The National Capital Authority has sought to remove 38 Lombardy poplars from the library's forecourt following an arborist's assessment their condition was not recoverable.

The NCA planned to replace the Populus nigra 'Italica' with the same species, however, the plant is prohibited under ACT law.

The fastigiate English oak has now been proposed to replace the trees, chosen for their likeness in shape, size and leaf colour to the poplar.

NCA's landscape development manager Duncan MacLennan said the English oak has also been proven to grow well in the Canberra climate.

"The poplars have declined so much in recent years and unfortunately we can't replace like with like because they're prohibited plants," he said.

"They're banned because when they're growing near water they proliferate by suckering from their roots."

He said the growth pattern of the "environmental weed" resulted in the blockage of water courses.

Mr MacLennan said assessing a suitable alternative had been a careful process, with both characteristics and climate suitability taken into consideration.

"The poplars are just over 50 years old and they're beautiful trees but they tend to grow quickly and then decline quickly as well.

"In Canberra's drying, warming climate they're becoming less and less suitable. The oaks have been assessed as the most suitable alternative because they bring the same heritage values - or as close as possible heritage values - as the poplars," he said.

Mr MacLennan said the NCA had identified a number of trees in Canberra which were no longer sustainable due to the warming climate.

"In most areas we'll try to replace like with like but where a species has become unsustainable and is just not going to survive in the future climate of Canberra, we'll try and select something that is more suited," he said.

The NCA has also proposed a reduction of the number of trees in each of National Library's four rows. The aim of which was to ensure they grow to maturity quicker and compete with each other less, while retaining the symmetrical lines synonymous with the NLA Forecourt.

"It's a beautiful part of the Parliamentary zone and it's looking sad," Mr MacLennan said. We want it to compliment the building and look as beautiful as the building itself," he said.

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Mr MacLennan said the replacement of all the trees simultaneously will allow the NCA to replace the topsoil and irrigation system as well as install a drainage system for the new trees.

The work was expected to take place in the second half of the year over a 12 week period.

Mr MacLennan said there would be no disruption to pedestrian access. Disruption to traffic was still being assessed, he said.

The NCA has referred the proposal to the Department of Agriculture Water and the Environment. The NCA will conduct a formal consultation process once the Department has completed its assessment.

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