From 1993 onwards I directed a programme of diagnostic and performance field observations, which were conducted by the soil mechanics laboratory at Imperial College on behalf of London Underground.
This involved the Jubilee line near Kingsbury. One of the things done was to develop a miniature piezometer system capable of measuring suction and positive pressure, which could be installed by hand between the trees on vegetated slopes. This was used extensively in site investigations.
The piezometers on seven sites with trees were reactivated in 2000 when preliminary measurements indicated that it would be an unusually wet winter. The pore pressures remained much lower than they would have been without trees.
A lot is now known about the effect of vegetation on clay slopes. The presence of trees will reduce pore pressures in a rather irregular manner and their removal is likely to allow them to increase with time to values greater than they had while trees were present. The removal of trees will have a destabilising effect leading towards slope failure, not a beneficial one as reported in you article. It is likely to lead to an increase in slope failure.
Prof PR Vaughan (F), 101 Angel Street, Hadleigh, Ipswich, Suffolk IP7 5DE