Viikki Street Tree Research Project


Why should we plant street trees?
Streets and parks are visible and important parts of public space in cities. Trees have a long life span and because of their relatively large size they often have a major impact on the cityscape. Plants and trees provide important ecosystem services in cities and also have well-known positive effect on city residents' physical and mental health. It takes several decades before a tree reaches its' expected size. Mistakes made in planting and establishing phase are very difficult and expensive to amend afterwards.

Built environment and trees
Built environment is stressful and completely artificial environment for trees. Suitable space for tree roots under ground is restricted and often unable to offer necessary water and nutrient resources for the tree. Several research papers have shown negative effects of restricted rooting volume on tree growth and health. Trees in built environment often experience poor soil conditions in addition to stress from e.g. vehicle damage and harsh microclimate. Stressed trees are more susceptible to pests and diseases and look otherwise unhealthy. If trees are doing poorly they cannot fulfil the ecosystem and aesthetic functions they were planted to provide.

Why urban tree research in Helsinki?
In Helsinki, more than a thousand street trees are planted every year; number of existing street trees is somewhere around 25 000 (2010). Almost half of the new street trees are planted in paved areas. As the city grows and land-use intensifies the amount of trees planted in paved areas increases. Additional structures (e.g. tree gratings, trunk guards, soil ventilation tubes) have to be set up as trees are planted in intensively built areas. Tree planting expenses include e.g. site preparation, associated structure cost and installation, transportation, planting and aftercare in addition to the cost of the plant itself. The price of the plant may be only a small fraction (roughly 10-15%) of the whole cost.

Tree trunks must be protected from vehicle damage. The trunk guard is installed on cast-iron tree grating, which in turn must have a proper footing in soil. Unfortunately most trunk guard types do not provide much protection against heavy winter maintenance vehicles.

Sometimes tree establishment is unsuccessful. Newly planted trees are e.g. quite sensitive to drying out and irrigation must be carefully scheduled. Replacement of dead street trees is very expensive. In addition to the planting costs, all the structures and the pavement around the tree must be reinstalled. Poor performance of an existing tree causes increased maintenance costs and decreased benefits. After the building phase of the infrastructure in a given area is finished, different soil amendment treatments tend to be difficult and expensive to carry out in intensively built areas.

These are some of the reasons why Helsinki City Public Works Department wishes to develop street construction to be more tree-friendly. The aim is to develop street construction techniques that lead to better tree establishment and performance. In addition, these solutions have to be cost-effective and easy to use. The resources set up at the planting stage have to be such that no expensive amendments are needed over the (hopefully prolonged) life span of the tree. Easy and reliable tree health indicators would make tree maintenance more cost-effective.

Skeletal soils
In a city, the microclimate is determined mainly by building intensity and height, and it is quite difficult for tree planters to change the microclimate tree experiences above ground. Below ground, however, changes are possible if care is taken that certain criteria (concerning e.g. drainage and load-bearing properties) are met. Structural soils provide means to increase the space available for tree roots, extending it under streets and sidewalks. In Netherlands, sand-based structural soils were researched in the end of 1970's. Structural soils have been investigated also e.g. in Germany and Denmark (Kristoffersen 1998, 1999). Cornell University researchers developed and patented a specified structural soil mix called CU-soil (Grabosky & Bassuk 1995, 1996). Structural soil in general consists of a load-bearing "skeleton" or matrix of crushed stone (e.g. well graded gravel, 30 to 60 or 60 to 150 mm in diam.) and fine rootable soil spread in the empty spaces of the stone matrix.

In Finland, the effects of intensive city building on the long-term development of trees are poorly understood. Many street trees established in conventional tree planting pits during the last 30-40 years are doing poorly. Structural soils have been used only for a few decades, and long-term behaviour of the soils and the effect of this soil type on tree performance remains to be seen. Yet in everyday city planning, reliable knowledge about e.g. soil volume requirements of trees and pavement type effects is needed.

Street trees are crowded both above and under ground. Space is in good use in Norkkokuja.

Research objectives

The main objective of the research project is to develop street construction techniques to better correspond with environmental demands of trees. Structural soils are considered a promising solution to soil quantity and quality problems faced by street trees. For this reason, we are interested in the function and behaviour of structural soils in different conditions. We also aim to develop new tools for tree health assessment, and tools and guidelines for planning and building street tree sites.

In this research project we are monitoring the performance of trees growing on different structural soils. Tree water use and photosynthesis system function is measured regularly. We also follow moisture conditions and summer and winter temperatures and oxygen levels in the soil, and certain microclimate variables. By combining these data we can pinpoint the features of the built environment that have most pronounced effect on tree well-being. We also aim to develop reliable and quick indicators of tree vitality and find ways to identivy reasons behind poor performance.

Grabosky, J. & Bassuk, N. 1995. A new urban tree soil to safely increase rooting volume under sidewalks. Journal of Arboriculture 22: 255-263.

Grabosky, J. & Bassuk, N. 1996. Testing of structural urban tree soil materials for use under pavement to increase street tree rooting volume. Journal of Arboriculture 21: 187-201.

Kristoffersen, P. 1998. Designing urban pavement sub-bases to support trees. Journal of Arboriculture 24: 121-126.

Kristoffersen, P. 1999. Growing trees in road foundation materials. Arboricultural Journal 23: 57-76.