Urban tree expert Justin Morgenroth says city forests can help combat climate change. We just need more of them.

Associate Professor Justin Morgenroth from the University of Canterbury is a leader of the New Zealand Urban Forest Initiative and has been studying urban trees and the critical roles they play in cities around the world. Here, he speaks to Britomart’s Jeremy Hansen about how New Zealand cities can nurture urban forests and enjoy their benefits.

JUSTIN MORGENROTH I grew up in Canada. I was studying towards a Master's degree in forest conservation at the University of Toronto, and happened to take a course in urban forestry. It really interested me for a couple of reasons. First of all, I was intrigued by how it is even possible for trees to grow and function in these really harsh urban environments which are so different from where they grow naturally. The second aspect of my interest was when I started to realise that forestry, and urban forestry in particular, is actually all about people. Most of us are never going to see the wild natural forests way out in rural areas: that park or that street tree is the extent of our regular interaction with green spaces or trees. Anyway, that interest grew and I was lucky enough to get a scholarship to study a PhD in urban forestry here at the University of Canterbury. 

JUSTIN Absolutely. And over the last five years I'm noticing a lot more grassroots groups, business communities, and various levels of government showing a huge interest in how urban forests can be used to mitigate some of the negative impacts of climate change.

JUSTIN Maybe I'll answer that a little bit by going back to 2006 when I first came over here. My only frame of reference at that stage was urban forests in Canada and in particular in Southern Ontario, around the Toronto area. And when I came to New Zealand, I was absolutely amazed by, first of all, how green the cities were, the size of the trees. The trees here are enormous compared to the trees that I was used to seeing in southern Ontario, even in the middle of cities. 

And I guess as I continued to study urban forestry, I started to think about New Zealand's particular situation. I think that New Zealand's urban forest has really benefited from a lack of development pressures in general. And I realise that that might not be the same in downtown Auckland, but generally speaking, New Zealand's urban areas have had a lack of development pressure and a very friendly environment: soil moisture is typically high, there's enough rainfall, it's never particularly hot, it's never particularly cold, trees grow in many cases year-round. 

So I think urban forests here have really thrived, but as the country has continued to grow in population over the last 20 years or so, I think we're starting to see those development pressures negatively affecting individual trees and urban forests as a whole. This particularly affects those mature trees that urban foresters like to see retained. That’s not always possible, of course, so it's good to have alternative plans for planting replacement trees elsewhere when older trees do have to get removed.

JUSTIN It’s one of the most challenging topics that we're dealing with in New Zealand today. I don’t have a solution to this, but I can certainly discuss some of the areas that I think are pertinent. First of all, I'm an optimist, so I do believe that that good development – medium- and high-density development – can occur in areas while retaining green space for people. 

But we do have a problem in this country with inequitable access to urban forest, not just to trees themselves, but the benefits that they give. There tends to be a concentration of urban forest canopy and large mature trees – which provide the greatest benefits – in wealthier areas. 

The use of trees as a tool to block development is interesting. Many developers – and I don't mean to cast all developers in the same light – can see trees as a bit of an obstacle to development. So it is ironic that they can be used as a tool from the other side to say, ‘Well, this tree here is going to prevent you from developing this site.’ I think the reality is cities are for people, and I strongly believe that the need for housing is absolutely critical. But I likewise believe that we can do it in such a way as to ensure medium- to high-density environments are also pleasant to live in, offering the benefits of green spaces and urban forests to the people that live there.

JUSTIN Well, every city in New Zealand is bound by the regulations that are set out in the Resource Management Act, which prevents cities from protecting urban forests in any sort of blanket way. There used to be blanket tree protection as part of the Resource Management Act, but that changed 15 years ago or so. Now, cities have the ability to identify individual trees and list them in their district plans. That level of protection is better than no protection, but it can certainly be challenged as well; even protected trees can be removed under certain circumstances. 

JUSTIN Yes, the Bill has passed its third reading and is set to become law. The Bill recognizes the important roles played by urban trees and greenspaces and so it seeks to offer them a greater level of protection than they currently have under the RMA. I’m not a planning expert, but if I understand it correctly, Unitary and District Plans will now need to regulate tree protection on private and public property. At a local scale, I think this will play out in different ways. The literature is clear that there is no one-size-fits-all solution to tree protection, so some degree of autonomy is needed to allow for local solutions to tree protection. 

I think there's room for other complementary approaches to protecting trees in cities. Instead of regulations, there could also be incentive schemes. I say that because one of the things that I sympathise with people over is that while the benefits of urban trees tend to be shared, the costs of retaining a tree on private property are borne out by that private property owner. No question, trees provide a huge number of benefits, but they also have costs: cleaning up leaf litter, pruning branches, getting aborists to undertake risk assessments, root incursion into pipes and other utilities. So I believe there's room for incentive programs for landowners to help support them to do the right thing and retain trees where possible.

JUSTIN Most cities in New Zealand have no idea where their trees are and how much urban forest cover they have – although Wellington, Christchurch and Auckland do have that data. Those cities are aware of their current urban forest cover and where that is within their boundaries, but I believe Auckland and Wellington only have a single data point for that – meaning they’ve done an assessment of their urban forest cover only once. So we don’t know how things are changing in those cities. In Christchurch, there have been two measurements, one in 2015 and one in 2018. And over the course of those three years, citywide canopy cover decreased by about 2 percentage points, from roughly 15 percent to roughly 13 percent. Auckland’s canopy cover is up near 18 or 19 percent. And Wellington’s is at roughly 31%.

So part of the problem is getting these repeated measurements. That's a useful tool to better understand where trees are, where trees are being removed, where trees are being planted. Broader canopy cover surveys can be costly, both from a time and money perspective, but internationally cities tend to commit to those types of surveys on maybe a three to 10-year timescale.

JUSTIN You're absolutely right. I think the current rules for the Emissions Trading Scheme have some scale conditions: your forest has to be a minimum size of, I believe, one hectare. There are also height requirements (minimum of 5 m at maturity) and not all species are eligible. The forest also has to have been planted since 1990. So there are some minimum requirements, but I don't know whether there's anything that specifically says contiguous areas of urban forest need not apply. 

JUSTIN I'm definitely aware that councils are looking to redress that issue. I've spoken with urban forestry folks in all three of the major councils in New Zealand – Auckland, Wellington, and Christchurch – and those are the types of things that they're interested in improving as they move forward. That means identifying areas with low canopy cover and setting in place tree-planting plans for those neighborhoods to increase canopy coverage there, while at the same time ensuring that other areas don't decline too much. 

JUSTIN To a degree. I'll hedge that by saying that there are virtually no examples worldwide of cities that have set a high canopy cover target and achieved it. Let's say a city identifies its current canopy cover at 20 percent and aims to reach 30 percent coverage by 2040. No city that’s set such an aggressive goal in the past has ever actually met its canopy cover target. This isn’t to say cities can't increase their canopy cover at all. Some cities have. The problem is that the scale of change that cities are trying to achieve in some cases is not achievable over relatively short timeframes. There are so many more questions that need to be answered before you can put a spade in the ground, including what land is it that cities actually own? Twenty percent of land in a city might be public land, managed by a council, but there may be very little space within that area that can actually be planted. 

People like open green spaces: they want to walk their dogs in grassy areas; they don't want to play rugby or football in a forest. So there needs to be balance in the different types of land uses that councils are managing. So am I optimistic? I am, and I believe we can make changes, but I think probably the thing that we are going to be better at achieving is more equity. I think it's probably easier to redress that equity imbalance than it is to, for example, get Auckland from 18 percent to 30 percent.

JUSTIN I think education is really important because it helps people to understand why urban forests are beneficial and should be considered as critical green infrastructure alongside the pavements, pipes, and buildings that comprise grey infrastructure. It’s easy to make trees an afterthought if you only think of them as decoration. So I’d suggest people seek out ways to educate themselves on the benefits of urban trees and the challenges they face in cities. Get in touch with your local university or library and talk to your local advocacy groups (in Auckland The Tree Council is one that comes to mind). When you’re feeling well-informed, touch base with your local MP or local board members and tell them why urban forests are important to you. Tell them that urban forests are infrastructure, not aesthetics, and that they need to be prioritised in planning and development.