What’s an Urban Forest?

We may not think of the trees along our streets, in parks, along rivers, and in yards as part of a forest, but they are. All the trees, other plants, and animals in a city, town or village are part of an urban forest.

A forest is an ecosystem. An ecosystem is all the living and nonliving things in an area interacting with each other. In an urban forest, the increased influence of humans means that in addition to trees, other plants,  animals, sun, and soil there are people, buildings, asphalt, pets, utilities, and more. This makes the urban forest a unique type of forest, but still one that is connected to other ecosystems.

It is important to note that rural forest ecosystems are relatively large areas compared to urban forests. Rural forests are not devoid of human influence. However, there is a difference between urban and rural forest ecosystems in the degree of impact that people have on forests.

When thinking about the interactions that occur in an urban forest, it may help to keep in mind that things that don’t really seem “natural” to us are still part of other ecosystems, concrete and asphalt are part of urban forests. A squirrel in a rural forest relies on nuts and seeds to eat. In an urban forest, they may also eat bread that people throw for birds or even discarded french fries.

Trees face different challenges and receive different benefits from their locations. In a rural forest, trees compete for nutrients, sunlight, space and water. A well-maintained tree in an urban forest may be watered and fertilized so it doesn’t need to compete for those things. However, an urban tree may also be subjected to more air and water pollution. Trees in both places may have to compete with other trees for sunlight. Urban trees may also have to compete with shade from buildings.

Water flows and moves through ecosystems. This water cycle connects all parts of earth. There are infinite paths a water molecule can follow. For instance, water may be absorbed by plants from the soil, but there are also aquatic plants that absorb water from rivers, lakes, and oceans. A cycle really has no beginning or end, but we’ll start with rain falling from the clouds. A raindrop falls from the clouds and lands in a rural forest on plants, rocks, soil, animals or water bodies. From this point, it could flow into a river or lake and eventually end up in the ocean, it could soak into the soil and end up in groundwater, it could be absorbed by plant roots and be released into the air during photosynthesis, it could be consumed by an animal, or it could just evaporate.

There is far less area in an urban forest covered by soil or water bodies that can readily absorb a raindrop. Concrete sidewalks, asphalt streets, rooftops, and cars shed rain as it falls. That rain flows until it can be absorbed by soil or into a water body. Gutters and storm sewers carry water away from hard-surface areas to prevent flooding. This water is eventually directed into lakes and rivers. Even rain falling on soil may run off into storm sewers if it falls too fast for the soil to absorb it.

Rain that falls on the leaves of trees and other plants is slowed. The water will still fall to the ground or run down a tree trunk eventually, but with it falling slower, the soil has more time to soak it up. Plants use water in photosynthesis too, so they can help dry out soil between rains. The puddles formed by a rain shower can be used as a source of water for wildlife in an urban area.

It is important to remember that water we use in our homes and businesses also fits into this cycle. Municipalities may get their water from large natural bodies of water, such as Lake Michigan, from groundwater that is then stored in a water tower, or from a reservoir filled with rainwater. That water is pumped into our homes and businesses. Some is put on lawns; it soaks into the soil and eventually back into groundwater. Some is used inside in our sinks, toilets, showers, dishwashers, etc. That inside water flows down the drain and into the sewage pipes. The sewage pipes carry water to a sewage treatment plant where waste is removed by a series of complex processes. That water, with the contaminants removed, is discharged into a nearby lake or river.

Adapted from LEAF Urban Forest Lesson Guide

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