It helps not to be allergic to maths when grappling with the idea of New Zealand planting 1 billion trees in the next 10 years.
Those who have absorbed the Government's grand plan for 1 billion trees by 2028 tend to make their case with a blitzkrieg of big numbers and lots of division and multiplication.
Forest Owners' Association president Peter Weir is among the maths gymnasts, but mercifully not before he cuts to the chase: "We've done it before".
"We did more than that [in the 1990s]," says Weir.
So there it is. An expert's answer to the most frequently asked question about Forestry Minister Shane Jones' grand vision – is planting 1 billion trees do-able?
Now for the maths.
Half of the billion-tree target – 500 million trees over 10 years - is 50 million trees a year.
That's about the replanting rate by commercial foresters now, so 1 billion trees becomes a game of two halves, says Weir.
"One billion trees is 100 million trees a year and half of that is replanting. So 50 million a year is business as usual and another 50 million is new planting. That means 50,000ha of new planting a year will get them there."
To put that in context, an estimated 1.7m ha is now in production forest.
What we won't see is commercial foresters jumping in to help get the other 500 million trees in the ground in the next decade – no matter how many seductive grants or loans the Government offers.
"We had this strategy [1 billion trees] in the 1990s but land prices were different then," says Weir. "They're now out of reach of commercial forestry owners – no large companies have got unplanted land in the land bank."
That leaves farmers, Māori landowners, Crown Forestry, local councils and Joe Public to plant the rest.
Here we go with the maths again.
"You have to ask what are the things that will make a farmer or owner of unplanted land suitable for afforestation plant it," says Weir.
"One is telling those farmers they are coming into the ETS (emissions trading scheme) and by planting they can grow their own [carbon] offsets so the farm is carbon neutral.
"You don't have to motivate all that many North Island hill country sheep and beef farmers to be planting 50 million trees a year. That's actually only 50,000ha at 1000 trees per hectare a year.
"If you have 50 farmers planting 1000ha, or probably more realistically, 500 farmers planting 100ha, you've done it."
Then there are hundreds of thousands of hectares of unplanted Māori-owned land, though no-one seems to be able to put a figure on the exact area.
Still need persuading? The Ministry for Primary Industries (MPI) has a planting progress clock on its website. It shows that between July 9 and July 23, 10 million trees were planted, taking the total since the 1 billion tree programme was announced to 49.2 million trees, 13 per cent of them native species.
Nearly 67.5 million tree seedlings have been sold since the programme began.
The Government has directly funded 6.5 million seedlings committed for planting this year.
For the purposes of the count, a "tree" is defined as a woody perennial plant that will grow to at least 5m.
The next most-asked question: why plant 1 billion trees?
The short answer is because trees absorb carbon dioxide (CO2) from the atmosphere and turn it into wood, which holds carbon for as much as hundreds of years. Trees absorb CO2, protect the soil, improve water quality and create wildlife habitat.
The long answer is because New Zealand has committed to reduce greenhouse gas levels which contribute to climate change. It has three reduction targets – for 2020, 2030 and 2050.
The ETS, now under review because it is seen as not up to the job of reducing New Zealand's net carbon emissions to zero within 32 years, is currently the main tool to help cut emissions. Trees play a critical part in achieving those targets, and changes to the ETS can be expected to provide incentives to plant more.
Complementing the billion-tree programme, a new business unit was this year established within MPI, called Te Uru Rākau Forestry. The unit is to "develop a comprehensive plan" and administer planting grants and incentives. Crown Forestry, created in the 1990s to manage Crown commercial forestry leases on Māori land, is a commercial trading organisation within Te Uru Rakau.
But the billion-tree programme is about much more than offsetting emissions, says Dr Warren Parker, chairman of the forestry group advising Shane Jones.
"One of the strengths of forestry is multi-functionality which generates a range of benefits," says the former chief executive of Scion, the Crown research institute for forestry, and former head of Landcare Research.
Planting trees, Parker says, doesn't just fight climate change by storing carbon, but also helps stabilise land against erosion. He's the first to note the irony here, after tonnes of felled trees recently washed down from East Coast forestry plots, damaging farms and rivers. Parker says many of those trees were planted to stabilise slopes after Cyclone Bola in 1988, when whole hillsides collapsed.
"We have a significant commitment with the Paris 21 [agreement] and the practical and sensible way to make progress on that and generate co-benefits is to plant trees."
(The Paris Agreement aims to limit global temperature increase to 1.5C-2C above pre-industrial levels. Under the agreement, New Zealand's target is to reduce greenhouse gas emissions by 30 per cent below 2005 levels by 2030).
"The second area of benefit is around water quality," says Parker. "Planting trees can reduce runoff of sediment and phosphate runoff. It has less impact on nitrogen, but generally can improve habitats around watercourses and makes a better habitat for biodiversity."
The programme isn't just about planting swathes of radiata pine.
"It's more sophisticated than that. Figures are still developing, but up to 20 per cent of planting is expected to be native trees."
Manuka for the high-end honey export market is likely to get a big push.
As New Zealand and the world move to a renewable bio-economy, trees become a very important feedstock, says Parker.
"Everything you can make from oil, essentially you can make from a tree. Globally there's a shortfall in trees and wood fibre and good old radiata has a real strength there in that its fibres tend to be long. That suits the making of bio-materials, replacements for plastic. It also means we can make componentry for machinery, cars and automobiles.
"The other big development is that more buildings will receive wood. Steel and concrete, which tend to be associated with higher emissions, will be replaced. You're starting to see that emerge with prefabrication of buildings and engineered wood products."
"This transition from oil over the next 30 to 40 years is going to call upon more tree stock.
"So in the short term, trees will help restore carbon and in the long term they will help underpin a bio-economy. All packaging, for example, is moving to wood-derived renewable cardboard and paper-strengthened materials.
"This will help to position New Zealand in the long term to have a secure supply of wood."
But wait, there's more.
Forestry could also help regional development, creating new businesses and jobs.
"We need to make sure we put the right tree in the right place with this programme, but what it does do is create jobs both in the establishment and silviculture of forests and later on in processing," says Parker.
The billion-tree programme also has a nice synergy with New Zealand's goal of being predator-free by 2050, he says. The job of eradicating the hares and rabbits which eat young trees would help smooth out the ups and downs of forestry work.
Parker is not suggesting a re-run of the 1990s.
"When planting exotic species like radiata, we need to be thinking about where's the market for this tree. Generally … you need to be within 100km of a port or sawmill for it to be viable. The regime for your forest will reflect the final market."
So, planting radiata on very steep land won't always be the answer.
"This is a real opportunity to have designed forests," says Parker. To make sure riparian margins have the right trees and maybe land with steep sidings that are uneconomic and difficult to harvest should be established with permanent species.
"We could also look strategically across lowlands where we have lost a lot of biodiversity,
and work with parties like the QEII Trust and others to expand planting of rare native species or create habitats to encourage birdlife and other flora and fauna."
The new programme will also fill a looming gap in the log supply, says Weir.
"We're two-thirds up that wall of wood now. We've gone in the last few years from 20 million tonnes a year to 30 million tonnes of harvest. Within five years we'll peak out at 40 million then go back down into a hole – because of that burst of 1990s planting."
Parker says an important lesson from the 90s is not to plant all the trees in one or two years. "On the East Coast, we've ended up with pretty much all of those trees coming to harvest at the same time."
For big commercial forestry companies, says Weir, today's carbon prices are too low to compensate for land prices, and allow them to be active participants in the new programme.
"Yes, we're supportive of it in principle but some other parameters would need to change. The carbon price lifts the internal rate of return in forest investment nicely and ... $200 a tonne [of carbon] would absolutely get a lot of planting going, but the current $20 a tonne – and the price in the ETS is capped at $25 a tonne at the moment – is not sufficient top-up to pay the current asking prices for land.
"Farm foresters and farmers already have the land, as do iwi, so for them this is looking fairly attractive," Weir says.
But there are challenges ahead. Māori landowners, Weir says, have typically lacked the capital to invest in large-scale forestry.
"Another issue for them is the quite high levels of biodiversity on their land … it's all regenerating into native forest because it's not being actively grazed. So whether under the [Resource Management Act] they can actually get rid of the native forest on it and plant is looking highly questionable to me – particularly when the Government has proposed a national biodiversity statement."
For their part, farmers are jittery.
Federated Farmers vice president Andrew Hoggard says hill country sheep and beef farmers fear their properties will be turned into forests, a concern fuelled by the Government creating a loophole in its anti-foreign investor policy for forestry investment.
There is fear about the social impact of creeping forestry dominance on rural communities and schools, says Hoggard.
But he supports the idea of "the right tree for the right place", as long as that can be achieved without removing livestock, and concedes that some farm gullies cleared in the 90s "should have been left in trees".
Shane Jones acknowledges farmers' anxiety "that forestry is going to drive them out of business", though he's mystified at how they came to that conclusion.
"Professionally, my job is to work with my team to defang those misapprehensions," he says.
Jones says he also has his work cut out reassuring some "deeply fearful" Māori landowners.
Many of those landowners now harvesting trees on land returned from the Crown "got fitted into arrangements that have not left them in a stable financial situation", without clear ownership of the trees, he says.
As for whether existing infrastructure can support a wood bonanza, no-one seems perturbed.
Weir says roads are already being "stress-tested" and have done well with the exception of those in "poorer" district council areas like Wairoa, Gisborne and Ruapehu.
Port of Tauranga, the country's main timber export port, is comfortable that its growth plans will meet future requirements, says chief executive Mark Cairns. Likewise for Northland's Northport.
As for keeping the value of increased forestry onshore this time, Parker says in fact, only half of logs are exported raw.
"Where mills have got scale or have put in place modernisation, they are competitive. The key for them is to have appropriate source within 100km.
"The evolution to engineered wood products does lend itself to more value-added being onshore. But this an area that requires more research and development."