Over the next few weeks we will be discussing how a variety of factors need to be considered when specifying trees within residential schemes. These factors will include:
• Zones of Influence (Part 1)
• Drought Tolerance (Part 1)
• Rooting Volumes (Part 2)
• Protecting Services (Part 2)
• Soil Compaction (Part 3)
• Protecting Hard Surfaces (Part 3)
• Pollution Tolerance (Part 4)
• Pests/Diseases (Part 4)
• Transplanting (Part 5)
• Initial Planting Sizes (Part 5)
Zones of Influence
The ‘zone of influence’ is a theoretical area where trees have the potential to influence foundations due to changing moisture content of soils, resulting in shrinkage and swelling. If the zone of influence of a proposed tree overlaps a proposed building, the foundations of said building will need to be deepened accordingly.
For calculating the zone of influence structural engineers often reference Table 3 in the NHBC Standards 2020. Chapter 4.2 Building near trees.
‘Tree identification can be assisted by reference to a tree recognition book. Information may be obtained from suitable alternative authoritative sources for trees not listed in this chapter.
When the species is known but the subspecies is not, the greatest height listed for the species should be assumed.
Where hedgerows contain trees, their effect should be assessed separately and the height of the species likely to have the greatest effect should be used.’
The zone of influence can be calculated by multiplying the mature tree height by the water demand multiplier: Low x 0.5, Moderate x 0.75, High x 1.25.
For tree varieties not referenced within the NHBC table, use the water demand multiplier for the genus (or assume high water demand if the genus is not present in the table either), and use tree heights referenced by a reputable source such as the RHS website (https://www.rhs.org.uk/).
The RHS website also mentions some other tree species and their water demands not mentioned in the NHBC table, such as Liquidambar and Ginkgo.
As developers often want to avoid the extra costs of deepening foundations, landscape architects are often requested to propose trees with smaller zones of influence. Unfortunately, this often becomes the leading factor in species selection for tree planting in residential schemes, resulting in an abundance of smaller, less impactful tree species selected.
Drought is considered to be one of the primary causes of death for newly planted trees in urban landscaping schemes. This can probably be attributed to poor watering regimes during the establishment phase of tree planting.
Some drought tolerant tree species may not necessarily be better adapted to drought within the establishment phase of planting, due to the stresses of transplanting and other factors. It is important to consider the conditions of the planting area within the post-establishment phase and select species accordingly.
Trees in urban environments are often surrounded by impermeable surfaces. The lack of access to water could mean they are subjected to more extreme conditions (drought and waterlogging) in comparison to trees planted within parks or open spaces.
Climate change promises more climate variability including wetter winters and drier summers, which emphasises the importance of drought tolerant tree selection. Choosing trees adapted to these extreme conditions will improve their chances of survival in urban environments.
The table below from the Forest Research BPG Note 20 Drought-Tolerant Tree Species for Land Regeneration categorises trees by drought tolerance.