Valuing our forests

Imagine that you are the last person on earth. Imagine that everyone else is gone, and that soon you will be too. To pass the time, you take it upon yourself to cut down every tree you can find. Eventually, you arrive upon the last tree on earth. Is it wrong for you, the last person, to cut it down?

This thought experiment from the 1970s helped to crystallise the field of environmental ethics. It was developed by a New Zealand-born philosopher, Richard Routley, who later renamed himself Richard Sylvan.

As you might guess from his chosen surname, Sylvan thought that it was categorically wrong for the last person on Earth to chop down the last tree. He also believed that this conclusion exposed the shortcomings of conventional Western ethics.

Richard Routley was born in 1935 in Levin, New Zealand, but lived much of his adult life in Australia. In his philosophical work, he focused on logic and metaphysics. However, after seeing deforestation in Taranaki as a child, he developed a strong environmental ethic, which he elaborated in the 1970s with his contributions to environmental ethics. Along with his partner Viv Plumwood, another influential environmental philosopher, he also co-wrote a book on forestry policy, The Fight for the Forests (1973), which criticised the clearing of native forest for pine plantations in Australia. When he died in 1996, he had purchased five large tracts of forest for conservation.

Western ethics, he argued, is dominated by the view that people should be free to do whatever they like, as long as they don’t harm other people. However, if you are the last person on Earth, there is no one else to be harmed by your actions. No one to be upset or disadvantaged. Consequently, the last person should be free to pursue any whim, even to exterminate every last tree.
Or at least so says Western ethics.

You don’t need to be a philosopher to share Sylvan’s intuition that this is morally repulsive. Indeed Māori perceived these flaws in Western ethics well before the 1970s. You might have had a similar intuition in recent years, seeing the removal of old urban trees from around Auckland, or the clear-felling of forests from erosion-prone slopes in sensitive catchments, such as Uawa / Tolaga Bay. The value of trees and forests is being depreciated by some decision makers – but not by everyone.

In Manaaki Whenua’s 2019 Survey of Rural Decision Makers, non-foresters were asked for their reasons for planting trees in the near future. Above all else, what motivated landowners was aesthetic value, beauty, the pleasing sight of a flourishing landscape. This was followed by habitat for native biodiversity, water quality, and animal welfare. Then personal wellbeing and kaitiakitanga. And then material benefits like erosion control, land resilience, wood products, and carbon sequestration.

Clearly, a lot of rural New Zealanders already have an environmental ethic, a land ethic. We might empower and enable them further with the right financial and regulatory frameworks – but do we have those frameworks? I would argue not yet.

Let’s narrow in on the Emissions Trading Scheme (ETS) which, until recently, was commonly framed as New Zealand’s primary policy response to climate change (at the expense of other supplementary policies).

The ETS focuses on just one value: carbon sequestration. By monetising this, the ETS creates a strong incentive for tree species that are fast to grow, and cheap to plant. Typically, that means Pinus radiata.

This remarkable, but controversial tree, is a victim of its own success. In respect to two goods – high-volume supply of softwood timber, and rapid carbon sequestration – Pinus radiata is hard to beat. But as we’ve just seen, these aren’t the only things that rural communities value. They value the things that other tree species, including native tree species, are better able to provide. And these trees, which are often slower growing and more expensive to establish, are disadvantaged by the ETS.

”“letting the ETS do its thing” cannot be sufficient as a national climate strategy

Some people treat this as a failure of the ETS, but it is simply doing what it was designed to do. The objective of the ETS, to be clear, is to create least-cost emissions reductions. It was not designed to do things like improve biodiversity, nor even to create climate resilient landscapes. And this is one reason why “letting the ETS do its thing” cannot be sufficient as a national climate strategy, even if we still need the ETS to put a price on carbon.

To prevent warming of more than 1.5 or 2 degrees, we must radically reduce global carbon dioxide emissions to something close to net zero within the next 30 years. Afforestation can absolutely play a part in this (with the important caveat that it should not be a substitute for earnest, urgent efforts to reduce gross emissions).

But what is also vital is creating a secure supply of negative emissions in the second half of next century, which takes us below zero. All pathways to 1.5C involve removing carbon dioxide from the atmosphere; and an important source of these will be nature-based solutions, including forests. A holistic, far-sighted climate strategy will not only value forests that grow quickly, as the ETS already does, but also forests that store carbon securely over the long run.

However, I draw your attention to this chart from a recent paper in Nature.

On the y-axis, the lifespan of 110 different tree species, on the x-axistheir growth rate, and then this red line: a negative correlation. In short, the faster the tree grows, the sooner it dies, a near universal phenomenon across almost all species and climates. Live fast die young.

Of course, this doesn’t matter much if we’re harvesting these trees for timber in 25 or 30 years. But if we’re looking for long-term carbon stocks, this ought to give us pause. Carbon farming can be done responsibly, but the door is open to cowboys looking to make a buck by creating least-cost, high-risk, carbon sinks.

And these risks are exacerbated by climate change itself. Under ordinary conditions, forests have natural resilience. But under the extraordinary conditions of a heating world, the risks of pests, wildfire and climate stress multiply. Forest carbon stocks may degrade and diminish as a consequence.

My worry is that New Zealand’s current frameworks are rather prone to creating landscapes that are susceptible to such risks, because they encourage policy and investment decisions that focus on maximising a single value – whether it is financial returns, pure conservation value, or carbon sequestration. They encourage a compartmentalisation of the landscape, and then the intensification of land uses within each compartment, such that the pursuit of one value comes at the expense of others. I call this the Siloed World.

Contrast this with what I call the Interwoven World, or an integrated landscape approach, where diverse forestry and agricultural systems are interwoven through the landscape, to strike a positive balance of environmental, social and economic impacts. On this approach, carbon is being reintegrated into the landscape without entirely displacing other non-forestry land-uses. And the landscape is diversified, in terms of species and management systems, so that our eggs aren’t all in one basket. This is a landscape that’s prepared for future shocks, whether environmental or economic, because it spreads risks, rather than concentrates them.

So if this is the ideal, how do we get there? There are, of course, many levers to pull – but let me champion one: a biodiversity payment, as proposed recently in the Aotearoa Circle report, Resetting the Balance. This could complement the ETS, by valuing and monetising the things that the ETS does not. Crucially, it would help rural decision makers to plant the trees that they genuinely value, which also means that these trees are much more likely to get planted and maintained.

I finish with another philosopher, Amartya Sen, whom our current finance minister is apt to quote because of his work on wellbeing. Sen argues that the way to improve wellbeing is by enhancing human capabilities – that is, our capabilities to live the kinds of lives that we value, and that we have reason to value. The ETS, for all its limitations, does provide landowners with the capabilities to make a living from forests that aren’t being harvested for timber. It provides more than an incentive: it is an enabler, and it can help to create some elements of a flourishing landscape.

”A biodiversity payment would help rural decision makers to plant the trees that they genuinely value, which also means that these trees are much more likely to get planted and maintained.

But other landscape elements need other enablers – through knowledge, culture, skills, infrastructure, regulation, finance, and policy innovation. For our landscapes to reflect what we really value, our systems need to too.

This essay is adapted from a keynote speech given to the Climate Change and Business Conference 2020 (available for viewing here). The speech was dedicated to Dr Ed Hearnshaw (1977–2020).

Dr David Hall

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For a small country like Aotearoa New Zealand, this is a potential obstacle to upscaling impact investment.

But the land sector is something of an exception. Agriculture and forestry face big challenges, especially in a warming world. Consequently, there are big opportunities to deliver solutions.

In Our Forest Future, I wrote about opportunities for the forestry sector, in particular the 1.1 million hectares of moderately to severely erosion-prone land in pasture that would benefit from afforestation. That’s more than 4 per cent of the entire country. Impact-oriented investors could play a role in providing the capital to enable land use change.

But consider also agricultural greenhouse gases. These amount to nearly half of New Zealand’s gross emissions. For the nation to meet its Paris Agreement targets under current accounting rules, these emissions need to decline to some degree, or to be offset. If we presume that this doesn’t involve purchasing international units – which is prudent given the uncertainties about future price, supply and international rules – then it will involve changing our agricultural systems. The recent IPCC Special Report on 1.5°C says that global methane needs to decrease by 35% by 2050. Again, impact investment could play an enabling role in the transition to more sustainable agriculture.

But let’s get more specific. Conversations about impact investment often suffer from too much hand-waving. It’s too often about the promise of impact investment, and not enough about the product. Nevertheless, it’s the availability of product – that is, investable propositions that combine social, environmental and financial benefits – that will drive the upscaling of impact investment.

The opportunity for impact investment can be helpfully seen as a spectrum, which runs from purely commercial propositions at one end, toward propositions that involve some blending of public/private finance at the other end.

Commercial propositions are those that succeed within the market as it’s given, by meeting investor expectations on their own terms. One example would be a carbon farming proposal that uses fast-growing Pinus radiata to deliver a return-on-investment that satisfies typical investor preferences. Another example might be equity shares in a successful impact enterprise, such as an agri-tech company, that delivers sustainability outcomes as well as financial returns for shareholders.

But both examples face obstacles to growth. Carbon farming with Pinus radiata might succeed financially, but scalability is constrained by environmental trade-offs, long-term resilience, and a shortage of social license. In regards to profitable impact enterprises, these are sweet deals when you can get them, but there are relatively few around.

Far more numerous are “the little deals that could”, the investable propositions that don’t quite meet the financial expectations of investors, in regards to risk, return, scale and tenor. Yet from an impact perspective, these deals might be precisely the ones that deliver the greater social and environmental benefits. For example, carbon farming using native tree species is unlikely to match the carbon sequestration rates of Pinus radiata, but it may deliver a richer stack of impacts such as biodiversity, community support, cultural value, and so on.

Getting such deals over the line is where the most immediate opportunity for growth lies. As I’ve argued elsewhere, this is best achieved through a systems approach, which considers supply- and demand-side factors, as well as the wider enabling environment. For example, tightening environmental standards will increase the cost of capital for activities that transgress those standards, thereby reducing their comparative advantage over more impact-oriented propositions.

But here I’ll focus on the role that financial innovation might play, especially through combining public and private finance. Public money, rather than being distributed through grants, could be used in more subtle ways to monetise non-market benefits, de-risk investments, or aggregate smaller deals into larger propositions. In this way, it can deliver on the promise of “blended finance for integrated impacts”, which delivers a balance of social, environmental and financial benefits.

One product that I’ve been working on – with support from Foundation North – is the Native Forest Bond Scheme. It is an environmental impact bond, whereby the government guarantees to pay for the successful establishment of continuous native forest on erosion-prone land. This payment, which is triggered when impact targets are met, reflects not only the value of sequestered carbon, but also other environmental values such as erosion control and native biodiversity. Consequently, it enables outcomes which otherwise mightn’t attract investment.

But there are other ways that blended finance could drive impact investment. Credit guarantees, for example, could be used to de-risk investment into alternative agricultural or forestry systems, such as low-input farming or continuous cover forestry. That insurance against insolvency might make all the difference, yet might not cost taxpayers anything if the project delivers its expected return on investment.

Another example: The Nature Conservancy has proposed Sustainable Land Bonds at the international level, where developed countries pay the bond coupon (or interest payment) as long as impact targets are met. A similar structure could easily be used at a regional level, or even a catchment level, to enable change at the landscape scale.

Impact investment cannot deliver all the change that needs to occur in the land sector. Regulation, compliance, environmental taxes and other strategies have a critical role to play. But by mobilising and “crowding in” private capital, impact investment can bring further resources to the challenge. Financial innovation is just another front in the fight to reduce net greenhouse gas emissions.

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In Aotearoa New Zealand, many of us see forests as a binary choice. It’s either native conservation forests, managed under the principle of look-but-don’t-touch, or it’s exotic plantation forests, typically Pinus radiata managed under a clear-fell system. Not everyone sees forests this way, yet this siloed thinking pervades public conversations.

Indeed, I hadn’t realised this until I published Our Forest Future in 2016. Some accused me of advocating for 1.3 million hectares of intensive exotic forestry, not an appealing idea to many people. Still others accused me of advocating for native forests only, which they regarded as a whimsical, even irresponsible, suggestion, because slow-growing natives are expensive to plant and poorly aligned to short-term net emissions targets. In short, I was two different enemies to two different groups of people.

Actually, what I called for was appropriate diversification. It was a refresh of that old forestry adage: “Right tree, right site.” My view then, as now, is that the forests we need are the forests that create the greatest public value: social, environmental and financial. And this is bound to change from site to site, depending on landowner preferences, financial constraints, soil type, slope, microclimate, access to seedlings, and so on.

A more diverse future forest is the obvious way forward. Yet it is also obvious that we don’t do enough to explore innovations and opportunities, to see what mixes of species and management systems might strike a better balance of social, environmental and financial outcomes. Instead, our regulatory, financial and research frameworks remain largely beholden to a siloed way of thinking, where the landscape is treated as either native conservation forest, exotic plantation forest, or pastoral agriculture – with little in-between. This siloed vision of the landscape looks something like this. 

Today, courtesy of The Policy Observatory at AUT, I’m publishing a new discussion paper, The Interwoven World | Te Ao i Whiria. Something of a sequel to Our Forest Future, it lays out an alternative vision of our landscape which deliberately interweaves and intermingles a variety of land uses in order to create a more prosperous landscape. The image below ought to make clear that the Interwoven World isn’t about either/or, but all and more.

Of course, some landowners are already doing this. They are already experimenting with wood lots, carbon sinks, mixed native/exotic systems, riparian margins, silvopasture, and more. Yet many more landowners would probably like to experiment in this way – but can’t. They can’t find the capital, time, capacity, or technical advice. Or they can’t make these land uses financially viable, even when this diversification would create the most value for rural communities, or avoid environmental harms.

These barriers shine a light on who this discussion paper is really written for. Not so much the folks on the ground, making tough decisions about what to do with their land. Much more the folks who make decisions about the wider regulatory framework, about policy design and enforcement, about research funding and agendas, about investment and supply of finance, about who to insure and on what terms.

We can’t ask landowners to make land use changes that would risk, or induce, insolvency. So, if we want a more interwoven, well-integrated landscape, we need to precipitate a systems change that makes reforms in all these areas (and more), that better aligns what is financially sustainable with what is socially and environmentally sustainable.

Today there is a unique opportunity to instill that change. Of course, one of the new Government’s flagship initiatives is the One Billion Trees Programme, funded through the Provincial Growth Fund. But what is more important than meeting this target is creating the conditions by which this target may be sustainably met. Here lies the path to genuine transformation of the landscape, not just the superficial alterations that we might precipitate by throwing grants at the problem.

And we are seeing the foundations of this systems change. The Zero Carbon Bill, currently under public consultation, could create the stable, meaningful carbon price that would make permanent forest a financially viable land-use choice. The $15 million boost to the Sustainable Farming Fund could accelerate research into the diverse forest systems that might break us out of our path-dependency on clear-felled Pinus radiata. New National Environmental Standards for Plantation Forestry, as well as proposed reforms for the Resource Management Act, aim to lift minimum standards. Finally, the resuscitation of the New Zealand Treasury’s Living Standards Framework, augmented by Statistics New Zealand’s work on environmental accounting of natural capital stocks and flows, can help us to reimagine how we evaluate national success.

Change isn’t only being driven by Government either. Trees That Count, which mobilises grassroots conservation groups around the country, has counted the planting of over 14 million native trees now, recently boosted by the Provincial Growth Fund. Meanwhile, throughout New Zealand, we are seeing the acceleration of a rebranding and reorientation of financial flows, variously known as “climate finance”, “sustainable finance” or “green finance” (see the recent Climate Finance Landscape report for the Ministry for the Environment that I co-authored). The most prominent examples – such as Auckland Council’s certified climate bond for low-carbon transport infrastructure – are not in forestry. But it is only a matter of time until investors start demanding products, such as continuous cover forestry, that deliver the right mix of social, environmental and financial returns.

I’ve been working on one such project, the Native Forest Bond, with support from Foundation North’s GIFT fund, to mobilise private capital for establishing continuous native forest on highly erosion-prone land in pasture (a concept paper is available here). But this is just one example of how innovative investment vehicles could function as “enablers” in the low-emissions transition, in particular the creation of a climate-aligned landscape. How can we facilitate what the Productivity Commission is calling an “inclusive transition”, a shift to a low-emissions economy that doesn’t leave people behind?

The key is to change our systems, but first we need to change our minds. My new discussion paper is available here.

Dr David Hall

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That’s worth keeping in mind whenever you’re boggled by the enormity of the climate challenge. It’s also helpful whenever you’re wondering how to get everyone lined up behind the transition to a low-emissions future.

When I wrote the Our Forest Future report earlier this year, I didn’t focus on trees because I thought that tree planting alone could solve anthropogenic global warming. To be clear, climate change is a multi-dimensional problem that demands a multi-dimensional response, with forests being one of many strands.

The reason I focused on trees is that, when it comes to “wicked problems” like climate change, we shouldn’t try to tackle everything at once. Instead, we might break the problem down to it constituent parts, then focus on what we can.

I first gleaned this insight at a talk in 2010 at the Oxford Union. This was the same venue where, in 1985, David Lange established New Zealand as a moral leader in the world – not just a “fast follower” – by railing against the global threat of nuclear weapons. I arrived two and a half decades later to write my doctoral thesis on political decision making, just months after the Copenhagen Climate Change Conference had reached a diplomatic stalemate.

The speaker was Professor Gwyn Prins. He was making the case against the Kyoto Protocol, not because he denied the reality of climate change, but because he denied that one-size-fits-all policy solutions are adequate for wicked problems like climate change. His speech drew on The Hartwell Paper, a report that he had co-authored with thirteen other natural and social scientists. He argued, in essence, that complex problems require complex responses – and he trotted out a metaphor that stuck firmly in my mind:

If one seeks long-lasting impact, the best line of approach may not be head-on. “Lose the object and draw nigh obliquely” is a dictum attributed to the famous eighteenth century English landscape gardener Lancelot “Capability” Brown. Brown’s designs framed the stately home at the entrance, but only briefly. After allowing the visitor a glimpse of his destination, the driveway would veer away to pass circuitously and delightfully through woodland vistas, through broad meadows with carefully staged aperçus of waterfalls and temples, across imposing bridges spanning dammed streams and lakes, before delivering the visitor in a relaxed and amused frame of mind, unexpectedly, right in front of the house. That displays a subtle skill which has manifest political value: the capacity to deliver an ambitious objective harmoniously. “Capability” Brown might be a useful tutor for designers of climate policies. His advice would be to approach the object of emissions reduction via other goals, riding with other constituencies and gathering other benefits.

This final line captures the central message. When we design climate policy, we needn’t try to solve it all in one foul swoop. Rather, we can progress “via other goals” – that is, we can address more manageable, more tractable problems that have a positive impact on climate mitigation and adaptation. By doing this, we appeal to things that people genuinely care about – “riding with other constituencies and gathering other benefits” – while also addressing some element of the wider carbon challenge. 

A map of Lancelot “Capability” Brown’s famous Blenheim Palace Grounds, near Oxford, without a straight road in sight. Source: Wikipedia Commons license.

Afforestation is a good example. It is easy to conceive the impacts of trees, far easier than conceiving the impact of the billions of tonnes of invisible and insensible gases that we hoist annually into the atmosphere. Even dedicated climate sceptics will recognise the benefits of tree planting – which include shade, shelter, natural habitat, water quality, land stabilisation, or timber. But no matter what reason a tree was planted for – whether for climate change or some other reason – it will still absorb carbon from our atmosphere. From the perspective of the global climate, a climate sceptic’s tree is as valuable as a climate activist’s.

We also stand to see progress in our lifetimes, to reap the rewards of positive feedback. This year alone, we’ve seen positive developments such as the Trees That Count initiative to boost the number of native trees being planted nationally, or Climate Change Minister Paula Bennett’s acknowledgment that we can spare future generations the costs of purchasing foreign offsets by creating domestic offsets through new forest planting.

This is the spirit of The Hartwell Paper, which argues that the Kyoto logic “needs to be inverted, so as to provide near-term, concrete, politically attractive benefits for near-term investments.” If we eradicate black carbon, we deal not only with a major contributor to global warming, but we also promptly improve the well-being of millions suffering from respiratory diseases. Similarly, if we increase the carbon content of soil, we not only remove it from the atmosphere but also increase horticultural and agricultural productivity. As such, we respond to climate change in spite of ourselves, while also convincing people that a low-emissions world is – in many ways – a better world to live in.

And, let’s face it, when it comes to winning hearts and minds, the climate movement has a mixed record at best.

The United States just nominated a President-elect who tweeted that “[t]he concept of global warming was created for and by the Chinese in order to make U.S. manufacturing non-competitive.” Admittedly, Donald J. Trump has recently adopted a more conciliatory tone, saying that he has “a totally open mind” on climate change. But being open-minded about whether climate change is occurring or not already flies in the face of the weight of empirical evidence and scientific prediction.

But this problem is bigger than Trump. He is, after all, an elected politician. He is where he is because many voters also doubt climate change, or don’t think that it’s so important.

Recent polling found that only 48 per cent of Americans believe that Earth is warming mostly due to human activity. Among the most conservative Republicans, belief in climate change drops to only 15 per cent. Moreover, New Zealand survey results from 2013 don’t fare much better: only 53 per cent of New Zealanders professing to believe in anthropogenic global warming, with 30 per cent undecided.

How did an ostensibly scientific issue become so polarised? In my view, the problem is that people still need reasons to be concerned.

It isn’t enough to bombard people with scientific information. People needs reasons that genuinely speak to their values, aspirations, loyalties and life projects.

Nor is it enough – as the political elite in the UK and US are belatedly discovering – to treat non-believers with carelessness or contempt. If folks hear you say – as Hillary Clinton did in Columbus, Ohio – that “we’re going to put a lot of coal miners and coal companies out of business”, they won’t care what your broader point was. They’ll care only that you’re planning to wreck their livelihoods, all due to something called “climate change”. And they’ll push back.

But if climate change touches everything, then it must touch upon the aspirations of these folk too. Climate advocates need to find new ways to speak – with respect and sincerity – to all kinds of people. It isn’t enough to simply appeal to justice, rights, equality and nature. Tradition, civic duty, security, prosperity, responsibility, purity and sanctity are relevant too. So too are reasons of religion and culture not commonly associated with climate change. Psychologists tend to describe such appeals as “framing”, as if it’s a kind of manipulation. But I think of this as simply taking political diversity seriously, as providing people respectfully with the reasons that speak to them even if those reasons don’t speak to us.

Of course, this won’t solve climate change in itself – but it will create the conditions in which solutions take hold. In our polarised times, that’s worth cultivating. Ultimately, it’s better for us to be arguing over what to do than whether we need to do anything at all.

Dr David Hall

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Whatever your answer to that old philosophical chestnut, what’s obviously true is that when a tree falls in a crowded city, a lot of people hear it fall. Conversely, when a tree takes root in a city, a lot of people reap the rewards.

That’s why, in my Our Forest Future report, I identified urban forest as one of three target areas for New Zealand’s future forest. What makes urban forest so special is that, for all the positive environmental impacts that each tree generates, the social impacts are multiplied because there are simply more people around to enjoy them.

Some folks already get this. Auckland mayoral candidate Phil Goff recently announced his Million Trees Programme, a pledge to plant one million trees and shrubs over three years if he becomes mayor. Wellington already has its Two Million Trees initiative to plant two million trees by 2020. Then there’s the grand vision of the Avon-Ōtākaro Network to “green the red zone”, to establish an ecological and recreational reserve around the Avon River on red-zoned land.

Of course, some will dismiss these projects as nice-to-haves, as “feel good” policies that ultimately aren’t essential. But there are plenty of hard-nosed reasons to invest in tree planting in cities. Trees and shrubs are part of a city’s “green infrastructure”, an integral aspect of urban planning that adds to a city’s value, cuts long-term costs, and provides services that are no less tangible than a city’s “grey infrastructure”.

Take storm water, for instance. Cities are especially vulnerable to rain runoff because of the prevalence of impervious surfaces like concrete, tarseal, and asphalt. An area with just 10–20% of impervious surface will have double the volume of stormwater runoff compared to forested catchments. 35–50% of impervious surface leads to three times the runoff (which is roughly the state of Auckland at 42% impervious area in 2010).

To compensate, we invest in the “grey infrastructure” of gutters, sewers and holding ponds to funnel this stormwater into our waterways and harbours. But it also carries sediments and contaminants such as oil, rubber and metals from cars, pesticides and herbicides from gardens, plastics, garbage, and miscellaneous other chemicals. These contaminants, especially heavy metals and polycyclic aromatic hydrocarbons (PAHs), then accumulate in the tissues of marine life such as shellfish. This potentially creates problems for our kaimoana and those who eat it.

Measurements from 2009 to 2013 concluded that twelve per cent of Auckland Council’s monitoring sites had high rates of contamination, especially from zinc, copper and PAHs. At those concentrations, negative ecological effects are expected to occur. A further twenty-six per cent of sites were moderately contaminated to the extent that ecological effects might appear. The most problematic sites were associated with highly urbanised catchments like the Upper Tāmaki Estuary and Central Waitematā Harbour sub-estuaries like Motions and Meola.

But trees and other kinds of vegetation alleviate these problems by reducing the volume of runoff and pollutants. Firstly, trees intercept rain in their canopy, capturing anywhere from 15% to 27% of annual rainfall depending on the age and species of the tree. Airborne pollutants are captured, filtered and stored through this process. The absorption of water and pollutants is then repeated at ground level by the root systems of trees.

This latter point is especially relevant for Christchurch. The lands around the Avon River are already flood prone, with risks increasing as sea levels rise. By planting trees in flood prone areas, however, Christchurch could create extra leeway by increasing the land’s capacity to absorb water.

And intercepting water isn’t the only benefit. Here’s a few fun facts about urban trees:

• A 2008 analysis found that Christchurch’s urban trees remove around 300 tonnes of air pollutants annually. Reduced exposure to PM10 particulates created health benefits worth $19.2 million.
• A 2007 study in fifteen New Zealand cities found that people would be willing to pay about $184 annually to avoid a 20 per cent reduction in their local tree estate.
• A study in Modesto, California, found that tree shade improved the resilience of asphalt to fatigue and distress, saving US$7.13 per metre² in maintenance costs over 30 years.
• A series of studies support the hypothesis that trees can reduce incidence of car crashes, including a Texas study which showed on average a 46 per cent decline in crash rates around ten sites following landscaping projects.
• A study in Minnesota estimates that a 10 per cent increase in tree cover within 100 metres of a home will increase its sale price by 0.48 per cent (an extra US$1371 in that study).
• A study of Toronto, Canada, found that having ten more trees in a city block, on average, improved residents’ perceptions of health in ways that were comparable to an increase of personal income of $10,000, or being seven years younger.
• A UK study estimated the value of British green space through improved air quality and increased opportunities for recreational exercise, putting the value at £1.44 billion per year for human health benefits.
• An American study estimated that US trees removed 17.4 million tonnes of air pollution in 2010, with human health effects valued at US$6.8 billion, or the avoidance of 850 deaths and 670,000 incidences of acute respiratory symptoms.

New Zealand is way behind in this. There’s some laudable ambitions – like those mentioned earlier – and good research emerging. But there’s not enough locally relevant knowledge being corralled into ambitious and feasible city strategies.

A good role model is Toronto’s Every Tree Counts report. By getting a handle on the value of trees, the report concluded that the annual cost of managing Toronto’s urban forest was exceeded by the CAD$28.2 million in ecological services that the trees provided every year. Now Toronto is committed to increasing its tree cover from 28% to 40% within 50 years.

Another example is Melbourne. Its comprehensive Urban Forest Strategy aims to increase urban forest canopy cover from 22% to 40% by 2040. This is supported by credible pathways and a clear exposition of the prospective benefits, especially providing shade and reducing urban heat. Famously, its interactive Urban Forest Visual also enables Melbournites to forge relationships with neighbourhood trees by sending them affectionate emails.

Meanwhile, British cities are joining up to the Urban Tree Cover project to create inventories of their urban forests. Drawing on this, the new Mayor of London Sadiq Khan has pledged to increase urban canopy cover by 5% from a 20% baseline.

What all these initiatives have in common is the i-Tree programme. Developed by the USDA Forest Service, it is a suite of free tools for assessing the status of urban forests and their economic value. It isn’t the only way to plan urban forest, but it is now the international standard.

Some preliminary work on i-Tree was undertaken in New Zealand a few years ago. An analysis of trees in Auckland’s Wynyard Quarter found that each tree delivered $2.60 in air pollution removal benefits. For a full roll-out, however, someone needs to create parameters that are specific to New Zealand’s native trees. Meanwhile, official interest in i-Tree appears to have stalled for no good reason. We could – and should – do better.

An urban forest strategy has three basic elements: firstly, we need to know what we’ve got; secondly, we need to conserve what we’ve got; and thirdly, we need to grow what we’ve got.

Knowledge will involve engaging with tools like i-Tree. Conservation will involve assessing how the Resource Management Act and local rules are impacting on our urban forests, as well as defending our urban trees from weeds and from pests, both animal and human.

Finally, growing green assets like our urban forest will require investments of land, labour and money. But these immediate investments can be justified in anticipation of long-term returns. We can also make these investments effective by returning to knowledge. For example, a LIDAR study of Auckland found that newer suburbs have less dense canopy cover than older suburbs, which suggests that we’re already missing a trick when it comes to urban planning. Surely it’s cheaper to plant trees as Auckland grows, rather than make the space retrospectively. With a little more foresight and long-term planning – something a recent Productivity Commission report shows that we’re struggling to get right in New Zealand – we could steer our cities toward a greener, leafier, more prosperous future.

Dr David Hall

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