As soil becomes more compacted from pedestrian and vehicular traffic, the oxygen content of the soil becomes lower and lower over time. The majority of tree roots inhabit the depth where oxygen levels are optimal; by compacting the soil the available rooting volume for the tree lowers, which can be detrimental to the tree’s health
In hard landscaped areas, compaction of growing media can be avoided by providing a tree grille where pedestrian traffic may occur. Providing root cells beneath adjacent hard landscape can also prevent the soil from becoming compacted.
Some tree species are better adapted to compaction than others. Species being able to tolerate higher oxygen deficiencies and having a higher root penetration can exploit more compact areas of soil.
The tables below from Trees Built in the Environment, categorise trees by their tolerance to compaction and oxygen deficiencies.
Protecting Hard Surfaces
In urban landscapes there are often concerns whether trees will cause damage adjacent to hard surfaces.
Trees with shallower root systems are more prone to pavement damage. Also, trees confined to spaces with inadequate rooting volumes are more likely to break through the barriers restricting them, causing damage to hard surfaces.
Where soil is compacted, roots are likely to be higher in the soil profile, potentially causing damage to hard surfaces.
Damage to adjacent hard surfaces can be avoided by:
- Selecting tree species with root systems other than surface root systems.
- Making sure the tree species has enough rooting volume available.
- Avoiding excessive compaction of soil.
- Installing root barrier (300mm should be sufficient) in places where trees are in close proximity to hard surfacing – however the tree must still have enough rooting volume available, or it will die.
The table below from Trees Built in the Environment categorises trees by rooting depth.
For trees categorised by root system types refer back to table 3.2 above under Soil Compaction.