Subsidence News and Updates in August 1999 referred to the start of this research project, which is being undertaken at Horticulture Research International, funded by a consortium headed by the Association of British Insurers and by a grant from Department of the Environment Transport and the Regions under the Horticulture Link Programme. Giles Biddle is one of the collaborating sponsors, attending the regular meetings to review progress.

The aim of the project is to improve the understanding of how large isolated amenity trees use water, and to determine sustainable and economic ways to reduce tree water use. Two methods are being evaluated - canopy manipulation and root restriction.

The work on root restriction only started in 2000, and no results are available yet.

Canopy manipulation has involved two pruning techniques, crown reduction and crown thinning [Tree root damage to buildings, page 284], in all experiments reducing the leaf area by 30% compared to the controls. In 1999 replicated experiments were started on cherry trees, with the treatment repeated in 2000 on some of these trees whilst others were left to recover their leaf area. Further experiments were started on cherry in 2000, and also on mature plane trees. A variety of techniques are being used, including sap flow gauges, neutron probe (to measure soil moisture content [Volume 2]), and measurements of leaf water potential [p.15], and stomatal conductance [p.14]. These measurements on the trees are being correlated with data on the environmental conditions which influence rates of evapotranspiration [p.121], and with rates of evaporation from artificial leaves placed in the canopies.

Details of the results are confidential and cannot be released, except to the extent that preliminary information from 1999 has been released in executive summaries and during seminars.

These results have shown a statistically significant reduction in water use by the trees which have been crown reduced, but no benefit from crown thinning. The reduced water use allowed the soil beneath crown reduced trees to remain significantly wetter than under the thinned or control trees, and to return to field capacity earlier in the winter. The difference between reduction and thinning is probably explained by differences in the layer of moist air which surrounds the leaf (called the boundary layer). The crowns of thinned trees are more open so that their leaves are exposed with a thin boundary layer allowing rapid water loss, whereas reduced trees have a small compact crown which shelters the leaves, thereby creating a thicker boundary layer and reducing water loss through the stomata.

It is emphasised that these results are preliminary and may not be representative of other trees or other seasons. They do, however, entirely support the opinions expressed in Chapter 17 of Tree root damage to buildings [p 281 - 289].

Although the results are statistically significant, their practical significance appears limited. Thus, the peak soil moisture deficits were 112mm under the reduced trees and 135mm under the thinned and control trees. This 83% reduction in the soil moisture deficit was only being achieved by a severe crown reduction, and would be of little benefit if a tree was already causing unacceptable foundation movement.