Establishing new forests is not always straightforward

All over the world, governments and organisations are working to plant new forests – with the aim of counteracting climate change, doing something positive for nature and restoring some of what has either been lost or is about to be lost.

However, afforestation requires much more than just throwing acorns on a field and waiting for oak trees to sprout.

Using land for agriculture or mining for many decades can strongly affect how easily trees can establish themselves.

A new study sheds light on how the biological and physiochemical characteristics of soil that is often depleted after many years of agricultural use affect how well trees can grow. The research also shows how these biological and physiochemical characteristics are important for the insects that feed on the foliage.

“Soil previously used for farming and minis is often not immediately suitable for afforestation as it is dry, compacted and often the nutrients have leached from the soil and therefore not optimal for afforestation. We have therefore also seen examples in which afforestation has failed. This study examines what is required to change the composition of soil so that it can support afforestation,” explains a researcher behind the study, Konstantinos Georgopoulos, PhD Fellow, Institute of Biology, Leiden University, Netherlands.

The research has been published in Applied Soil Ecology.

Many factors affect tree growth

Various factors affect tree growth when land previously used for other purposes is afforested.

The biological characteristics of the soil are crucial. For example, does it have the right mix of microbes for decaying organic material and converting it into something that stimulates tree growth? Microbes are also involved in processes that produce many nutrients, such as nitrate, which then enriches the soil.

The abiotic characteristics of soil include acidity, humidity and salt content.

Finally, herbivores feed on the foliage. These can be large mammals but also insects and snails.

“All these factors influence each other. The microbes influence the abiotic conditions in the soil, and the abiotic conditions also determine which microbes thrive in the soil. All these factors affect how well trees can grow,” says Konstantinos Georgopoulos.

Trees planted in soil from recently afforested areas

Konstantinos Georgopoulos and colleagues collected soil from a series of replicated forests 10, 15 or 25 years old and used it to grow black alder (Alnus glutinosa) as a test tree since it is a pioneering species that can rapidly establish itself in new areas.

The researchers conducted experiments in which they cultivated black alder in either live soil taken from the forests or in sterilised microbe-free soil that still retained the same abiotic characteristics.

The trees grew up to 0.5 metres during the 18 weeks of the experiment. During that time, the researchers investigated the microbiological and abiotic conditions of the soil to determine how they affected the growth of the black alder.

Finally, they also investigated what happened when the larvae of the cabbage moth (Mamestra brassicae) were allowed to feed on the foliage of the growing trees.

“Black alder is inherently interesting because studies have indicated that it can fix nitrogen in the soil through symbiosis with nitrogen-fixing bacteria. This helps to enrich the soil with nitrogen, which in turn can help other trees to grow,” notes Konstantinos Georgopoulos.

Minor but meaningful differences

The study found age-dependent differences in the soil environment but not much of a difference in the associated microbes.

Konstantinos Georgopoulos thinks that this may be because the forests were all very young. He suspects that if the researchers had taken soil from a forest older than 100 years, the age-dependent microbial diversity would have been greater.

However, the researchers found some differences between the microbes and the trees.

The black alder planted in the 10-year-old forest soil had thinner stems when grown in sterilised soil versus soil containing microbes. This shows that microbes influence tree growth.

The researchers also found that trees in soil from 15- and 25-year-old forests had finer root networks and thicker trunks when the soil was microbe rich versus sterile soil.

Overall, the results indicate that the microbial environment has an age-dependent effect on the trees’ growth and growth patterns.

The researchers also found no major differences in the composition of the microbial environment related to the age of the soil.

Streptomyces and Rokubacteriales were more abundant in the roots of trees growing in 15-year-old soil, and the presence of these bacteria was positively associated with tree growth.

“This indicates that the individual forest’s tree growth depends on the unique microbiome in the soil but is not necessarily related to the age of the forest,” says Konstantinos Georgopoulos.

The researchers also observed that the larvae of the cabbage moth fed most on trees grown in 10-year-old soil.

Examining older forests

Konstantinos Georgopoulos says that the results provide insight into the potential conditions in the soil in a new forest and how they can affect tree growth.

This knowledge may improve understanding of how to create optimal conditions for afforestation on a field or other land that may not originally be best suited for the purpose.

The research also elucidates the intricate relationship between soil conditions, microbial communities and plant–herbivore interactions and how these affect tree growth during afforestation.

The next step will involve the researchers examining major changes in the soil from the earliest stages of a new forest until it is well established.

To achieve this, the researchers plan to repeat their experiment by also taking soil from forests that are hundreds of years old.

“Further research is needed before we can provide specific instructions for establishing a new forest. We also think that examining black alder more closely as a species is interesting, since it has some properties that could be especially useful in relation to afforestation. The species grows rapidly and can potentially enrich the soil with nitrogen, which will strengthen the growth opportunities for other tree species in nutrient-poor soil,” concludes Konstantinos Georgopoulos.