The conservation charity cares for around 500 tree avenues such as the Spanish Chestnuts at Croft Castle in Herefordshire and Beech Avenue at the Kingston Lacy estate in Dorset.
The avenues, which cover hundreds of miles, were planted to frame views and have since formed countryside wildlife corridors and habitats for fungi, beetles, bats and lichens.
The survey findings will help the organisation to make informed decisions on where to prioritise funding.
National Trust ancient tree adviser Brian Muelaner explained that the survey would also help to identify care strategies.
"Many of our tree avenues are under threat from different diseases, so this survey will pinpoint where the issues are and help us decide how we can address them," he said. "We have an extensive tree-safety management programme, but avenues as a whole are not currently taken into consideration."