That was because the biodiversity benefit of the land outweighed its lost food production, senior scientist Dr John Dymond, of Palmerston North, said.
NZ had another 700,000ha of shrub land suitable for protection from clearance, he said. This land had a high carbon sequestration over stock-carrying-capacity ratio.
Dymond’s research team of Landcare Research staff is looking at a way to identify land suitable for ecosystem service offsets.
Ecosystem services are the benefits humans receive from nature, including clear air, pollination of crops, and appealing landscapes.
In the 18 years from 1990 and 2008 about 51,000ha of indigenous forest had been cleared, mostly on private land, he said.
This loss occurred despite programmes designed to encourage protection and restoration. The Environment Court now accepted biodiversity offsets, which should help balance the loss of indigenous forest with restoration elsewhere, he said.
Protection on private land involving covenants meant owners forgo the right to convert forest to a different land use in future, which was an opportunity cost to the landowner.
Restoration by de-stocking and retirement was also a cost through loss of agricultural production.
Trade-offs between environmental benefits and agricultural had to be made when looking for opportunities to protect and restore indigenous forest, Dymond said.
His team was not suggesting new forests can compensate completely for the loss of old forests elsewhere; rather it was identifying the best areas if compensation was thought necessary, he said.
Last year a study by fellow scientist Fiona Carswell found significant increases in carbon stocks and plant biodiversity after 100 years of succession in lowland forests.
Dymond and his team created national maps of agricultural production using land-use maps and stock-carrying-capacity maps for each district across the country.
It multiplied stock numbers by regional averages of production and converted production to dollar values/ha.
“Restoration and protection opportunities were identified by finding land with environmental benefit over cost ratios greater than given thresholds,” Dymond said.
“These thresholds were chosen so that the benefit is about the same as the loss in farm profit to the farmer and represents the point at which land use might take place, if there was a market for ecosystem services.”
He said there were limitations to the approach his team used, such as assuming all pasture areas could actually revert to indigenous forest. It also omitted the cost of pest control in the analysis.
Both islands had large areas of shrub land, with 1.1m ha in the North Island and 0.9m ha in the South Island.
The research was published recently in the Journal of the Royal Society.