Eloise Gibson is Newsroom's environment and science editor. She's written for the New Zealand Herald, Stuff.co.nz, The Listener, and BBC Future.com. Twitter: @eloise_gibson.

environment

Number of natives under one billion trees anyone’s guess

How many of the one billion trees planted in the next decade will be native species? Government tree planting agency Te Uru Rakau has clarified that it can't hazard an estimate.

The Government’s tree planting agency, Te Uru Rakau, says it can’t estimate what proportion of the one billion trees programme will be native species, saying a previous figure it gave to Newsroom was meant to be purely "illustrative”.

The illustrative figure was used to calculate the estimated climate benefit from the tree scheme, which Te Uru Rakau has put at 384 million tonnes of carbon dioxide over the trees’ lifetimes.

The Government plans to foster planting of half a billion new trees over a decade, using a mixture of grants, education and co-funding forestry projects with other entities.

The one billion figure that gives the programme its catchy name was reached by counting another half a billion plantation pine trees in existing pine forests. These likely would have been replanted anyway by commercial foresters after harvesting, and these trees won’t receive any government assistance.

Te Uru Rakau has a goal of funding two-thirds of native trees, but the agency has clarified that this target applies to only a small proportion of the trees: an estimated 60 million that could be paid for by $120 million in grants to landowners, from money set aside from the Provincial Growth Fund.

If the two-thirds target is met, that means roughly 40 million of the trees planted with direct landowner grants will be native species - roughly 4 percent of the billion trees programme, or 8 percent of the half a billion genuinely new trees that will arise during the programme.

That's just one part of the mix. As for the overall proportions of new trees, the agency says that’s largely up to landowners who will choose what trees they want. It expects people to continue voluntarily planting natives with no subsidies, but the agency says it's impossible to forecast how many indigenous trees we will end up with.

Te Uru Rakau previously told Newsroom it had estimated the carbon sequestration benefits of the billion trees scheme (by assuming the total mix of species) would be 70 percent exotic and 30 percent indigenous, equating to 384 million tonnes of CO2 sequestered over the forests’ lifetimes.

The mix of native and exotic species is important for working out the carbon sequestration, because pine absorbs more carbon, by forestry scientists’ measurements.

That’s despite pine usually being harvested and replanted about every 27 years, which allows native forest to slowly catch up with the pine over more than a century. (These greenhouse estimates don’t count everything - for example, they exclude how much heat the forests absorb or reflect, and the carbon cost of harvesting machinery and trucking, something native forest advocates argue should be studied and factored in properly).

To work out how much carbon the scheme would sequester, the agency assumed the average carbon stock of pine was reached at age 21, while indigenous forests’ maximum storage is reached at maturity (after 200 or so years). Te Uru Rakau also assumed indigenous forest had twice as many trees per hectare. Despite that, it put pine’s average carbon sequestration at 650 tonnes per hectare compared to indigenous forests’ 257 tonnes.

When Newsroom sought to clarify the mix of species used to reach that figure, the agency said it could not really estimate what proportion of trees planted would be indigenous, given the uncertainties.

Sucking in carbon is not the planting programme's only stated goal: it is meant to create employment, mitigate climate change, support Māori values and aspirations, and protect the environment, as well.

To try to level the playing field between native and exotic trees, landowners such as farmers are paid for planting native trees at a higher rate than pines and other exotics, reflecting the higher costs of buying and nurturing indigenous species.

Some planting programmes and researchers are also looking at low-cost ways to establish more indigenous forest, such as helping it self-regenerate, to make natives a cheaper option.

While pine would earn more carbon credits, much of the stream-side planting and planting on marginal land that is already happening on farms uses indigenous species because that’s what the landowner wants on their property.

Te Uru Rakau expects people would have planted about 10 million native trees during the programme even without any incentives.

How that number will change remains to be seen.

So far, Te Uru Rakau says 149 million trees have been planted since the scheme's announcement, 24 million of them directly government-funded. The mix so far is 12 percent native to 88 percent exotic, the agency says.

Here’s what Te Uru Rakau said when we asked them to clarify what mix of species they expected from the grants and the rest of the billion trees programme:

“Planting carried out through the one billion trees fund (direct grants to landowners and partnerships that are co-funded) is the only part of the entire one billion trees programme where the Government can directly influence native planting. It is estimated 60 million trees will be planted through the fund, of which the aim is for two-thirds natives.

“The earlier response from TUR that mentioned a 70/30 exotic/native split across the entire programme was an estimate given by way of an example to illustrate the complexities of sequestration. It is, in fact, not possible to quantify what the final exotic/native percentage split might be over the full programme out to 2028. There are so many dependencies and we can’t provide a definitive number.”

As for how the “billion” was reached, Te Uru Rakau said:

“You are right that at least 500 million anticipated plantings will be through projected replanting of exotic forests after harvest by the commercial forestry sector. Private landowners, government agencies, NGOs, iwi, regional councils, nurseries and the private sector are the key to planting the other 500 million trees.

“The Government has launched the Crown Forestry joint venture programme and the IBT grants fund to kick-start this new planting. These will establish up to 24 million and 60 million trees respectively. It is more difficult to estimate the indigenous/exotic split for these plantings as it depends on the objectives of the landowner. As well, changes to the Emissions Trading Scheme and other forestry settings and regulatory changes are expected to drive an increase in new tree planting by private landowners once they are in place from 2021.

“We estimate around 10 million trees from business as usual planting will be native and continue through the life of the one billion trees programme. We also anticipate some of the new planting that will be driven through the Emissions Trading Scheme will be native, but this is more difficult to quantify.”