Researchers investigated how the soil biota around tree roots affect tree growth and the emission and absorption of greenhouse gases. The researchers also determined how the effects of the biota differed between young and mature forests. A researcher involved in the study says that all these factors are important in planning afforestation.
There is an increasing global desire to restore many of the forests that have been cleared in favour of agriculture and fuel production.
Understanding how to create optimal growing conditions for new forests is important – and how various conditions can negatively or positively affect tree growth, so that newly planted trees can grow as rapidly as possible into a new forest.
A new study shows how the soil biota around the roots affect how trees grow and how greenhouse gases are absorbed or emitted.
This insight improves researchers’ knowledge of what to consider and what to ignore in establishing a new forest.
“This improves understanding of the complex interactions between trees and the soil in which they grow. Even when we simplify all the soil communities, trees are still picky, and their roots associate with specific biota,” explains a researcher behind the study, Konstantinos Georgopoulos, PhD Fellow, Institute of Biology, Leiden University, Netherlands.
The research has been published in Soil Biology and Biochemistry.
Investigating growth in mature and young forests
The researchers investigated how soil conditions affect the growth of black alder trees (Alnus glutinosa) and greenhouse-gas fluxes, which are the net result of the CO2 soil and trees absorb and emit. The researchers therefore obtained soil samples from both mature forests (more than 100 years old) and young forests (about 15 years old).
The difference between these types of forest floor is that the soil biota in a mature forest has had longer to develop and become more diversified compared with the soil biota in a young forest.
Further, the researchers fractionated the soil biota from the various forests by sieving it through sieves and filters with smaller and smaller pores (250, 20, 11 and 3 μm).
The fractions contained less and less biota as the pore size declined, and the fraction passed through the sieve with the smallest pores contained primarily bacteria, whereas the others also contained various fungi and other organisms, including soil microfauna.
Finally, the researchers investigated how black alder grew in soil with the fractions from the various forests.
“Biodiversity in soil is recognised as affecting the growth of plants because the roots interact with the microbiota in complex interaction. We simplified these communities of microbiota by fractionating it to determine how the size fractions affect growth and the absorption and emission of greenhouse gases,” says Konstantinos Georgopoulos.
Trees are very selective in how they respond to microbial communities
Cultivating black alder in soil biota from fractions from forests of different ages did not affect greenhouse-gas fluxes. The soil and trees emit and absorb similarly regardless of whether they grow with communities from young or mature forests and regardless of how much these communities have been simplified by sieving.
The fractions that had only passed through the largest sieve pores also contained the same soil biota that could pass through the smallest sieve. However, this fraction contained many more different microorganisms that were filtered out in the fractions from the sieves with the smallest pores.
Analysis of the composition of soil biota around the tree roots also showed that the trees responded better to the same biota, regardless of how the researchers had sifted the total biota.
“This shows that trees are very selective in how they respond to the soil biota around their roots. This is a main result of the study,” notes Konstantinos Georgopoulos.
Young trees struggle with complexity in mature forests
The research also showed that black alder grew better in soil that contained biota from mature forests versus young forests. However, the largest fraction of microbes from mature forests negatively affected tree growth.
According to Konstantinos Georgopoulos, this probably results from greater competition between several microbes in the largest fraction versus the smaller fractions and the presence of more soil fauna in the larger fraction, and this affects the interactions between microorganisms and trees, which then affect tree growth.
Perhaps the trees must be old enough to be able to handle very complex soil biota.
“This challenges the theory that greater soil biodiversity is always better. We found that this does not always apply to new trees with soil communities from mature forests,” explains Konstantinos Georgopoulos.
Considering many aspects
According to Konstantinos Georgopoulos, the study improves researchers’ knowledge on how to optimally afforest. Taking microbiota from mature forests promotes growth.
Further, differences in the microbial composition of the soil do not affect whether trees emit or absorb more greenhouse gases.
“More research needs to be performed before this knowledge can be used in afforestation, but the research contributes to the overall sense of what needs to be considered and what is important. Many factors need to be analysed, and they can all influence how rapidly a new forest can be established,” concludes Konstantinos Georgopoulos.
