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.

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