Introduction
Street trees are a relatively recent introduction in England. Prior to the 19th century there were relatively few trees that were either planted or allowed to grow within the curtilage of the highway.
For the purposes of The Highways Act 1980, the ‘highway’ was defined not just as the carriageway (the road surface), but the adjacent footway running from the kerb edge all the way to the back edge of the pavement area. It also includes footpaths and public rights of way maintained at public expense.
Trees growing in the highway are usually the responsibility of the local highway authority and are managed and maintained just like any other element of the highway infrastructure that requires maintenance or replacement from time to time: lamp columns, bollards, kerb edging, signage, etc.
The current statutory requirement is for tree branches to be a minimum of 5.2m above the carriageway and 2.3m above footpaths, although where they combine with cycleways 2.5m is more appropriate.
However, unlike other highway infrastructure trees do not remain static, they grow, enlarge over time, both above and below ground.
They shed leaves and branches, and in some situations may fall over presenting a hazard to users of the highway.
This growth and life cycle which may be perfectly benign in a natural setting is problematic when the tree interacts with the built infrastructure around it in a highway setting.
Trees can cause maintenance issues for kerbs, footway paving, carriageway surfaces, adjacent shallowly founded structures (direct damage) and in certain conditions damage building foundations as well, when growing in a shrinkable clay soil (indirect damage).
Despite all these issues, highway trees are a critical element in the overall canopy cover of the urban forest within towns in Britain.
As such, they make a significant contribution in respect of visual amenity and the many aspects of ecosystem services they provide to people living in towns.
Larger landscape scale trees provide the greatest overall benefits. When managing highway trees, this will be done in accordance with the guidelines in the Well-managed Highway Infrastructure guidelines (WMHI).
Engineered highway solutions
Unfortunately, it is often the larger trees, which, if not adequately maintained in the highway context, may cause the most issues for the built infrastructure in their immediate vicinity.
However, there are a range of engineering and maintenance solutions that can easily be applied throughout the trees’ life cycle that allow both the tree and the highway to mutually co-exist, each providing the benefits to society that are implicitly valued by residents and businesses in towns; safe, usable highways and a high quality, well managed and maintained urban forest.
Some examples, not exhaustive, are:
- use of flexible tree pit sizes, rather than relying on a small range of pre-specified dimensions
- use of narrow kerb profiles to accommodate trunk flare and buttress roots
- dispensing with kerb edging when possible in appropriate streets
- use of bonded gravel in the tree pit to provide an inclusively accessible surface
- use of tarmac inserts around the base of trees where the footway surface has become deformed due to root growth
- use of flexible rubber crumb along pavements and as inserts where previously tarmac or paving has been deformed due to root growth
- integrating tree pit locations into parking bays as kerbside buildouts
- creating kerbside buildouts to accommodate tree growth or redirect pedestrian footfall
Tree maintenance solutions
Root pruning of non-structural surface roots to accommodate the laying of new paving, tarmac inserts, rubber crumbing surfacing.
Root pruning of non-structural roots to accommodate relaying of kerb edging.
Placing barriers around trees to discourage parking on verges.
Regular pruning of the tree to control its water uptake and limit its root and trunk annual incremental expansion.
All of the above solutions will require the co-ordinated response and guidance of an experienced and qualified Arboriculturalist working closely with an experienced Highway Engineer.
Some, such as the rubber crumb surfacing bring multi-purpose benefits in that larger areas of rubber crumb surfacing can act as a Sustainable Urban Drainage (SUD) element within the footway, by desynchronising flash flooding and providing additional available water to the tree itself.
Where there is potential for direct infrastructure damage as well as indirect foundation damage, the need for regular pruning of highway trees in particular can be difficult to justify to residents and incurs a cost.
However, tree management and maintenance is a long term management activity and many tree managers take the view that when necessary, it is better to retain a large species, large landscape tree and its root system in situ, but maintained at a reduced size.
This is done in the expectation that at some future date due either to; a technical innovation that solves the issue of building subsidence, a change in climatic factors (increased winter rainfall rehydrating the soil sufficiently annually) or a cultural shift that means property owners tolerate minor cracking (as was the case prior to 1971 before mortgage lenders and insurers covered subsidence as an insured risk) the trees could be allowed again, to redevelop their larger canopies quickly if the worst case scenarios for climate change were realised in the 2050’s to 2080’s.
Thereby providing a quick response to the need for increased canopy cover, restoring them at just the moment in time when they will be needed most and without the time lag and challenge of replanting from scratch.
There is a wealth of technical advice and information available across the relevant sectors that provide practical and comparatively low-cost methods of achieving the objective of permitting highly valued trees to exist within a well maintained and modern inclusive highway.
These solutions will require effective communication to Elected Members and Managers, as well as to the public, as to why they are suitable and necessary for achieving good highway tree management practice.
