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.


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


Do urban and peri-urban forests fulfill their hydrological function?

Last year, Janez Kermavnar finished his Master’s degree thesis at the Biotechnical faculty, University of Ljubljana. The topic of his thesis is the hydrology of selected urban and peri-urban forests in the City of Ljubljana. He wants to expose ecological benefits of forests for water, which are often taken for granted.

Urban and peri-urban forests influence on drinking water quality and quantity. With the rainfall interception, they mitigate many negative consequences of extreme weather phenomena (stormwater runoff, soil erosion, flooding etc.).

A study network of green infrastructure in and around Ljubljana provided an array of different ecosystem services. Slovenian capital has vital urban forests. Moreover, woodlands are abundant even on the city`s periphery, where peri-urban forests serve their specific aims. They are often unduly neglected, although they protect human settlements against floods and act as a buffer zone between river and neighboring agricultural land.

Healthy drinking water provisioning is one of the key factors when considering well-being of urban society. Worldwide, water quality in urbanized areas is now days often questionable. Fast urbanization and its negative effects (air, soil and water resources pollution) are onerous for whole natural environment. On the contrary, forests act as protective and selective layer between atmosphere and ground. Without that layer, there is no natural cleaning plant and rainwater (enriched with pollutants) just run off from impervious surfaces, such as asphalt or concrete, into nearest watercourses. With deep  root systems and rainfall interception in canopies, forests also prevent erosion.

Urban forests give us many things on a regular basis. What can we do in return?

Solid information on the role of urban forests in fulfilling the ecosystem services, related to water, is still limited. In order to better understand rainfall interception, we did an interesting research study. So the answer is: explore and keep the public informed about new discoveries.

Slovenian Forestry Institute (Department of Forest Ecology) measured rainfall interception in three different natural urban and peri-urban forest stands: mixed urban forest in Landscape park Rožnik, Tivoli and Šišenski hrib and in two peri-urban forests along Sava river: riparian pine forest and floodplain hardwood forest.

SFI monitoring plot_Kermavnar
Monitoring plot for measuring rainfall interception in mixed urban forest positioned just beside Slovenian Forestry Institute in Ljubljana.

Results showed that:

  • rainfall interception mainly depends on (horizontal and vertical) forest stand structure and its tree species composition;
  • mixed urban forest interception was the highest, due to its dense canopy cover and trees with greater dimensions (height, diameter);
  • in riparian pine forest and floodplain hardwood forest, shrub and understory vegetation influenced rainfall interception;
  • meteorological conditions, like rainfall intensity, can play an important role in estimating rainfall interception.

Some of these findings might be useful for sustainable forest (close-to-nature) management and urban planning. Bottom line, silvicultural plans and measures have to promote multiple forest functions (recreational activities, biodiversity, …) at the same time. Urban and peri-urban forests protect available renewable water resources. Majority of them is located in the forests or in the forest edge. Despite the fact that forest ecosystems are inevitably essential, management practice are not always in favour of providing those benefits.

This blog post is authored by Janez Kermavnar and is a part of the #EFUF2016 blog competition.