Ebb and flow
Our native forests are doing more for us than we realise (11 minute read)
When I stand beneath one of New Zealand’s giant trees, a towering kahikatea perhaps, or a graceful rimu, the world around me seems to still. Footsteps are muffled by the deep layer of leaf litter. Sounds are softened and winds deflected by millions of leaves. I know such a tree was here before me, before my parents, before my grandparents – in fact, centuries before them. I’m in the presence of something which feels ageless, both ancient and capable of enduring long after I’ve gone.
It's tempting to view such a tree, and the forest it inhabits, as static. Particularly in New Zealand’s evergreen forests, nothing much appears to happen on time scales which humans can perceive. But there is a lot happening in our forests that we don’t see. Among the most crucial processes is the ebb and flow of carbon dioxide. It’s in constant motion, with daily cycles, seasonal cycles and responses to unpredictable events such as storms.
To understand what is happening with carbon dioxide a forest, it helps to know about two crucial chemical reactions which occur in the cells of plants. The first is photosynthesis, where plants use sunlight to drive a reaction between carbon dioxide and water molecules to create carbohydrates, with oxygen released as a by-product. Plants get the carbon dioxide they need from the atmosphere, meaning that they are absorbing some of the carbon dioxide we are emitting by burning fossil fuels. This is why planting trees is often promoted as a solution to climate change, but it’s not as simple as that, because trees also release carbon dioxide.
That’s where the second chemical reaction comes in1. This second reaction is effectively the reverse of the first – plants break apart the carbohydrate into carbon dioxide and water, releasing energy as a result. The released energy powers every process inside their cells, and it’s actually the same process we use when our cells break up the carbohydrates we absorbed from our food. Because this process is constantly happening in plants, they are emitting carbon dioxide as well as absorbing it.
However, not all the carbon dioxide absorbed by plants goes straight back into the atmosphere. Some of it is used to build the plant’s structure, in the form of carbon-based molecules such as lignin and cellulose. Lignin is particularly concentrated in wood and takes a long time to break down, unless the wood is burned. So, as long as a tree is alive, and even for some time after it dies, it holds onto some of the carbon it absorbs from the atmosphere.
There are many different ways that carbon moves. It can move quickly, from carbon dioxide in the atmosphere, to carbohydrate in a leaf, then back to carbon dioxide in the atmosphere. It can move more slowly, from the atmosphere to a leaf, then to lignin in a tree trunk, only making its way back into the atmosphere when the tree dies and decomposes. It may end up in the soil, then get washed into a river, out to sea, become incorporated into the shell of a marine creature then sink into deep ocean sediment and turn into limestone. And, of course, it finds its way into the atmosphere after being buried for hundreds of millions of years deep underground when we burn fossil fuels. The direction and rates of these movements are known as carbon fluxes, and they are crucial for understanding climate change.
When a tree is actively growing and making new wood, a large proportion of the carbon dioxide it absorbs is turned into wood. Young forest is therefore considered a carbon sink, that is, somewhere which absorbs more carbon dioxide than it emits. For some years, the prevailing view was that as they get older, trees absorb less carbon dioxide. Mature forest was thought to emit about as much carbon dioxide as it absorbs, or in some cases to even emit more, as trees died and decomposed. That is, as they age, forests were thought to stop being carbon sinks, and could even be the opposite, a carbon source. However, these conclusions were largely based on overseas studies, particularly for managed plantation forests.
More recent studies have shown a much more complex picture. The rate at which a tree absorbs carbon doesn’t necessarily decrease as it gets older. Usually, it increases. This has been shown for a wide range of trees, tropical and temperate, including some New Zealand natives. What, then, is happening in our native forests as a whole?
A recent study led by atmospheric scientist Dr Beata (Bea) Bukosa2 has given us some answers, but also raised some new questions. She worked with scientists from a number of different organisations to look at carbon dioxide on a national scale for New Zealand. This work has delivered some unexpected findings, particularly in relation to the extensive areas of native forest on the South Island’s West Coast and Fiordland. I spoke with Bea to learn what she did, what she found and what it means for our understanding of climate change.
Before we talked about her research, we spoke in more detail about what is happening with carbon dioxide in trees. Bea explains that “During the day you have sun, so you have photosynthesis, and the tree is taking up more carbon dioxide than it’s releasing. But during the night, when there is no sun, the tree is just releasing carbon dioxide back into the atmosphere. There is also a seasonal cycle. During summer, or during the growing season, it’s taking up a lot. During winter, when photosynthesis slows, then they release more carbon dioxide than they take in.”
This discussion leads to another point I hadn’t appreciated. The figure we see quoted for carbon dioxide in the atmosphere (422.7 parts per million in 2024) is an average. But carbon dioxide levels are not the same in every location. Bea tells me that the two long-term monitoring sites in New Zealand have different measurements. “Baring Head, near Wellington, is the longest running greenhouse gas monitoring station in the southern hemisphere. It’s quite a famous measurement site. And then the second one is Lauder in Central Otago. Lauder is consistently lower, which means that somewhere around it is a big carbon sink.”
To better understand what was happening, Bea and her co-authors took a top-down approach to studying the movement of carbon dioxide, which is the opposite of the usual bottom-up methods. She tells me about these methods first, so that I can understand what was different about her work, and why the unexpected result is so important. “One of the bottom-up methods is through inventories. Under the Paris agreement, every country needs to report on their greenhouse gas emissions. The method used is to measure the different carbon dioxide sources, combine that with remote sensing data and models, and then scale that up to larger regions. The other bottom-up method is a process-based approach, which measures localised changes in carbon dioxide fluxes, for example at the level of a paddock. Again, you add all that up to understand what is happening overall.”
The top-down approach, however, starts with the big picture. “We go from the overall story and try to track it back down to see what the sources were and what led to the atmospheric carbon dioxide values we are seeing. We don't use every measurement point for carbon dioxide. We use measurement points where we have better mixing, where there are stronger winds, for example. Because in those cases you don't only see what's happening in a few kilometres around a measurement station. You see a quite big region.”
The recently published study by Bea and her colleagues used data only from Baring Head and Lauder. But how can measurements from just two sites tell us what is happening over a large area? “We take the measurements, which have a high precision. Then we combine that with information about transport of air, that is, wind speed, wind direction, atmospheric mixing etc. We also use information about the background levels of carbon dioxide, how much is around New Zealand, to remove that from the actual calculations, so we can just identify what’s happening with carbon dioxide on New Zealand’s land. That information allows us to calculate how much carbon dioxide was emitted or absorbed by different regions, and then map it in space and in time.”
When Bea and her colleagues looked at the maps they created, they noticed something unusual. In the North Island, the maps were reasonably consistent with the results from bottom-up modelling. But for the South Island, there was a crucial difference. “We saw a large sink happening along the west coast, especially in the southern part, Fiordland. When you look at the maps of land use, you see that those areas are native forest.”
One previous study, published in 2017, had given a similar finding, but it was based on only three years of data. This new work used at ten years of data. It also gives us a seasonal picture. Like all temperate forests, New Zealand’s take in more carbon dioxide when temperatures are warmer and they are more actively growing. Bea explains that “it seems as if during summer Fiordland’s native forests are taking in carbon as we expected. But then, during winter, there is less being released back to the atmosphere than we expected. It’s not necessarily that they're absorbing more. It may be they're releasing less.”
While this gives us more confidence that these forests are much better at absorbing carbon than previously thought, it doesn’t tell us why. While we don’t yet know, Bea and her colleagues have given these questions considerable thought. “We looked quite widely in terms of ideas and explanations. One possibility is that the West Coast and Fiordland get quite a bit of rain. There are landslides, there’s erosion and this may mean carbon burial in the soil. It could also be the forests regenerating after this kind of disturbance. There are also pest control efforts happening along the coast, so that might be leading to healthier forest in some regions and therefore more carbon dioxide uptake. It may be that with landslides and increased rainfall some carbon buried in the soil is being transported into rivers, and then out in the ocean. This will appear as a carbon sink because it's disappearing from New Zealand. However, it’s quite tricky if that’s the case, because we don't know where the carbon is going. Is it going to end up in the sediments, where it’s locked away for centuries? Or will it come back into the atmosphere somewhere else, in which case it’s not really a carbon sink? It's more like a carbon loop.”
There’s another possibility too, another kind of loop. “It could be due to climate change. It might be that these environments are responding to climate change in a specific way that’s leading to additional carbon uptake.” This is an interesting point, and deserves closer attention, because it sometimes appears in discussions where people are arguing that climate change is less of a concern than most of us think.
Carbon dioxide is essential for the growth of plants – this is as undisputed as saying oxygen is essential to humans. It’s also well-documented that increasing atmospheric carbon dioxide has increased plant growth under some circumstances. This may moderate some of the increase in carbon dioxide resulting from the burning of fossil fuels, up to a point. It’s not happening everywhere, and it doesn’t cancel out the many negative impacts of increased carbon dioxide which are being felt around the world. While the fertilisation effect does need to be accounted for in climate change models, it’s not going to solve the problem for us. There is no solution to climate change which doesn’t involve deep and immediate cuts to carbon emissions.
It's tempting to see research telling us that our native forests are better at absorbing carbon than we thought as a validation of the way we like to see ourselves, as a clean and green country, despite so much contrary evidence. Depending on your viewpoint, it may also be tempting to see this as a way to make the numbers look more favourable, and to argue for reducing our international obligations. Alternatively, some might think that focusing on this as good news is a distraction from the urgency of the problem and the very real consequences we are facing.
These are all very understandable, human responses, but this research doesn’t necessarily support any of these views. What it says is that over the last decade or so, some large areas of mature native forest have been absorbing more carbon than we realised. We don’t know why.
And ten years is still short in the life of a forest. We don’t know whether this is a long-term trend, whether it’s the result of how the forest is being managed, with increased pest control, or something else. We need to know this. If it relates to something like control of introduced mammals, we need this information to make decisions on conservation funding. If it’s something else, we need to know that too.
Bea and her colleagues have proposed further research to answer these questions. But we will need to wait for answers. In the meantime, we do know that our forests are important. Protecting existing forests, as well as other natural ecosystems which store large amounts of carbon, such as wetlands, is far more important than planting new trees. If nothing else, this research has shown us how lucky we are to still have such precious forests remaining.
This reaction is known as cellular respiration, or respiration, but it can be a confusing term because respiration also means breathing.
Almost all of the carbohydrate we eat comes from plants either directly or indirectly. However, there are exceptions, because much of the photosynthesis in the marine environment is done by seaweeds and microbes (or plankton) which are not classified as plants.
At the time the paper was published, Bea was working for New Zealand’s National Institute for Water and Atmospheric Research (NIWA), but this organisation is now part of the New Zealand Institute for Earth Science, which is shortened to the Earth Science Institute (ESI).
Why are carbon credits allowed for pinus radiata platations which are milled or pulped and exported every 25 years or so. The end use, including export, will ultimately see the sequestered Carbon Dioxide being released.