University of Florida

City Forests & Water

Forests in urban areas can play an important role in providing natural habitat in a city environment. Besides serving as a natural space for humans and wildlife, urban forests also can help in managing stormwater, as well as soil and water quality.

The tree canopy is the most important way that forests help with water regulation. The canopy is made up of all the branches and leaves that make the tree crowns in a forest. When rain falls, it first hits the tree canopy where it stays and evaporates or falls to the ground. In this way the tree canopy acts as an “interception point” for rainfall.

Erosion

Water that falls from the canopy can drip or flow down tree trunks. This reduces disruption of the ground’s surface by rainfall. Since it is being slowly released by the tree canopy, water is easily absorbed by leaf litter on the forest floor. The soil stability provided by tree roots and absorption of water by the leaf litter help reduce erosion in the area.

However, foot traffic, removal of leaf litter, vehicles, and roads all can cause the soil to become compacted. When the soil is compacted, water is not absorbed and becomes stormwater runoff, which increases erosion.

Stormwater

Stormwater is the water that collects and runs over city surfaces such as lawns, sidewalks, and streets during a storm. Typically, stormwater is not absorbed into the soil because the surface is impervious (e.g., compacted soil, parking lots, roofs, driveways, roads), the water is flowing too fast, or there is too much water at one time to be absorbed. This means that stormwater drains into gutters and storm drains or natural waterways.

When rainwater is intercepted by a tree canopy, this reduces and slows down stormwater accumulation. When stormwater accumulation is reduced, it also reduces the load of drainage systems and the risk of flooding.

Pollutants

When stormwater travels over urban surfaces, it picks up heavy metals, fertilizers, bacteria, pesticides, trash, and other harmful substances. While reducing stormwater, urban forests help lessen this pollution problem as well.

The soil, plants, and tree roots in the forests can remove pollutants, nutrients, and sediment from the stormwater, preventing these substances from getting into groundwater and local waterways.

A Beneficial Process

Individual trees are not as effective at reducing and slowing stormwater as forest cover. Minimizing tree and vegetation clearing preserves the benefits that urban forests provide and minimizes soil compaction. Proper management of trees and vegetation in the urban environment is an important part of urban water quality.

Try these other tips:

  • Maximize growing space and vegetation around a tree.
  • Preserve established trees and minimize soil compaction and erosion around them.
  • Do not over-fertilize or over-irrigate your trees and lawn.
  • Route stormwater flows through vegetative buffers and soil beds to reduce pollutant loads and erosion.
  • Include vegetative strips in parking lots to help collect and treat runoff.
  • Maintain and increase the amount and width of urban forest buffers around urban stream, lakes, and wetlands.
Adapted and excerpted from:

J. Seitz and F. Escobedo, Urban Forests in Florida: Trees Control Stormwater Runoff and Improve Water Quality (FOR184), UF/IFAS School of Forest Resources and Conservation (rev. 03/2011).

Cityscape

Related Sites & Articles