Johnson said he fears for the London plane but "gingko is the toughest tree and could replace it, and Lagerstroemia is becoming an option for a street tree", while sycamore is being recommended to replace ash.
"We need to look more broadly," he added. "In the city the problem is not about lots of street trees but about getting trees to somewhere near their mature size with restrictive pit sizes and utilities in pavements."
On the threat of pests and diseases, Johnson, who was due to speak at last week's Landscape Show, said everyone knows about ash dieback but most people do not know about emerald ash borer (EAB), which has reached Ukraine and could kill even disease-resistant ash.
With limited resources to inspect plant passports, he said: "We have to develop an island mentality again because if we do manage to find a strain of common ash that's resistant to Chalara the chances are they are going to be wiped out by EAB."
The "industry needs to wake up" to threats such as EAB and Ceratocystis platani, which has reached Paris, he added. Horse chestnuts are also having a tough autumn because of leaf miners that have thrived in this year's warm weather. Trees are producing large numbers of conkers in an effort to reproduce as they are being weakened by the miner and killed by canker. "If people don't clear leaf litter, chances are it's going to overwinter and explode next year. It's one of the worst years yet for it."
Johnson said all imported trees should be held on UK nurseries for a year before being planted.