David Hall knows that trees can save us from climate change disaster - we just need to think about what we're planting, and where we put them.

DAVID HALL There have recently been some rather flamboyant claims made on behalf of trees, that they can deliver a large proportion of the mitigation that's required around the world. Their benefits are enormous, but there are all sorts of considerations. The key thing is it needs to be the right kind of tree in the right place for the right reason. The other thing is that tree-planting shouldn't be used as an excuse not to do emissions reductions within your own supply chains as an organization. It's a both/and, not an either/or situation.

You want to also be thinking about things like biodiversity and climate adaptation. And in an urban environment, you want to be thinking about things like flood risk mitigation, and where the trees can play a role in supporting stormwater systems to reduce the chances of big flooding and big overflows. A really general rule of thumb is that trees will prevent about a third of that water flow from actually going into the ground. About a third will get captured in the crown of the tree, and then return to the atmosphere through evapotranspiration.

Below: The native trees for Britomart's tree giveaway in Takutai Square come from the nursery at The Landing, a 1,000 acre Bay of Islands estate under the stewardship of Cooper and Company, the same  team behind the regeneration of Britomart. The Landing has been the focus of a 20-year wetland restoration and native forest rehabilitation programme; the before and after shots below show some of the results of this.   

DH Absolutely. And I think one of the great things with nature-based solutions like trees is that the benefits are really diverse. And they connect to a lot of non-material and non-financial things, like our mental wellbeing, our sense of spiritual connection to nature, and a sense of whakapapa for Māori, who are related to particular species depending on where their ancestral links lie. That's something that concrete culverts or piping can't offer – they’re not going to bring birds back into the city. They’re not going to give a sense of wonder or solace when you're sitting in your COVID lockdown. Whereas having a beautiful pūriri out your front window with birds coming in might be just the thing that keeps a smile on your face, under difficult circumstances.

DH I don’t know the answer to that. When I was a teenager I was working as an aborist in North Canterbury, but I was actually cutting down more trees than I was planting. But it was really when I was doing my doctoral research that I became interested in how you design climate change policy which speaks to people's hearts and not just their wallets. Because most climate change policy design is based around economic instruments and rational choice theory, and the assumption that if you put a price into the economy that everyone will just do the right thing. And that seems to me hopelessly naive. I think there is an empirical record now to demonstrate that is the case.

New Zealand's net zero pathway relies heavily on trees: about 30 percent of our gross emissions get offset by forests, so it's a really vital lever. But it wasn’t long ago that we were actually in a state of net deforestation; we were actually losing more forest than we were gaining. That was a really problematic trajectory.

Since then, changes have been made, and net forest is growing again. If you're getting forests into vulnerable catchments, and alongside waterways, you're starting to deal with some of the problems around water quality, and biodiversity, and erosion sedimentation into waterways. There are a lot of co-benefits in that space.

Below: Native saplings and grasses growing in the nursery at The Landing.

DH All sorts of trees have different virtues. Pinus Radiata is an incredibly fast-growing and fairly resilient tree that can perform really well in a lot of different circumstances – it’s kind of an amazing species in that regard. It provides quick timber – not the best quality timber – and quick carbon sequestration. It's pretty hard to beat. But that's not everything we value out of a forest. We also value things like biodiversity, with cultural connection, as I mentioned. And I think we are increasingly going to value forest resilience: its capacity to adapt to climate-related change, its ability to withstand droughts and extreme weather like major wind events and fires. It’s not clear that monoculture forests of just Pinus Radiata which are all planted at the same time are necessarily the most optimal forest across all of these other kinds of values. Because it is quite fire-prone, and prone to getting blown over in high winds.

And, from a biosecurity perspective, if you've got all your eggs in one basket of one species, if you get a pest that comes in, like a bark beetle or something, then it could impact hundreds and thousands of hectares of forest if they're all the same species. So, generally, you want diversity in your national forests.

One of the challenges is working out how to finance forestry well. If you could, it would teach you real lessons around how to do long-term infrastructure better. Commercial forestry business models have to last over 25 years, but when you're starting to look at native trees as a commercial crop, for instance – which I think is the right way to go – then you're talking 50 years plus. In the current economic environment, how do you enable business cases to exist over that period of time? It's essentially impossible under business as usual, but that just teaches you some valuable lessons about why the economy is structured in the wrong way.

DH No. The problem is, when you've got the Emissions Trading Scheme as something which financially rewards rapid carbon sequestration, there's always an opportunity cost in planting slower-growing trees because you could have planted them with pine trees and got quicker and better returns. A lot of landowners are in a bind because their heart might say that they want native trees, but the wallet steers them towards the pine trees. And a lot of people are struggling with the cognitive dissonance of that, because they often want to plant the native trees for all the right reasons of resilience and biodiversity and land stewardship and kaitiakitanga, but they may not financially be able to justify that decision.

DH One option which the government is looking at now is to clip the wings of the ETS a little bit and try to funnel that financial incentive towards the native trees. The other option would be to come up with a complementary economic instrument which rewards biodiversity improvements. Native forest could access financial incentives from that, because it delivers greater biodiversity value. From a classic approach to public policy, that's the more rational way of doing it, to create instruments that start to value the specific benefits that different forests deliver.

Below: A grove of tī kouka trees at The Landing. 

DAVID HALL New Zealand has some of the most modified landscapes in the world – especially the Canterbury Plains, where I grew up, where native species are unable to naturally regenerate without specific support from humans. The bird species and the plant species have been so substantially displaced that there's no possibility for them to regenerate in a natural way. There are some really difficult questions there, because the future is going to be different from the recent past because of the amount of warming that we've already locked into the coming decades. So, it's not clear that the native species that grew in particular regions are going to grow as well as they used to. Plus, Aotearoa New Zealand is now full of all sorts of species. I think we have to be practical and work with all of them, both native and exotic species, and use them in the best ways for the best possible outcomes. At the end of the day, we want resilient landscapes, and having diversity is really the key to resilience. You might get one forest die off, but you'll have other forests which aren't vulnerable to those same stresses.

DH The trick is to do something positive about climate change. The trick is to be involved in climate action, because then you feel like you're doing something. There are all sorts of different ways you can contribute to the solution. Finding those opportunities and making them happen, is the best way to get over the anxiety. I've been researching or involved in climate change for more than 20 years now and in some ways the last five years have been easier, because everybody's concerned about it now. Prior to that, it was quite a niche concern. Now people generally appreciate the significance of the problem, and the basics of what needs to be done. So there's a sense of widespread solidarity which didn't used to exist and that's heartening.

Also, the projections used to be bleak. We were looking at five or six degrees of warming, which would've been seriously apocalyptic. Now, if you look at the policy that's been implemented, it's looking more like three, three and a half degrees. And then if you look at the pledges which have been made, it's more sort of two, two and a half degrees. So if the policy starts to match up with the pledges, we are starting to get closer and closer to those ambitions that were put into the Paris Agreement.

DH Yeah, exactly. And not just our political representatives, also our business leaders and our local politicians and all decision-makers throughout the whole system, really.

Watch David Hall in conversation with Jeremy Hansen at our free Zoom event, Why Trees Matter, by clicking on this link. Their talk is part of our ongoing Good Citizen talk series. 

The talk is part of our Greening the City event series, which includes a giveaway of thousands of native trees in Takutai Square and planting workshops to teach tree-planting basics. You can read more about the events (and sign up for them, they're all free!) at this link.