Researchers fear hopes of a green Christchurch arising out of the rubble may be stymied for economic reasons.

Dr Justin Morgenroth from the forestry school at Canterbury University and Tony Armstrong from Christchurch City Council have studied the impact of earthquakes on the city's urban forests.

In a study published in the journal Urban Forestry & Urban Greening the authors said the earthquakes had provided an opportunity to improve the urban area for future generations.

But the human tendency to want to quickly return to pre-disaster conditions could stymie those plans. Arguably the most important factor for determining the future greenspace of Christchurch was the impact of land ownership on land use.

That had been highlighted by other studies which contrasted the urban greening of Tokyo after it was firebombed in World War Two, with that in Hiroshima after the atomic bombing. Private property ownership in Tokyo had prevented ambitious plans for turning large areas into public greenspace, while Hiroshima had expanded upon standards set out in its plan.

Christchurch was in a position to learn from past successes and failures, the study said.

Arguably, the single largest opportunity was in the 350 hectare of land in the red zone, comprising about 5000 properties, mainly along the Avon River.

The importance of the government's offer to buy all those properties must not be overlooked, the study said.

The government was in a unique position to develop public parks or greenspace without the hindrance of private land ownership. Community groups and residents had called for a riverside park connecting the city centre to the sea.

"Though the public, on the whole, have asked that their city be a green city, the economic disincentives of such a decision may compel decision makers to rule against it."

The study authors said thousands of trees in Christchurch were scheduled for removal following the quakes.

By June 2011, 384 had been removed from city parks and along the Avon and Heathcote River corridors. Figures for tree removals from other public land and private land were unknown.

Tree loss had been caused by mass soil movement, soil liquefaction, rockfalls, and land slips. In some areas local canopy cover had been dramatically reduced.

It was not known how many trees were in Christchurch, but an inventory of the 792 public parks found about 65,000 trees, with the city's urban forester estimating a further 65,000 to 67,000 street trees. Private tree numbers were not known but typically exceeded public numbers by between 2.5 and 10 times. Urban tree canopy cover was estimated at 22 per cent.

While many of the benefits of urban forests were well understood, trees and greenspaces provided additional benefits during earthquakes and in the aftermath, the study said.

Planted trees in the hills surrounding the city had limited the damage caused by rockfall to property and people downhill.

Spreading root systems were believed to have bound soil together, preventing or at least minimizing soil slips, while large rocks and boulders were found at rest at the foot of shelterbelts.

Homes in the suburb of Morgans Valley were protected by a shelterbelt made up of pine, macrocarpa, eucalyptus and cedar, the study said.

The shelterbelt curved around Morgans Valley for 1km between the houses and rock cliffs above, ranging from 25-85 metres wide.

"The force exerted by rockfalls snapped many individual trees, but the wide shelterbelt succeeded where fences (constructed uphill from the shelterbelt) failed."

Many rocks were stopped by the fence, but others bounced over it or tore through it. The vast majority were slowed and stopped by the shelterbelt.

- © Fairfax NZ News