• David Hall is senior researcher at The Policy Observatory, AUT, and author of the Our Forest Future report for Pure Advantage in 2016 which advocated 1.3 million ha of new forest in New Zealand. With support from Foundation North's GIFT fund, he is developing (with Sam Lindsay) an environmental impact bond to fund permanent native forest.
What is a tree worth? This is not a rhetorical question, not when the new Government is promising to plant one billion new trees over the next 10 years.
Given the scale of investment and potential impact, this question deserves a serious answer. Our future forest could be done well, contributing to land resilience and long-term prosperity. But it could also be done poorly, with the opposite effect.
Fortunately, there is already a vast scientific literature on the environmental, social and economic benefits of trees.
Trees can reduce terrestrial erosion and soil loss which is no small thing for a country that loses an estimated 192 million tonnes of soil into its waterways every year.
Trees also remove carbon dioxide from the atmosphere. A recent international study led by the Nature Conservancy estimated increased carbon storage in forests, grasslands and wetlands could provide up to 37 per cent of the mitigation needed between now and 2030 to prevent global temperatures rising over 2C.
Relevantly for Auckland, trees also intercept rainwater. As a rule of thumb, about one-third of rainwater is captured by tree canopies but this varies significantly by tree species, age and leaf cover. Root systems absorb further water while also filtering out pollutants, including the heavy metals that wash off our roads into our waterways.
This has economic implications.
Auckland's "green infrastructure" of trees and vegetation reduces the need and urgency for more expensive "grey infrastructure" such as drainage pipes and retention ponds.
Conversely, the current spate of tree removals throughout Auckland, unleashed by changes to the Resource Management Act in 2012, is heightening the stress that heavy rainfall events put upon the wastewater system.
So protecting, maintaining and expanding our forests makes good practical sense. The challenge is how to quantify the expected return-on-investment, given the complexities involved.
There are tools available. The i-Tree programme which is used throughout North America, Australia and Britain puts a price tag on urban forest benefits. A preliminary analysis of Auckland's Wynyard Quarter found each tree delivered $2.60 in annual air pollution removal benefits. Further analysis would augment this valuation.
At the national level, new forest avoids future costs by reducing the volume of carbon credits that New Zealand needs to purchase to meet its international emissions targets.
The Ministry for the Environment estimates that meeting our 2030 target could cost between $14 billion and $36b. Worse, there are no guarantees international carbon markets will even exist then. Creating forest carbon sinks now protects our future against these forward liabilities.
In other words, the fiscal case for planting trees is consistent with the previous National Government's actuarial approach to social investment.
In urban settings especially, trees and green spaces improve community wellbeing, thereby reducing public health costs. There is a prudent fiscal case for the public sector to intervene today to reduce expenditure in future decades.
But it also needs to be recognised that not all benefits can be quantified.
Gary Taylor, chief executive of the Environmental Defence Society, recently argued in this newspaper that native trees should feature prominently among the Government's billion trees. Many landowners and voters will agree with him.
There are good strategic reasons for including indigenous species, especially to create resilient permanent forests that future generations will want to steward over coming decades and centuries.
More simply, though, New Zealanders put greater value on native species, and in ways that aren't easily captured in dollar terms.
What is the cost to the country of forever losing pohutukawa to myrtle rust, or kauri to dieback? There would be little, if any, impact on GDP. Yet New Zealand would undoubtedly be poorer.
Whether we put this in terms of duty, beauty, kaitiakitanga or national character, these trees contribute to our total value as a nation.
Capturing this value is part of the challenge we face today — to develop reliable frameworks that support investment in positive social and environmental impacts. It is our children and grandchildren that need us to plant tomorrow's forest today.