Miles of pines

Is our most planted tree friend or foe? (12 minute read)

When Wellington’s vicious southerly picks up, a row of trees stands between me and the worst of the gale. They’ve been there for decades, clinging to the top of a cliff, indifferent to the salty wind. Their branches are a dark tangle, hacked and trimmed again and again to stop them from taking out the power lines. It’s not possible to apply the same constraint to their roots, and the driveway swells and cracks where they’ve grown too large. On the other hand, those same roots are holding the top of the cliff together, so my neighbours and I put up with the damage.

The trees are radiata, or Monterey, pine and they are New Zealand’s most multipurpose tree. Timber plantations, carbon sequestration, wind breaks, erosion control… we plant them for every purpose. Then we cut a lot of them down, exporting them as logs, treating them with chemicals to make them suitable for construction timber, pulping them to make particle board and paper, and decorating them at Christmas. New Zealand grows more radiata pine than anywhere else in the world, around 1.6 million hectares. That’s more than Australia, more than Chile, more than South Africa and much, much more than California, which is their home.

Is it really a good idea that New Zealand has planted around 6% of its land area in this single pine species?

I have a lot of questions about this. I have questions about whether pines are affecting our water, soil and native species. I have questions about what happens when trees are cut down – very pertinent questions after what happened in Tairāwhiti during Cyclone Gabrielle. I have questions about what happens when we don’t cut pine trees down – if we leave them as permanent forest. And I have questions about what happens when pine trees go wild.

There are wider issues with pine trees as well – the social impacts of changing land use, local versus foreign ownership, the impacts on communities and landscapes. I’m going to focus on the scientific issues, but I have to acknowledge that these wider questions exist and they affect how we feel about pine trees. We should be discussing these issues, but I’m not going to discuss them in this article.

New Zealand’s plantation pine forests are almost all radiata pine, but radiata is one of more than 100 pine species. All are native to the Northern Hemisphere – only one species, from south-east Asia, extends south of the equator naturally. They are a remarkable group of trees. At a secret location in the White Mountains of California lives a specimen of the bristlecone pine which has been dated at more than 5000 years old. Some ponderosa pines can grow to over 80 metres tall, significantly higher than New Zealand’s tallest tree, the kahikatea. Some species are extremely cold-hardy; Scots pine can be found growing north of the Arctic Circle in Scandinavia, while the Swiss pine grows high in the mountains of Central Europe. Yet there are other species of pine which grow in tropical regions, even in rainforest.

The survival of plants depends on how they compete with other plants to get access to sunlight, water and nutrients. They can’t pick up their roots and walk, so where they grow is crucial. In fertile areas with the right amount of water, there are always lots of plants growing, which means the competition is intense and getting to sunlight is difficult. In less ideal sites, where there is too much or too little water, extremes of heat and cold, or the soil is infertile, there is less competition. But to live in those places, the plant must be able to survive the unfavourable conditions.

A close look at a pine tree reveals clues to how it deals with this problem. First, there is the bark. In most species, including radiata, it is thick and tough, protecting the living cells beneath. Second, there are the hard, woody cones encasing the seeds. These are two features which allow pines to tolerate or recover quickly after a fire. The fires reduce competition from other plants and allow pines to become a dominant species in many places where they grow. A natural pine forest is not like the lowland forest of New Zealand, or tropical rainforest, where the canopy is a mix of different species. Natural pine forest often looks very much like the plantations we have in New Zealand – a canopy of pines as far as the eye can see.

image credit: Melanie Newfield

But a forest is more than just a canopy. A healthy forest teems with life, from fungi in the soil to insects and birds on the wing. I’ve been lucky enough to spend a lot of time in our native forests, including some in which invasive species control has allowed rare birds, insects and plants to thrive. I know what a healthy native forest should look like. But I’ve spent very little time in pine forests. My impression of them is that they are monotonous and sterile, but that may be my biases speaking. So, I decided to talk to an expert.

At a symposium a couple of months back, I met a scientist from Scion Research, Yvette Dickinson. She told me that she has researched the non-timber value of forests, including pine forest. So I found her card and sent her an email.

She told me that what lives in a pine forest depends on where it is. “There are some pine forests, particularly in some of our drier areas, where there aren’t many understorey plants. But there are other parts of the country where you walk into the understorey and you wouldn’t even know that you’re in a pine forest, unless you looked up. There’s just such a thick understorey of tree ferns and native shrubs. But we do have some places that have invasive species in the understorey too – gorse and Himalayan honeysuckle, for example.

“We also have some places where there’s native wildlife living quite happily in the pine forests. For example, we have pine forests that have brown kiwi living in them and doing quite well. And in the central North Island, we have some places where we’ve got New Zealand falcon living in pine forests. They benefit from the structure of the forest, with nice big openings to hunt small mammals and birds in, with relatively large pine trees on the edges where they can nest. We also did some research where we found that there’s quite a number of native insects, like beetles, that were doing well in some of the plantation forests. There’s quite a bit of decaying wood that creates nice homes for them.

“It varies a lot. I don’t think I would ever argue that native species are doing better than they would in a healthy native forest. But it’s not a shopping centre parking lot. There are trees, there’s food, there’s shelter, there’s places to nest, there’s clean water and healthy soil.”

Pine forest, then, is not necessarily a desert. And I know there are other benefits from pine forests – an intact canopy of pines protects the soil from erosion, and growing trees sequester carbon. But there is still the question of what happens when the trees are harvested. Can a pine forest do all that and still be harvested for timber?

Again, the answer isn’t straightforward. “There are trade-offs. I think it’s really important that we understand what those trade-offs are, and are really conscious about them when we make those decisions. If we’re looking over a 30 year period, maybe it’s okay to have an erosion risk for a short period of time, but certainly there are places that risk is not acceptable at all.”

Dickinson has made the point about trade-offs in relation to erosion, but I can see a similar point in what she says about biodiversity. “Some species absolutely need that stable canopy, and if you did harvest, they’ll disappear. But there’s other species that do well in big openings and shrubby environments and you won’t find them in the continuous cover forest. So what you want is a mix of both of those together, a mix that varies through time.”

When it comes to storing (or sequestering) carbon, the situation is slightly different. Forests don’t “lock up” carbon, even if it is sometimes helpful to describe it that way. Instead, Dickinson explains, “it’s moving constantly. You’ve got carbon circulating through the carbon cycle. What we’re trying to do is take more carbon into the forest than what’s being released.”

There are two different factors which need to be considered when we talk about carbon storage. The first is the total amount of carbon stored. As a rule, this increases with time – the more mature the forest, the more carbon stored. There is also the rate at which the forest is absorbing carbon. As a rule, this is greater when the forest is younger. (There’s an excellent explanation of carbon storage in forests which I’ve linked to here.)

A fast-growing tree like radiata will absorb carbon at a faster rate than a slower-growing tree, giving it an apparent advantage when it comes to mitigating climate change, at least in the short term. This is why New Zealand’s Emissions Trading Scheme favours radiata pine over other species. But what happens when the trees are older is also important. When a forest is harvested, some carbon is released back into the atmosphere, but it also depends what happens to the wood, Dickinson tells me. “If some of that wood goes to wood products, it might get locked up in a building for quite a while. Or it could alternatively be turned into paper products that decay and get released back into the atmosphere pretty quickly.”

But what if the trees aren’t harvested? What happens to the forest then?

“There’s a perception that radiata gets to 30 years and then it dies, that’s not reality at all. The reason that the industry harvests at about that age is financial, those trees can grow for much longer. In its native habitat, radiata is known to have a lifespan of over a hundred years, maybe 150, 200 years, maybe a bit longer. The oldest stands I know of in New Zealand date back to about the 1880s. So we do have some individual trees and small stands of trees that are in excess of one hundred years now and they’re still growing.”

It's possible, then, that New Zealand could end up with long-lived radiata forests which do support a reasonable amount of native biodiversity, and protect water and soils – depending on where they are planted. But what about the risk of fire?

Dickinson’s answer may be surprising – “there’s no evidence to suggest that they are actually a much higher risk than any other kind of forest in New Zealand.” There is a difference between being adapted to survive fires, which radiata certainly is, and being flammable. In fact, radiata sits well below a number of native species in terms of flammability, including our native beech species.

There’s also the matter of where these forests are and how they are managed. “There are some environments where fire is a big risk no matter what because it’s dry. But I think any forest that is not managed is a potential risk. I think it’s just as concerning, the idea of planting native species and just walking away. To me some of those concerns are the same as the concerns that we should have for pine forests if we walked away from them.”

This is a crucial point, because one of the most flammable species growing in New Zealand is one of our most common invasive species – gorse. It’s a species whose reputation has been somewhat improved over the last forty years, and we have realised that areas of gorse which are left alone and not burned will eventually revert to native forest in many, but not all, parts of New Zealand. But any newly-planted pine forest (or native forest, for that matter) with a lot of gorse is certainly a fire risk.

One thing that I’ve been wondering for some time is whether the fast growth rate of pines in the short term translates to greater carbon storage in the longer term. According to the figures currently used by the Emissions Trading Scheme, it does, at least up to fifty years. But a group of scientists have recently written a report for the group Pure Advantage which challenges this. The figures used by the Emissions Trading Scheme for native forest are based on regenerating manuka and kanuka. When data is used for planted native trees such as tōtara and kauri, and they are compared at an age of around 50 years, the difference between pines and native trees is much less. It is also possible to increase the overall rate of carbon storage in native forest by planting a mix of fast-growing shrubs and longer-lived trees.

Radiata pine, then, does not have as much of an advantage over natives in terms of carbon storage as our current policies suggest. But nor is it necessarily as bad at encouraging fire or providing a home for native species as you might expect – at least when planted in the right place. Radiata pine is neither as good, nor as bad, as it’s often painted. It has its place.

But there’s one more question with planting pine trees which we need to consider, and it’s a big one, so I have left it until last. This is the question of what happens when pine trees go wild. Currently, New Zealand has around 1.8 million hectares of land where there are pine trees growing which have planted themselves. We spend millions of dollars a year controlling these trees.

But here’s the twist in the tale – many of these trees aren’t radiata pine, but other pine species. There’s a whole other story here, a story of some well-intentioned efforts which went awry and ended up threatening iconic landscapes. I’ve got some personal history here too. When I worked on invasive plants in the late 1990s/ early 2000s, wild pines were part of my job. It’s a fascinating and cautionary tale, and there are some implications for our current planting of radiata pine. I’m in the process of setting up an interview with a scientist who works on wild pines, so I will return to the story of these pines in a few weeks.