Scimex reports: Forests that are a combination of various different species that compliment each other could store over 70% more carbon than a single-species plantation, according to international researchers who say many countries’ forest restoration plans could under-perform. The researchers analysed studies comparing carbon storage in mixed and single-species forests, and combined the results with unpublished data from a global network of tree diversity experiments. Overall, the researchers say more diverse forests have enhanced carbon storage capacity, with the greatest increase in carbon storage found in four-species mixtures compared to single-species plantations.

 

Forests with multiple tree species are 70% more effective as carbon sinks than monoculture forests

Above ground carbon stocks are at least 70% higher in mixed forests than in monocultures, with the highest carbon stocks relative to monocultures in forests comprised of four species

Forests are excellent at absorbing and storing carbon and can play a role in meeting global net zero targets. As more countries commit to forest creation, but mainly plant single species forests, an international team of researchers has examined how carbon stocks in mixed forests and monocultures compare. They found that mixed forests store more carbon, and that out of the forests assessed those with four species had the highest carbon stocks relative to monocultures.

To slow the effects of climate change, conserve biodiversity, and meet the sustainable development goals, replanting trees is vital. Restored forests store carbon within the forest’s soil, shrubs, and trees. Mixed forests are especially effective at carbon storage, as different species with complementary traits can increase overall carbon storage. Compared to single-species forests, mixed forests are also more resilient to pests, diseases, and climatic disturbances, which increases their long-term carbon storage potential. The delivery of other ecosystem services is also greater in mixed species forests, and they support higher levels of biodiversity.

Although the benefits of diverse forest systems are well known, many countries’ restoration commitments are focused on establishing monoculture plantations. Given this practice, an international team of scientists has compared carbon stocks in mixed planted forests to carbon stocks in commercial and best-performing monocultures, as well as the average of monocultures.

“Diverse planted forests store more carbon than monocultures – upwards of 70%,” said Dr Emily Warner, a postdoctoral researcher in ecology and biodiversity science at the Department of Biology, University of Oxford, and first author of the study published in Frontiers in Forests and Global Change.

“We also found the greatest increase in carbon storage relative to monocultures in four-species mixtures.”

Species richness increases carbon storage potential

The researchers analyzed studies published since 1975 that directly compared carbon storage in mixed and single-species forests, and combined this with previously unpublished data from a global network of tree diversity experiments. “We wanted to pull together and assess the existing evidence to determine whether forest diversification provides carbon storage benefits,” Warner explained.

The mixed planted forests assessed in the study ranged in species richness from two to six species. In the dataset the scientists worked with, four-species mixtures were the most effective carbon sinks. One such mix was made up from different broadleaf trees which can be found across Europe. Mixes with two species also had greater aboveground carbon stocks than monocultures and stored up to 35% more carbon. Forests made up of six species, however, showed no clear advantage to monocultures.

Accordingly, the researchers were able to show that diversification of forests enhances carbon storage. Altogether, above-ground carbon stocks in mixed forests were 70% higher than in the average monoculture. The researchers also found that mixed forests had 77% higher carbon stocks than commercial monocultures, made up of species bred to be particularly high yielding.

Forests for the future

“As momentum for tree planting grows, our study highlights that mixed species plantations would increase carbon storage alongside other benefits of diversifying planted forests,” said Dr Susan Cook-Patton, a senior forest restoration scientist at The Nature Conservancy and collaborator on the study.

The results are particularly relevant to forest managers, showing that there is a productivity incentive for diversifying new planted forests, the researchers pointed out.

While showing the increased potential of mixed forests to store more carbon, the researchers cautioned that their study is not without limitations, including the overall limited availability of studies addressing mixed vs monoculture forests, particularly studies from older forests and with higher levels of tree diversity.

“This study demonstrates the potential of diversification of planted forests, and also the need for long-term experimental data to explore the mechanisms behind our results,” Warner said.

“There is an urgent need to explore further how the carbon storage benefits of diversification change depending on factors such as location, species used and forest age.”

• Link to research (DOI): 10.3389/ffgc.2023.1226514

 

Expert Reaction

Associate Professor David Evison, School of Forestry, University of Canterbury

“This is an interesting paper which may have some application to New Zealand – diversification to minimise risk is always a good idea. However the paper had no detail on the volume of carbon being stored in the forests being studied, so we can’t yet evaluate the major conclusion of the paper against what we can do in New Zealand.

In New Zealand, a new radiata pine forest harvested at 28 years might reach an average carbon stock of 400 tonnes per hectare, in about 16 years. It is a cost-effective, known technology that sequesters carbon rapidly. A forest managed for timber production will grow vigorously, reducing risk of attack by pests and diseases. At harvest the forest provides products for construction, packaging and energy that substitute for the current fossil fuel-hungry alternatives. That is a very powerful combination,  the significance of which is not widely appreciated in New Zealand. The people who say “let’s plant forests, but with other species” are not just giving away the cost and growth rate benefits of the commercial species but also the downstream benefits of utilising a versatile and well-understood timber.

If the overarching problem is the quantity of greenhouse gases in the atmosphere, solving that problem must take precedence. The Inter-governmental Panel on Climate Change has called for immediate and drastic action, using all the tools at our disposal to reduce emissions of all greenhouse gases (including methane). We will need to use technologies that sequester carbon. Why would we ignore the part new forests can play in New Zealand’s contribution?

Reducing gross emissions is the long-term solution to the climate change problem. But most solutions that reduce gross emissions have a long implementation time, because they involve investment in new technology and infrastructure, and behaviour change. So the biggest challenge we have is reducing net emissions in the shorter term (25 years). And the biggest benefit of our planted forests is that they sequester a lot of carbon quickly. It is feasible to get to net zero by 2050 using our existing planted forest technology, by changing land use on about 15% of our farmland. If we wanted to use slower growing and more diverse species we might need to plant 30% of our farm land (or more) to reach our climate change objectives. These are the trade-offs we need to consider and decide upon, urgently. For our mokopuna.”

Declared conflicts of interest:

No conflicts.

Sources:  Scimex and the Science Media Centre