Its researchers say the trees' worsening health is due mainly to attacks by winter moths and to a mysterious and sometimes lethal disease called oak dieback.
Each summer the commission surveys about 9,000 trees of five different species in sample plots around the country, assessing the density of leaves in their crowns.
Oak was the sickest of the five species - it and Scots pine have been in decline since 1990. More than 11 per cent of oaks were found to have less than half the normal foliage in their crowns.
But Norway spruce and beech, whose condition also worsened between 1990 and 1991, showed signs of recovery last year.
The commission said natural causes such as insect attacks, drought and storms were largely to blame for the changes in tree health, rather than air pollution. But it said the condition was 'probably temporary and many should return to full health'.