A heritage advocate is questioning why one of the oldest trees in Christchurch was cut down, rather than incorporated into the city's earthquake memorial.

Nine trees were removed along the south bank of the Avon River to make room for the memorial, which the Canterbury Earthquake Recovery Authority says were a mix of ages.

Professional forester and heritage tree advocate Mark Belton said at least one was 150 years old.

"A tree of that nature is notable because of it's age and it is part of the memory of the city so it is ironic to cut it down to create a memorial," he said.

"Those trees are not common, and are precious, and I just wonder about how much thought actually went into the design and setting of the memorial."

Mr Belton said the tree should have been designed into the memorial, rather than cut out.

The Canterbury Earthquake Recovery Authority said the earthquake memorial design chosen required less trees to be removed than other options it considered.

A spokesperson for CERA said six Red Maples will be planted in place of the other trees, and are a crucial component of the overall design that's received positive feedback from the bereaved families and seriously injured.

The memorial wall design did not require trees to be removed on the north bank side, as many other designs did.