tree age diversity in urban centers is often overlooked aspect while it is very crucial for the long-term health and resilience of urban forests. The age diversity of urban forests has relatively received limited attention in research papers as well as and have never been identified as critical information regarding to many issues such as pest infestation or  impact climate change on urban forest.

In natural forest ecosystems, forest age and distribution are critically valuable information for various aspect of forestry management decision makers and  include the assessments of biodiversity and habitat,  growth yields, pest infestation, fire management,  forest carbon science and modeling as well as other sustainable forest and land decisions. During the inventories tree and stand age are typically measured directly on site or estimated through visual photo interpretation. Somehow, urban tree inventories don’t collect information about age of trees that are growing within cities nor urban tree managers collect the age of trees that are removed for various reasons. There are several reasons why to collect or obtain information on tree age diversity in urban area.

  • Helps to evaluate resilience and environmental stressors: Age diversity contributes to the resilience of urban tree populations. A mix of young, mature, and old trees helps ensure that the urban forest can withstand various environmental stressors such as climate change, pests, diseases, compaction, salt, and extreme weather events. Older trees, for instance, may have developed resistance to certain pests, level of compaction, drought and too much moisture, while younger trees contribute to genetic diversity, making the overall population more adaptable.
  • Forecast pest outbreaks – as vast majority pests are relatively very narrow specialist, and each pest has very unique tree age requirements  to go from endemic stage to outbreak stage. The dominance of certain species within same age frame ( 5- 10 years) could decline the resilience and increased vulnerability to pest outbreak.
  • Assess the ecological and biodiversity values: Diverse age groups of trees create a diverse habitat that supports a variety of wildlife. Various stages of a tree’s life cycle provide specific ecological niches for various species of birds, insects, and mammals. A rich biodiversity within the urban forest contributes to the overall ecological balance of the city, fostering a healthier and more sustainable environment. The ecosystem services provided are intricately linked to the amount of live tree biomass, its growth rate, and the canopy cover and volume.
  • Forest fire prevention –in urban centers where natural forest dominates, age structure is crucial to evaluate potential of forest fires. Overmature and declining forest will increase fuel load and increases risk of forest fires. On other hand younger, species and age diverse forest reduce the risks of large forest fires.
  • Evaluation of ecosystem services: Urban trees offer a range of ecosystem services, including air and water purification, carbon sequestration, and temperature regulation. A diverse age structure enhances and strengthen these services. Young trees, for instance, are often more efficient in sequestering carbon, while mature trees provide extensive canopy coverage, reducing the urban heat island effect. Collectively, a mix of age groups maximizes the overall impact of these crucial ecosystem services.
  • Validate green asset values – Age is also a critical factor affecting both the increase in biomass and the structural development of an urban forest. More mature trees are structurally more valuable and proportionately overall increase the green asset values.
  • Helping to design long term care and maintenance plans – Urban trees at various age stages requires various level of maintenance, pruning, watering, and monitoring for diseases. Understanding tree age diversity will greatly help urban managers in developing long term tree care and maintenance planning and budgeting process.
  • Assessing species selection and planting practices: The tendency to plant a limited number of tree species and rely on uniform planting practices can result in a lack of age diversity. Moreover, certain species may have similar lifespans, making the overall urban tree population more susceptible to diseases or pests that affect specific age groups. Promoting a broader selection of tree species with various lifespans and varied planting practices can address this issue.
  • Forecast tree replacement: knowing tree age distribution will help to forecast tree replacement plans. The timely replacement of old or diseased trees with younger ones is crucial for maintaining age diversity. Also keeping records of age of removed and replaced trees will provide valuable information on cause of dead and overall lifespan of planted trees.

Urban trees age diversity is a critical component of sustainable urban forestry that somehow have not been overlooked. Conducting tree age information during tree inventories and monitoring programs helps track the age distribution of urban trees. By recognizing the importance of maintaining a balanced age structure within the urban canopy, cities can ensure the long-term health, resilience, and ecological functionality of their urban forests. Tree age diversity data will help inform decision-makers about the health of the urban forest as well as facilitates strategic planning for maintenance, pest outbreaks, fire risk, replacement plan and overall evaluate the resilience of urban forest.

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