The benefits that populations of healthy urban trees can create are significant, yet the environmental conditions found in urban spaces can often be a barrier to their successful establishment and long term survival.
In order to provide urban trees with the best opportunity to survive and thrive it is essential to consider the conditions of soil, climate and water availability that trees face when they are planted in towns and cities. The correct design and installation of tree pits will mitigate the negative effects of the urban environment.
GreenBlue Urbans's document, A guide to the benefits of urban trees, covers some of the key factors for consideration as follows:
It is generally accepted that a minimum of 3m3 of potential rooting volume is required to give most tree species any viable chance of establishing successfully. In nature, a mature tree root spread will occupy more than ten times this volume, but a 3m3 tree pit will give most tree species an excellent start and chance in life.
Where this amount of space is not available, tree establishment is still a possibility provided great care is taken with regard to species selection and root management. In all cases the deployment of soil structure modules will help to prevent soil compaction and ensure that the available rooting volume is fully utilised. This is particularly important if the tree is to be located next to a road or another engineered structure, as the soil structure requirements for hard surfaces capable of sustaining large weights are diametrically opposed to those of a healthy tree root network.
In addition to being provided with a sufficient volume for growth, tree roots must also be appropriately directed to ensure that they do not damage surrounding surfaces or underground structures. Paved surfaces and utilities are particularly vulnerable to tree root damage and various types of root management products can be specified depending upon the item that requires protection. For example, if a tree pit is to be located amidst a continually paved surface then the tree roots will need to be managed downwards by at least 300mm to remove the possibility of paving heave.
In order for a tree to thrive, the root network must be able to access an appropriate amount of water. If there is insufficient water then the tree will be unable to absorb nutrients from the soil and will perish as a result of the water loss that occurs during transpiration. However, if the rooting area is continuously waterlogged then the surrounding soil may become anaerobic, which is equally likely to result in the destruction of the tree. Gaseous exchanges that are vital for tree survival occur not only above ground but also in the root zone. These cannot take place if all air has been excluded from the soil pores by the presence of excess water.
In light of the above, consideration must be given to the substance and structure of the ground surrounding a tree pit to ensure that there is sufficient drainage to prevent waterlogging. However, since the majority of urban tree pits are covered by a hard, impermeable surface, tree pit designs should also incorporate the means to irrigate efficiently, particularly for the first three years.
An efficient irrigation system will account for the fact that tree roots may have been directed downwards as a result of root management measures and will deliver water directly to the rooting volume, rather than the area above it. Such an irrigation system will help with soil aeration at depth, but it is also prudent to leave sufficient open space around the trunk to allow for gaseous exchange between air and soil (and of course, tree growth). In urban areas, it is often impractical to allow for bare soil and a variety of grille systems or permeable resins may therefore be considered.
Whereas young trees in their natural environment are likely to be at least partially sheltered by the presence of surrounding vegetation, many urban trees are situated in exposed locations where they are vulnerable both to the high winds that can develop as a result of urban wind tunnel effects and to disturbance from pets, pedestrians and vehicles. An appropriate form of support is therefore required to help trees through the establishment phase. Underground guying is widely favoured for urban tree pits as it is unobtrusive. Staking and tying is a cost-effective alternative although this will require maintenance.
It is an unfortunate fact that trees in many urban locations are vulnerable to gratuitous vandalism. Damage by dogs is also a growing problem that affects the health of urban trees. Where either of these is a possibility, some form of above ground protection can be critical to tree survival. When you are designing tree pits you will need to decide whether there is a need for tree grilles, vertical guards or other protective measures.
By creating a tree pit design that successfully accounts for the issues above you will have attended to over 90% of the reasons for urban tree failure. GreenBlue Urbans’s Arborsystem products can be used in combination to address any or all of these factors as need dictates, giving you the confidence that your tree pit design will be efficient and successful, producing vigorous and healthy trees.