Arboricultural Consultant

P.G. Biddle

Dr P.G. Biddle, O.B.E., M.A., D.Phil., F.Arbor.A.
Registered Consultant of the Arboricultural Association
Honorary Fellow, Insitute of Chartered Foresters

Hortlink Project 212 Controlling water use of trees to alleviate subsidence risk

This detailed research project looked at the effects of crown reduction and crown thinning on the water uptake of wild cherry and London plane trees, using a whole variety of scientific techniques. 50% of the funding was provided by the government through Hortlink, with the remaining 50% from industry-based partners, primarily the Association of British Insurers. The main work was undertaken by Dr Neil Hipps at East Malling Research. Giles Biddle was one of the industry-based consultants.

The 114 page Final Report provides full details of the project and its results.  The following is taken from the Executive Summary:

Practical implications.
• Trees recovered their canopy leaf areas to pre-pruning amounts very quickly (1-3 years) following crown-reduction to normal industry standards.
• The re-growth after crown-reduction produced trees with greater leaf area density because they had larger leaves more closely packed together within a smaller crown volume compared to non-pruned trees.
• Crown-thinning reduced the leaf area density, and generally the trees took longer to recover their canopy leaf area than for crown- reduction.
• Total tree water use (transpiration) was reduced by crown-reduction and unaffected by crown -thinning in the year of pruning.
• Crown-reduction reduced soil drying by trees in the year of pruning, but the effects were generally small asnd disappeared within the following season, unless the reduction was severe, in which case the effects were larger and persisted for up to two years.
• Crown-thinning did not reduce soil drying.

Recommendations.
• For practical soil moisture conservation, severe crown-reduction (70 - 90% of crown volume) would have to be applied. Reduction of up to 50% crown volume is not consistently effective for decreasing soil drying.
• To ensure a continued decrease in canopy leaf area and maximise the period of soil moisture conservation, crown reductions should be repeated on a regular managed cycle with an interval based on monitoring re-growth.
• Crown-thinning is not an effective method to control soil drying by trees.

Tree management implications.
If severe crown reduction is required to alleviate subsidence risk, those trees which pose a potential risk must be identified so they can be treated. There are approximately 100 million trees in the urban environment. Of these, a large, but undefined, proportion is in sufficient proximity to a building to pose a perceived risk of damage. However, even in a drought year, the number of actual cases of subsidence is only about 50,000. The risk of a tree causing subsidence damage (which is related to species, foundation depth and soil type) may therefore be less than 1%. If one could identify this 1% with any reasonable accuracy, they could be pruned accordingly. However, attempts in the past to develop methods of subsidence risk assessmsnt have not been successful. The Arboricultural Association method of "Subsidence Risk Assessment" was withdrawn, as it was considered to be ineffective. Royal and Sun Alliance's recent efforts to develop a statistically based model TreeRAT (Tree Risk Assessment Tool) have not been taken beyond an initial prototype stage.

If trees that pose a risk cannot be identified, then one alternative is to treat all trees, regardless of the risk they pose. The environmental consequences of this would be catastrophic; nor could there be economic justification for any such policy as the cost of recurrent pruning would far outweigh alternative methods of remediation. For example, even pruning 1% of the tree popluation could cost anywhere between £50-100 million. Thus, pruning universally is unlikely to be a viable method of alleviating subsidence risk.

Some trees, for instance many of the London plane trees in city streets, are pruned on a regular basis as part of their normal management. This project has indicated theat there is justification for modifying their pruning regime to reduce the risk of subsidence by reducing rather than thinning the crowns and using techniques which produce compact crowns.