“Vaccination” with microbes: How the German forest could become resilient

LThin crowns, dying trees: the German forest is bad. Surveys of the condition in recent years show visible damage. Four out of five of the common species beech, oak, pine and spruce are affected. The main reason for this crisis is climate change, with more heat and above all more drought. Experts have been discussing how to deal with the problem for years. A study has now shown that a kind of soil inoculation with microorganisms could make forests more resistant to drought.

“The forest is currently under pressure from many sides,” says landscape ecologist Martin Wilmking from the University of Greifswald. Example spruce: The dry years 2018 to 2020 had a massive impact on the trees and favored the massive infestation by bark beetles. Especially since the species was planted outside of its natural habitat, in monocultures, as Wilmking emphasizes.

Plants around the world are confronted with climatic challenges. In order not to become extinct, many species either have to move their habitat or adapt. But trees in particular, with their long generation times measured in decades, have a hard time with this.

Experts have therefore been discussing for years how the ecosystem and the forest as an economic factor could be adapted to the future climate. Should we rely on other tree species such as Douglas firs, which provide wood quickly but can withstand more drought? Whether this is sustainable remains to be seen. Planting more resilient varieties of a species, such as Balkan beeches, is also being discussed.

Another option could be on the horizon. In the journal science a research group reports that the resistance of trees to stress depends crucially on the microorganisms – especially fungi and bacteria – in the soil. If these are experienced in dealing with certain climatic conditions, for example, then the individual tree can cope better with it.

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Specifically: if trees are brought into contact with microbial communities that have experienced drought, they are more likely to survive in a dry climate. “Associations with specific groups of microbes can play a key role in helping plant populations adapt to extreme environments,” writes Cassandra Allsup’s group at the University of Wisconsin in Madison. “Understanding microbially mediated climate tolerance may improve our ability to adapt forest ecosystems to a changing climate.”

Fungal networks supply trees with water and nutrients

In the study, the team tested the influence of the soil microbiome experimentally: To do this, they collected soil samples from twelve locations in the north of the USA, each with different temperature and precipitation conditions. They transferred these samples, together with the microbial communities living in them, to tree seedlings that were grown in various areas of the US states of Illinois and Wisconsin, both outdoors and under controlled conditions in greenhouses. The trees included various species of birch, oak, linden and maple.

The group studied over three years how the microbes originating from different climate zones influenced the survival of the trees under different stress conditions – cold, heat and drought. In general, trees were more likely to survive certain stresses when they were in contact with soil dwellers familiar with that environment. In particular, trees had higher drought tolerance when the microbial communities were from arid zones. Even after three years, the introduced microorganisms were still detectable.

The team focused on specific soil dwellers, namely mycorrhizal fungi. These fungi, which are also widespread in Europe, penetrate the roots and form a symbiosis with the plants. While they provide the trees with water and nutrients, they get carbon from their hosts in return. And in contrast to these, the microorganisms can adapt to new conditions much more quickly due to their significantly shorter generation times.

“These results suggest that soil and root microbial communities may provide a pathway for forests to become more climate tolerant,” the group writes. This applies at least to forests in temperate latitudes. However, it remains to be seen how transferrable this is to other ecosystems.

The group admits that certain groups of microbes cannot be applied on a large scale to existing forests. But in the coming decades, trillions of trees would be planted anyway, typically as seedlings. Here you can use those microorganisms in tree nurseries that are adapted to certain climatic conditions.

in one “Science” commentary writes Michelle Afkhami of the University of Miami, Soil microbes may offer a strategy to strengthen forest resilience to climate change. Before that, however, one must clarify in detail what exactly the effect is based on. The biologist writes that knowledge of these relationships is necessary before attempting to actively use soil microbes.

“We don’t know what’s really going on”

Andrea Polle from the University of Göttingen takes a similar view. The results of the tests for cold stress are less convincing. “But the probability of survival under drought stress has improved significantly thanks to the soil transplant,” says the tree physiologist. “The microorganisms seem to have a positive effect.” However, the study initially only shows such an effect for three years, which is not enough. “Then trees are just out of baby shoes.”

In addition, there is a lack of understanding of the mechanisms behind the effect. “We don’t know what really happens.” Furthermore, the team only looked at fungi and ignored other microorganisms. What influence the bacteria in the soil samples had on the observed effect remained open in the study. Lars Opgenoorth from the University of Marburg also criticizes this. Mycorrhizal fungi have been better researched and are also much easier to characterize. “We know much less about bacteria.”

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Nevertheless, the plant ecologist finds the study extremely exciting, as it provides experimental evidence of the influence of microorganisms on seedlings. “We don’t know whether the mycorrhizal fungi are really adapted to climate extremes,” he says. “But we see an effect on the plant.”

The climate-changed garden

So could “vaccinating” microbial communities help local forests adapt to climate change? First of all, according to the Göttingen expert Polle, you need reliable evidence that such an approach also helps in the long term. In addition, the study shows the climate effect only for certain tree species such as maple, cherry or ash, which are not very common in local forests. At the moment, she concludes, the application is still a long way off – not least in view of the costs involved.

“I assume that this can be an option for seedlings in the future,” says the Marburg researcher Opgenoorth. “But for forest management, it will only be part of a broader answer.” Other options include using seedlings of the same species from regions that already correspond to future climate conditions in this country. The most important thing, however, is to promote the diversity of tree species in forests.

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