Urban Forests and Water

This year’s topic of the International Day of Forests on March 21st is “Forests and Water”. It’s a very important topic, especially in urbanizing cities, where reducing stormwater runoff by urban forests and trees has become important part of stormwater management. Read about this in a blog written by Dr. Urša Vilhar, research fellow at the Forest Ecology Department of the Slovenian Forestry Institute.

The ability to mitigate stormwater runoff in many urbanized watersheds around the globe has decreased. Stormwater runoff associated with an increased amount of impervious surfaces in the cities is the main cause of flooding, poor water quality, and deteriorating stream health.

How can urban trees reduce stormwater?

Urban forests and trees have a great potential for reducing stormwater runoff by enhancing soil infiltration and evapotranspiration, as well as regulating the amount of throughfall reaching the ground via rainfall interception by tree crowns. Trees can also absorb water in the soil by root uptake. Together, the roots and leaf litter stabilize soil and reduce erosion. Since the amount of impervious surfaces, e.g. parking lots, roof tops, driveways, and roads is increasing in many urban communities, rainwater cannot infiltrate and runs off as stormwater.

In urbanizing cities, management of urban forests and trees has become important part of stormwater management. Urban forests, trees, vegetation and pervious soils beneficially affect urban watershed hydrology by their ability to intercept, evaporate, transpire, infiltrate, and store rainfall.

Vilhar_vodnilrogWater cycle in urban forest and in the city. Canopy interception loss by urban forests or individual city trees accounts for 4 to 50 % of annual or seasonal rainfall. Trees and their associated tree pits may reduce surface runoff from asphalt by as much as 62 %.

How can urban trees improve water quality?

Research has found that water quality is strongly related to runoff. Stormwater flows into the community’s stormwater system or flows directly into the urban streams, lakes or wetlands. Before reaching a stormwater system or water way, stormwater picks up and transports loads of nutrients, heavy metals, organic pollutants, and other harmful substances from roadways, sidewalks, yards, and homes.

Tree roots, leaf litter, and vegetation can remove pollutants, sediment, and nutrients from the stormwater, lessening the amount of harmful substances reaching our ground or surface waters. Among plant types, trees have an exceptional ability to capture and filter multiple air pollutants, including ground-level ozone, sulphur dioxide, nitrogen oxides and particulate matter. Finally, tree canopy over streams and wetlands can reduce water temperatures, thereby increasing dissolved oxygen and reducing the formation of nuisance algae.

Using natural vegetation as a low impact development and best management practice can be an effective technique to control stormwater runoff on site, mitigating the impacts of urbanization on surface runoff and pollutants delivery at a local scale.

ecosystem_contribution_water

Relative contribution of different land cover classes in the City of Ljubljana to ecosystem services, related to regulation of water flow and water purification. The highest capacity to provide water flow regulation was indicated for wetlands and mixed forests. The highest capacity to provide water purification services was indicated for mixed and coniferous forests. The lowest capacity for both ecosystem services was indicated for urbanized areas. (Results of the Life+ project EMoNFUr).

Urban forests and trees are valuable parts of our urban ecosystem for the numerous benefits they provide to communities. Proper management of the urban forest reduces stormwater runoff and improve water quality. The following practices can help achieve this:

  • Maximize the amount of growing space and understory vegetation around a tree.
  • Preserve established trees and minimize soil compaction, displacement, and erosion around a tree.
  • Minimize clearing of trees and vegetation to preserve their benefits and minimize soil compaction.
  • Do not over fertilize or over irrigate trees, lawns or gardens.
  • Route excess stormwater to bioretention areas made of a vegetated buffer and a soil bed to filter pollutants, store water, and prevent erosion.
  • Include tree and vegetative strips in parking lots to collect, store, and treat the runoff.
  • Maintain and increase the amount and width of urban forest buffers around urban streams, lakes, and wetlands.

Author: Dr. Urša Vilhar, Forest Ecology Department of the Slovenian Forestry Institute

Advertisements

Monitoring of Urban Forests – LIFE+ EMoNFUr

The benefits of urban forests for citizens and the most important results of the LIFE+ EMoNFUr project are introduced by our invited blogger Dr. Urša Vilhar, research fellow at the Forest Ecology Department of the Slovenian Forestry Institute.

Forest, trees, parks and other green areas in urban landscapes are the irreplaceable part of the nature and our environment and especially important for citizens. Urban forests are important because they provide direct contact with nature to citizens, peace, relaxation, aesthetics and in Slovenia they are frequently visited for recreation. At the same time urban forests provide a great deal of ecosystem services that play an important role at insuring the health and improving the citizens’ quality of life. Namely, urban forests filter air, protect water quality, reduce soil erosion etc. In addition, trees and soils store carbon and reduce concentrations of carbon dioxide in the atmosphere. But the climate changes reduce the ability of urban forests to provide these benefits for the environment and people.

The most important results of LIFE+ EMoNFUr

Emonfur2

In the LIFE+ EMoNFUr project a monitoring network was established to assess lowland forests in Milan (Italy) and Ljubljana (Slovenia). Monitoring of urban forests was set up at five study areas in Milan and two in Slovenia during a 3-year period.

A network of permanent plots for monitoring urban forests was established in Milan and Ljubljana. The inventory of urban and peri-urban forests was preformed in Milan. Researchers from the Slovenian Forestry Institute assessed diversity of selected plant and animal species, monitored insects and diseases of forest trees and their health status in Ljubljana’s urban forests. They have also analyzed soil pollution, monitored visits to the urban forest, assessed air pollution, analyzed tree growth, assessed forest inventory, estimated carbon stocks in trees and forest soils, monitored water quality and quantity from forested watershed, etc.

Emnofur1
A part of the LIFE+ EMoNFUr project monitoring network in the Landscape park Tivoli, Rožnik and Šišenski hrib for monitoring urban forests. The Forest inventory revealed that 1 hectare of urban forest on average sequestrated 138 tonnes of carbon in above ground, below ground and dead wood biomass.

The results have shown that:

  • Tree health is especially important in urban areas – diseased or injured trees can pose threat to humans and property.
  • Urban forests sustain the quality of drinking water sources and have a large capacity for retaining excess stormwater and melting snow.
  • Forest soils in Ljubljana have proved to be well preserved and represent one of the cleanest environments in Ljubljana.
  • In urban forests, the air temperatures during the heat wave are appreciably lower than in the urban center.
  • the diversity of plant and animal species is an important indicator of biodiversity in the urban forest
  • At the same time urban forests serve as natural filter for pollutants, while in average 1 hectare of urban forest binds 138 tons of carbon.

The most important EMoNFUr project results are the online guidelines and the Protocol for monitoring urban forests, which can be used by any city in Europe and around the world. The documents include a wide range of recommendations and criteria for detailed descriptions of ecological, environmental and social values of urban forests.

Dr. Urša Vilhar, Slovenian Forestry Institute

life

Acknowledgement: the  LIFE+ EMoNFUr project was financially supported by the European Commission’s LIFE – Financial Instrument for the Environment.