Trees have potential

Christiane Weihe

When we think about nature’s capacity to absorb carbon, a forest is probably the first thing that comes to mind. And indeed, trees are very effective carbon reservoirs. In 2021, a German woodland the size of a football pitch had the potential to store an additional 1.8 tonnes of CO2 on average. But in Germany, the forests’ function as carbon sinks is under threat – from intensive use of wood and from the die-off of entire stands as a result of extreme weather events. The Carbon Inventory 2017 described our domestic forests as a carbon sink that removes 62 million tonnes of carbon dioxide from the atmosphere annually. However, researchers at the Oeko-Institut suspect that in the years from 2018 to 2020, the forests became a carbon source. Their research also aims to identify the forests’ most severe stressors and determine how our woodlands can absorb and store more carbon again in future.

“The carbon sink service provided by Germany’s forests is likely to have changed significantly since the last Carbon Inventory in 2017 – large numbers of trees died off between 2018 and 2020,” says forest expert Dr Mirjam Pfeiffer from the Energy and Climate Division. This was caused not only by heat and drought, but also by infestations of pests such as bark beetles and beech splendour beetles, parasites and fungi. If trees are weakened by climate extremes, their immunity is reduced. “It therefore seems likely that in sum, the coniferous forests have become a carbon source. However, we won’t know this for sure until the findings of the Fourth National Forest Inventory are published later this year.”

More coniferous wood

So what is the situation with regard to the forests and their capacity to sequester carbon? And how is this impacted by the use of wood as a source of energy and material? A recent study by the Oeko-Institut and INFRO compared the projected development of Germany’s forests to 2076 against expected demand for wood. It shows that demand for deciduous wood (hardwood) will decline, while demand for coniferous wood (softwood) will increase. “This applies particularly to wood from conifers such as spruce, which is used in a wide variety of products – in construction, furniture-making, packaging and paper,” says Dr Mirjam Pfeiffer. “Our findings from forest modelling show that this high level of demand for softwoods is depleting the mature stands. This means that from 2035 onwards, it will only be possible to harvest regrowth. That leaves an annual gap of 20-25 million cubic metres of softwood, which will have to be imported.” Demand for hardwood, mainly for energy production, can be met from domestic woodlands. “Indeed, we are expecting the amount of deciduous forest to increase significantly to 2050 as less hardwood will be harvested than regrows.”

The project “BioSINK – Impacts of the energy use of forest biomass in Germany on German and international LULUCF sinks” on behalf of the German Environment Agency (UBA) applies a reference scenario and three different scenarios on the use of wood for energy. As well as considering the use of wood as a material, e.g. in construction, it examines various developments in wood combustion for energy, as well as three different assumptions relating to natural damage caused by factors such as drought, storms and beetle infestation. The reference scenario is based on the predicted development. With regard to the forests’ capacity to sequester CO2, the researchers anticipate that  in sum, coniferous forests will continue to be a carbon source until 2035. “On this scientific basis, policy-makers will be able to assess the forests’ ecosystem services more accurately. This can also support the development of policy instruments relating to the use of wood and forest management and inform the National Biomass Strategy, for example,” says Dr Klaus Hennenberg, a Senior Researcher at the Oeko-Institut. These issues are explored in more depth in the DIFENs project, supported by the Forest Climate Fund. “Here, we look at demand for wood and at climatic changes, as well as natural disturbances and forest development. But we also focus on policy requirements and possible responses from the forestry and timber sector.”

Experiments with the forest

When it comes to our forests, we face major challenges, yet we have little experience of what lies ahead, says Mirjam Pfeiffer. “We are running headlong into a situation that cannot be compared to anything we have ever known. Extreme weather, growing pressure on resources, mass die-offs – never before have there been such major changes in such a short time. That’s why there is no patent remedy for the necessary restructuring of the forests; a degree of experimentation is required here.” One option is for forest research institutes to plant native trees of diverse origins to determine which of them exhibits the strongest growth. “But it is also worth looking at the genetics. If the beeches growing at a given site are suffering to varying extents, it is worth finding out why this is happening. In such cases, we should also place our trust in the process of natural selection to some extent.” Another option that is frequently mooted is to plant more non-native species such as Douglas fir. “It grows a good 20 per cent faster than spruce and has outstanding timber properties,” the forest expert explains. “From a nature conservation perspective, however, this is a controversial approach, because we are not able to predict how this will affect other ecosystem functions. So it’s important to thoroughly weigh up the benefits against nature conservation goals; Douglas fir should only be planted in a mix with native tree species such as red beech.”

Less wood-burning

The Oeko-Institut’s researchers have also reached a clear position on the question whether forest wood should continue to be used as an energy source. “This really should be phased out. In terms of greenhouse gas emissions, it is always better to keep the carbon locked away, either in forests – if they are healthy – or in durable wood products,” says Dr Klaus Hennenberg. “What’s more, wood-burning emits 367 kg CO2 per kilowatt-hour of generated energy – far higher than the emissions from natural gas or heating oil, which amount to 202 and 288 kg CO2 per kilowatt-hour, respectively. We need to exit fossil fuels first and then phase out wood combustion – also, incidentally, because it produces particulate matter, which is harmful to health.” A good alternative to the use of wood for energy, he says, is a heat pump that runs on renewables-generated electricity.

Where do we go from here?

As natural carbon sinks, forests offer major potential for mitigating climate change. “There is scope to build up the timber stock, principally in ecologically stable deciduous mixed forests at suitable sites. Spruce monocultures in unsuitable locations should be converted to climate-resilient forests. Harvested wood should be used to produce durable goods for which recycling systems exist,” says Dr Hennenberg. Careful management of the forests benefits not only the climate but also biodiversity. “In mature and structurally diverse forests, there are more habitats available for endangered forest species such as beetles, bats, birds and fungi.” And as Dr Mirjam Pfeiffer emphasises: “We don’t have much time left – after all, forests do not spring up overnight. Not enough is being done to protect and restore the forests. And the action that is being taken is far too slow.” To ensure that trees can unlock their potential as carbon reservoirs in good time and to the full, an appropriate policy framework is therefore also required.

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Geoecologist Dr Mirjam Pfeiffer joined the Oeko-Institut as a researcher in 2022.  She works on topics such as climate stability of German forests, sustainable wood and forest use, and future development of forest stocks. Biologist Dr Klaus Hennenberg has been employed in the Energy and Climate Division since 2007. He focuses inter alia on sustainability criteria for biomass production, and modelling of developments in the LULUCF sector.