Cutting down trees to save forests ... and clean heating?

Cutting down trees to save forests ... and clean heating?

Trees are often sold as the solution to all problems, because they convert carbon dioxide into oxygen and take care of climate protection all by themselves. But only if they are healthy and growing. But many forests are suffering and dying - in Germany especially coniferous forests that were once planted for timber construction. Hubert Röder would therefore like to actively convert these forests: "We should harvest high-risk stands and reforest them with trees that can cope better with stress," says the forestry expert from Weihenstephan-Triesdorf University of Applied Sciences in ntv's "Climate Lab".

If you believe Röder, we kill three birds with one stone with the conversion: a fit forest grows and binds further CO₂. In addition, felled trees remain a safe carbon store if they are used in construction. And the waste wood is suitable as a green heat source in pellet heating systems - much to the delight of the German Energy Wood and Pellet Association (DEPV), which supported the study : "I'm happy to take part in this debate," says Röder. "This is not commissioned research."

ntv.de: If we have understood your plan correctly, you want to cut down forests because it's good for the climate. Is that correct?

Hubert Röder: Absolutely not, I love forests far too much for that. But we have to look after the forest. They are suffering from climate change and urgently need support. The forest is a living and dynamic object. If we just stand by and watch climate change, it will be damaged. We are already seeing this in Germany and other countries with high timber stocks. That's why we should harvest high-risk stands and reforest them with a portfolio of different trees that are more resilient to climate change.

Where are these high-risk stands located?

Unfortunately, we have a high proportion of coniferous forests that consist of a single tree species: Spruce or pine. This is due to historical reasons. Spruce and pine are typical tree species of boreal coniferous forests, which were originally native to high altitudes or mountainous regions. But a few centuries ago, we started to grow them at lower altitudes because they were useful for many purposes - for example as construction timber. A spruce grows quickly and straight as a die. Deciduous trees, on the other hand, branch out early and only have a small amount of trunk wood that can be used as lumber.

We have cultivated large areas of coniferous forests because they are so good for building?

Correct. In Germany, there are no more primary forests and primeval forests, only commercial forests. Our ancestors began to adapt deciduous forests to their needs. They wanted to have a roof over their heads and be able to heat. This is how coniferous forests came about. But now we have the mess: coniferous forests are suffering particularly from climate change. We need to help these forests, replant them and rejuvenate them.

But now we have new needs, because trees and forests are good at regulating our climate. Locally, you always notice this on hot days when it is noticeably cooler in the forest. Globally, it is mainly the ability to store CO₂ that is appreciated. And there are many researchers who are convinced: These forests should be left alone. Nature knows best what it is doing and will also be able to cope with climate change.

I have little sympathy for colleagues who want to watch forests go to the dogs. That's not right. At the moment, the focus is on the storage function: forests bind CO₂ from the atmosphere. But what about the annual growth? What does a forest bind every year in addition to what it has already stored?

In addition?

Exactly. Every forest has stored carbon. But what's on top of that? The older our forests get, the less they actively store. The storage function gradually diminishes. This is because the trees naturally crowd each other over time and the light decreases. Some trees do not survive this competition and die. This is natural mortality. With thinning, we prevent this: we use trees that would have had little chance of survival anyway, build houses from them, stimulate additional growth in the trees that remain in the forest and help them to bind even more CO₂.

The great thing about this type of management is that on the way to a log being processed in the sawmill, there is some small wood that I can easily use for energy, i.e. for heating. This is a by-product that goes hand in hand. Timber construction and the energy supply also benefit from this forest management.

But as soon as I burn wood, the stored CO₂ escapes again. Then you have gained nothing.

Correct. The best CO₂ storage we have in forestry is wooden buildings. We should use wood as a building material much more often to store carbon for decades or even centuries. But only half of a tree trunk that comes into the sawmill ends up as a product. The other half ends up as wood chips and sawdust, in other words as waste wood. I can only use this waste wood as a building material to a limited extent, but it is excellent for energy. That's the synergy: a large proportion of the wood removed ends up in the construction sector in the long term and stores CO₂. In addition, waste wood can replace fossil fuels. The alternative would be to leave the wood in the forest, where it ends up as dead wood due to natural mortality, decays, rots and also emits into the atmosphere. The balance would be the same whether I burn wood or not - but when I burn it, I also replace fossil fuels!

But the time aspect is crucial: when we burn wood, large quantities of CO₂ are immediately released into the atmosphere.

This is the so-called carbon debt: a time lag between the emission and the point in time when new trees store the CO₂ in the forest again. This is the subject of interesting debate in scientific circles. There is a widespread assumption that I have to wait for a new tree before I can burn an old one so that the CO₂ that escapes during burning is immediately absorbed again. But that only applies to a single tree. If you look at the entire forest, this carbon debt is zero, because new biomass has already grown back by the time the emissions occur.

Because trees grow at all times?

Yes, trees are constantly growing. The carbon debt remains at zero as long as there is more growth and biomass in the forest than we emit. That is the case in Germany. We have no carbon debt. We calculated this in our study: What happens if we leave the residual wood to rot? What happens if we build wooden houses from it? What if we use the residual materials for energy to replace fossil fuels? The overall balance is best if we use the forest sustainably and do not set it aside.

Because you have to consider two components: we store additional CO₂ if we increase the growth in the forest. At the same time, we can still replace so many fossil fuels that the use of wood for energy is also positive for the climate overall.

But this calculation only works as long as we use fossil fuels? If we only use renewable energies, should we also heat with renewable energies?

For the next few decades, we will have a positive effect on the climate if we use wood for energy. This effect will diminish over time. When we no longer have fossil fuels in the energy mix, the use of biomass will be climate-neutral again.

This leads us back to the beginning of the argument: if we proactively use more wood, we can move the construction sector forward. We currently have a wood share of around 20 percent in construction. Countries like Sweden are at 55 percent. I can imagine the same for Germany, because the wood is sustainably available in our forests. And whenever I process a tree, there are always residual materials that can be used to produce insulation materials, wood-based materials or pellets. We currently produce around 3.6 million tons of pellets per year in Germany. That could easily be 5 to 6 million tons.

Your study was supported by the German Energy Wood and Pellet Association (DEPV). They must be delighted with the results.

They are not hiding this and I stand by that. The study was carried out according to reliable scientific standards. This is not commissioned research in which the results were predetermined, but rather a gap in scientific processing: the wood sector, the building sector and the energy sector are too often considered separately. Then, of course, people who love forests will try to prevent biomass from being used for energy. But you have to look at the sectors as a whole in order to take stock. I am happy to engage in this scientific discourse, I have no problems with it.

Pellet production would presumably also benefit from active forest conversion?

Sick stands are not doomed to die immediately, it's a long process. But we should proactively go into these forests, look after them and harvest high-risk trees such as old spruces, which are literally on fire, in good time in a careful and natural way. Then we can rejuvenate the forest with climate-friendly tree species that can cope much better with climate change. At the moment we are running behind climate change, it has overtaken us left and right. There is massive forest damage and clear-cutting on the land, which we cannot control. We should take this opportunity to use the high volume of wood in existing buildings.

In the medium term, would we then have more mixed forests in Germany again?

Yes, there is clear scientific evidence on this. We need a portfolio of many different tree species in one area. At the moment, we tend to have separate coniferous and deciduous stands because it is easier to manage them separately. But we need near-natural forests with three to five tree species. This provides more light, more biodiversity and also more productive forests that produce more growth.

How long would proactive conversion take?

That's difficult to say. Our current forests are 100 years old or more. We don't have that much time. We need to be faster. A normal timeframe for implementing a regulated forest conversion is 30 to 50 years. In our model, we take an optimistic approach and say that we can at least convert the older stands in the next 20 to 30 years. Then, by 2050, when the EU wants to achieve climate neutrality, we would already have forest structures that are much fitter and a building sector made of wood that serves as a second carbon sink. I clearly see this as a win-win situation.

Clara Pfeffer and Christian Herrmann spoke to Hubert Röder. The interview has been shortened and edited for clarity.

1. The study supported by the German Energy Wood and Pellet Association (DEPV) suggests that by strategically converting high-risk forest stands and replacing them with more resilient tree species, we can simultaneously promote forest health, increase carbon storage in construction, and utilize waste wood for pellet heating systems, all contributing to climate protection.
2. Hubert Röder argues that the focus on forests' carbon storage role is misguided, and instead, we should consider their annual growth and the potential to utilize waste wood for energy as part of a sustainable forest management strategy, which can help mitigate climate change.
3. The conversion of forests to include a diverse portfolio of tree species better equipped to cope with climate change is crucial, as Germany's coniferous forests, primarily consisting of spruce and pine, are particularly vulnerable to the impacts of climate change and are more susceptible to diseases, pests, and mortality.

Source: www.ntv.de