They spoke out after last week's launch of the Adaptation Strategy, which calls for a massive urban greening programme to beat floods, heatwaves and droughts.
According to Johnson, well over one million people, nearly 500,000 properties and 10 hospitals were at flood risk and efforts to make "drastic reductions" to global emissions had not worked.
"This document is one of the first comprehensive climate-change adaptation strategies produced by any major city worldwide," he said.
The report insisted greenery should become a front-line defence against urban heat-island effects through the use of trees, green roofs and green spaces.
A list of "priority actions" included leading a "citywide urban greening programme" to absorb and retain rain and to cool the city.
Tree consultant Jeremy Barrell said he would work with the Arboricultural Association on a response to Johnson's document and they would seek to play an advisory role.
"We are very keen to engage with London and the Government, but my main worry is there's always too much focus on numbers.
"Why plant one million trees if 900,000 of them die? It's about establishment. There are appalling tree-planting failures in London, as contracts focus on numbers not survival rates.
"They haven't identified why trees are being lost and how to prevent it. However, when people of the calibre of mayors talk trees to stay in office, it is food for thought."
Barcham Trees sales director Keith Sacre, who has studied trees in London, said the strategy was "commendable" and he would advise if asked by the mayor. "It's not just about numbers of trees, but their longevity and the management they need," he said.
The loss of one Victorian plane would need 60 young trees to compensate for canopy loss, he said. Success of the mayor's plans hinged on management budgets.
"There's this idea that a tree is established after two years, but it's more like five to seven years before it's thriving independently."
Friends of the Earth London campaigner Jenny Bates said: "Johnson has so far failed to explain how he will cut carbon dioxide emissions. He must provide a comprehensive action plan for reducing emissions that includes ways to make it cheaper and easier for Londoners to go green."
- Lead citywide urban greening programme to absorb and retain rainwater.
- Manage heat through street trees, green roofs and green spaces.
- Promote and facilitate compulstory water metering in all developments.
- Provide design guidance on developments to prevent overheating.
- Encourage rainwater harvesting and grey water recycling in new buildings.