Fresh concerns have been raised about the condition of trees along the Norwich Northern Distributor Road - with a replanting programme scaled back because of uncertainty over whether more of them have died.
When the £205m road was built, 6,000 trees were chopped down, but Norfolk County Council pledged to plant five for each which was removed - 30,000.
However, in 2019, the council admitted 3,000 newly planted trees and shrubs had died and two years later, the council said it needed to replant more than 500 more which had perished.
Council bosses have said it is not clear whether even more trees along the road, also known as the Broadland Northway, have died over the past year. But a replanting programme has been minimised while investigations are carried out.
A spokesperson said: "It was noticeable that during the drought and towards the end of the summer many of the trees dropped their leaves early – not just on Broadland Northway but generally.
"The advice from our consultant was that this is partly a defence mechanism and it is to be hoped that many of them will survive and come into leaf in the spring.
"With this in mind, it was decided to minimise replanting this current planting season (2022/23), just concentrating on particular areas, and carry out a further survey in the spring 2023 inspection.
"This will confirm the number of trees requiring replanting in the next planting season, 2023/24."
The spokesperson added most trees and shrubs planted in 2021/22 were "growing reasonably well and becoming established, despite the prolonged high temperatures in the summer of 2022".
Liberal Democrat county councillor Steffan Aquarone said: "It's extremely frustrating to hear the county council can't tell how many trees on the NDR have died.
"Tree planting is an important part of carbon offsetting, but it needs to be properly managed or it's a waste of effort."
And Mr Aquarone said it did not bode well for the council's plans for the £251m Norwich Western Link.
He said: "This administration can't afford to manage the roads it has got, let alone take on a new one by creating a concrete corridor through a conservation area to the west of Norwich."
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