Global change under the microscope
The authors explored the impact of global warming on the microbiomes of boreal, temperate, and tropical forests. Special focus was paid to fungi, bacteria, archaea, and other unicellular organisms that are important for the cycling of nutrients. Therefore, the soil has to be thoroughly studied in order to understand the effect of global change on our forests.
The main role of fungi in a forest is decomposition. They transport carbon from roots to soil and important nutrients, mainly nitrogen and phosphorus (made in specific metabolic pathways) from soil to the roots of trees. Bacteria decompose dead biomass as well and they are also important in nitrogen cycling, because they hold several important metabolic paths, for example, its fixation from gas. Archaea are also important because of their metabolism and protista are predators of bacteria and fungi. Together, these organisms maintain the stability of a forest ecosystem, but this stability is being disrupted by global changes.
One of them is global warming, which is causing changes in precipitation, droughts, and consequently fires. The concentration of carbon dioxide in the atmosphere is increasing, so the trees absorb it at an increased pace. As a result, more carbon is transported to the microbiome, which can subsequently increase its metabolic rate. In general, however, the microbiome does not seem to be significantly affected by increased amounts of this greenhouse gas. Another consequence of global changes is increased nitrogen deposition and phosphorus limitation in the soils. The availability of these nutrients is tightly linked to carbon storage in plants.
Some ecosystem trends are highly dependent on the biome, but there are others, that are general across tropical, temperate, and boreal forests. Soil respiration increases to a maximum at 25 °C and decreases if the temperature further rises. However, no alteration to this pattern in connection to global change was observed. Drought is also a common trend across the ecosystems as a result of global change. Primary production has decreased, mostly due to the length and intensity of the drought periods. It does not come as a surprise that fungi are better adapted to water deficiency than bacteria, mostly due to their mycelium.
Boreal forests are carbon sink which means they function as an important carbon storage. This is possible mostly because they experience low temperatures, where decomposition is very slow. Therefore they benefit from the warming of the climate and longer growth periods – the hostile environment is changing to a more habitable one. However, fungi benefit from climate change as well and are more effective in decomposing. As a result, boreal forests are losing their carbon storage capacities. Unfortunately, warming brings along droughts, which is one of the factors bringing higher occurrence of fires. They destroy not only trees and other plants in the forests but also the microbiome in the soil. Furthermore, the border zone between boreal forest and non-forest ecosystems is moving towards the north. Unfortunately, this border is shifting faster than the trees can grow. As a result, the amount of carbon stored in these forests is decreasing.
Elevated levels of carbon dioxide in temperate forests are causing faster decomposition of biomass, which leads to higher amounts of usable nitrogen. It also slows down the deposition of methane by microorganisms. In general, decomposition is faster due to global warming, mainly in winter, when climate in temperate forests is similar to boreal forests. Higher temperatures in summer are causing a decrease in the biomass of the microbiome and therefore slower metabolism, decomposition, and overall lower primary production of the forest. The number of fires is rising in temperate forests as well, but when compared to boreal forests, the recovery to a pre-fire state is much faster, also thanks to special adaptations of the species.
Tropical forests are very understudied in the context of global changes. They will be probably affected the most by droughts, which will lead to slower growth of trees and carbon deposition. Tropical forests don’t face the problem of pathogen outbreaks (which are common in both boreal and temperate forests) thanks to their high biodiversity. Nevertheless, tropical forests are also facing an increasing number of fires.
Boreal forests benefit from higher productivity, caused by global warming. On the other hand, they are threatened by drought, fire, and pathogens. Temperate and tropical forests will be facing drought in the future and subsequent heightened mortality of weaker species and individuals. As a result, it is recommended firstly to plant new tree species with consideration to the ongoing climate change, which would heighten the resilience of the forests. Secondly to leave deadwood in the forests, as it contains cycling carbon. And thirdly it has been suggested to start controlled fires, which would prevent spontaneous outbursts of destructive wildfires.
Baldrian, P., López-Mondéjar, R., & Kohout, P. (2023). Forest microbiome and global change. Nature reviews. Microbiology, 21(8), 487–501. https://doi.org/10.1038/s41579-023-00876-4