The Ecology of Urban Forests

If you live in a city or a suburb, you are probably a part of an urban forest. That’s because urban forests are any area with trees that is densely populated by humans. Urban forests have a very interesting ecology because they don’t just encompass trees and plants, but also humans and human-made structures.

But first, what is ecology?

  • Ecology refers to the interactions among living things and their environment. It studies the relationships between each organism and each facet of the environment.

Now, we can explore the following questions:

What is An Urban Forest?

An urban forest is all the trees and vegetation that grow in a relatively densely populated area. If we look at the urban forest as an ecosystem, we can break its components down further into these three categories: 

  • Living things: trees, shrubs, grasses, animals that live in trees
  • Non-living things: climate, water, sunlight, soil
  • Humans and human-made things: The human population that lives among the trees, the streets, sidewalks, houses, apartments, and businesses

While shrubs and grasses are important to the urban forest ecosystem, the main focus of urban forestry is on trees. Any tree in an urban area is a part of the urban forest including:

  • Street trees
  • Trees in public parks
  • Trees in private front yards and backyards
  • Trees in vacant lots

Understanding relationships among trees, and between trees and other living, non-living, or human-made elements of the urban ecosystem will allow us to understand the ecology of the system. Let’s now talk about how we can understand those relationships.

What is the Ecology of Urban Forests?

To understand the urban forest as an ecosystem, we need to look at an urban area as ecologists. So, we want to ask questions of the forest where the answers will reveal the relationships between all of the elements of the forest, both living and non-living. 

Ecologists find those answers by asking the following questions of urban forests:

  • Structure and Function: What is the structure of the urban forest and what is the function of each element within it?
  • Tree Cover: How much tree cover is there in a particular area?

Structure and Function in an Urban Forest

Urban forests vary widely based on what climate they are in, how densely populated they are, and to what extent they are maintained by governments. However, there are a few main structural elements that you will see across all urban forests like we said before:

  • Trees, grasses, shrubs
  • Other living things (animals)
  • Non-living things: sunlight, climate, soil
  • Humans and human-made structures

These elements can be set up in a variety of different ways depending on the urban planning used in that particular city. Each element has a different function for the ecosystem. 

For example, the function of trees in an urban forest include:

  • Providing food and habitat for animals
  • Increasing beauty and adding to structural designs
  • Providing shade

Humans in an urban forest function to: 

  • Plant trees
  • Maintain the health of trees
  • Manage trees that could be dangerous (i.e. cutting down dying trees)

The above examples are just one way to look at an urban forest functionally, but there are infinitely more relationships between living and non-living things in an urban forest that keep it alive.

Tree Cover in the Urban Forest

Another way urban forests are analyzed in ecology is through tree canopy cover. This number is given as a percentage based on an aerial view. So, if you were to be on a plane in the summertime looking out over a city, what percentage of that city is green?

To give context to this metric, here are a few cities in the United States and their relative tree canopy percentages:

  • San Francisco: 13.7%
  • Houston: 18%
  • Los Angeles: 21%
  • New York: 21%
  • Chicago: 15.5%

Tree canopy cover is an important metric because it can give cities a goal percentage of how many trees they’d like to have in a city. Generally, the more tree cover, the more robust the ecology of the urban forest.

Biodiversity and Dominance of the Urban Forest

Once tree cover is established, urban ecologists also wish to study the species present in the forest. This is done through tree inventories. During a tree inventory, an urban forester will survey a specific area of land and list each tree by its species, condition, and size. 

Inventories give foresters a way to calculate the biodiversity of the urban forest, that is, how many different species live there. Foresters can also determine any dominant species in the forest.

Tree inventories are an important ecological research tool that allow scientists to determine the overall health of the urban forest. For example, tree inventories across several years may reveal that a certain species of tree is in decline, and urban ecologists would then investigate why those trees are dying and how to reestablish them within the urban forest.

Urban Forests and Humans

The most unique part of an urban forest is that humans are a crucial part of their ecology. Humans benefit from the urban forest, as can the forest benefit from humans.This is how the urban forest functions as a socio-ecological system.

  • Urban forests as socio-ecological systems means that they are ecosystems interlinked with human society and culture.

This system has several benefits associated with it that humans living within them benefit from every day. Such as:

  • Improving mental health
  • Providing beauty
  • Providing shade and decreasing temperatures in the summer
  • Lessen flooding by naturally controlling stormwater
  • Improving air quality

In turn, humans provide the functions we discussed above such as tree planting and maintenence.

So, urban forests don’t just have a purely ecological function, they also have a socio-ecological function because they live within human society and culture.


  • An urban forest is all the vegetation and animals living within an area that is densely populated by humans
  • The ecology of the urban forest is studied through tree inventories, tree canopy cover, and structural and functional benefits
  • Urban forests are are socio-ecological systems that benefit human health and culture