The mapping, highlighted on the cover of the Nature, reveals how these associations between species that are fundamental to forest ecosystems are distributed across the globe
On forest floors, some species of fungi and bacteria associate with tree roots to grow together to obtain mutual benefits. Microorganisms help plants absorb water and nutrients from the soil, sequester carbon and resist the effects of climate change. In exchange, they receive carbohydrates essential for their development, produced by plants during photosynthesis.
A collaboration of more than 200 scientists from different countries, including 13 from different regions of Brazil, mapped the global distribution of these associations between organisms of different species (symbioses), fundamental for the functioning of forest ecosystems. Based on this mapping, it was possible to identify factors that determine where different types of symbiosis can arise and estimate the impacts of climate change on these symbiotic relationships and, consequently, on the growth of trees in forests.
If carbon dioxide (CO) emissions2) remain unchanged until 2070, there could be a 10% reduction in tree species that are associated with a type of fungus found mainly in colder regions of the planet, researchers estimated.
The work, highlighted on the cover of the magazine Nature, had the participation of Carlos Joly and Simone Aparecida Vieira, both professors at the State University of Campinas (Unicamp) and members of the coordination of the Research Program on Characterization, Conservation, Restoration and Sustainable Use of Biodiversity (BIOTA-FAPESP).
“It was already known that the association between microorganisms and roots is fundamental for some groups of trees to be able to establish themselves in regions where the soil is very poor and nutrients are slowly released by the decomposition of organic matter. Mapping allows us to understand how these relationships are distributed across the planet and the factors that define them,” Vieira told Agência FAPESP.
The researchers focused on mapping three of the most common types of symbioses: arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and nitrogen-fixing bacteria. Each of these interactions encompasses thousands of species of fungi or bacteria, which form unique partnerships with different tree species.
30 years ago, English botanist David Read, professor at the University of Sheffield in England, and a pioneer in research on symbiosis, drew maps of places in the world where he thought different symbiotic fungi could be found, based on the nutrients they exploit to allow plant growth.
Ectomycorrhizal fungi, for example, obtain nitrogen for trees directly from organic matter such as decaying leaves. Therefore, Read proposed that these microorganisms would be more successful in forests with seasonal, colder and drier climates, where decomposition due to temperature and humidity is slower and litter – a layer of plant remains – is abundant.
Arbuscular mycorrhizal fungi, in turn, would be dominant in tropical forests, where tree growth is limited by soil phosphorus and in which hot, humid seasonal climates increase decomposition.
More recently, a study by another group of researchers estimated that nitrogen-fixing bacteria would be more abundant in arid biomes, with alkaline soils and high maximum temperatures.
These hypotheses can now be tested by collecting data from a large number of trees, in different parts of the planet, gathered by the Global Forest Biodiversity Initiative (GFBI) – an international consortium of forestry scientists.
The consortium is made up of, in addition to Joly and Vieira, Pedro Henrique Santin Brancalion and Ricardo Gomes César, both from the University of São Paulo (USP), Gabriel Dalla Colletta, from Unicamp, Daniel Piotto, from the Federal University of Southern Bahia (UFSB ), André Luis de Gasper, from the Regional University of Blumenau (FURB), Jorcely Barroso and Marcos Silveira, from the Federal University of Acre (UFAC), Iêda Amaral and Maria Teresa Piedade, from the National Institute for Amazonian Research (Inpa), Beatriz Schwantes Marimon, from the State University of Mato Grosso (Unemat), and Alexandre Fadigas de Souza, from the Federal University of Rio Grande do Norte (UFRN).
In recent years, researchers linked to the GFBI have carried out inventories of more than 1,1 million permanent forest plots, covering 28 thousand species of trees, from more than 70 countries, located on all continents, with the exception of Antarctica.
Inventories bring together information such as soil composition, topography, temperature and the evolution of carbon fixed in these permanent forest plots over long periods of time.
“The plots inventoried by researchers linked to BIOTA-FAPESP are located in the Atlantic Forest and include regions on the north coast of the State of São Paulo, such as Caraguatatuba, Picinguaba, Cunha and Santa Virgínia, and Carlos Botelho and Ilha do Cardoso, on the south coast” , said Joly. “We also inventoried a significant set of parcels in the Amazon through projects in collaboration with other groups.”
From this set of inventories, the researchers were able to estimate the location of 31 million trees around the world, as well as the symbiotic fungi or bacteria associated with them. Using a computer program (algorithm), it was possible to determine how different variables related to climate, soil chemistry, vegetation and topography influence the prevalence of each symbiosis.
The results of the analyzes suggested that climatic variables associated with the decomposition of organic matter, such as temperature and humidity, are the main factors that influence the symbioses of arbuscular mycorrhizal and ectomycorrhizal fungi. Symbioses of nitrogen-fixing bacteria are probably limited by soil temperature and acidity.
“Any change that may occur in the climate in the northern hemisphere could displace ectomycorrhizal fungi to other regions and result in a loss or a very large decrease in the density of these symbiotic relationships,” said Vieira.
“This can affect nutrient cycling and, mainly, carbon fixation, which depends on this symbiotic association so that forest vegetation can absorb nutrients that are scarcely available or are not in the form they need,” he stated.
Effect of climate change
In order to estimate the vulnerability of global symbiosis patterns to climate change, the researchers used mapping to predict how they could change by 2070 if carbon dioxide emissions remain unchanged.
Projections indicated a 10% reduction in ectomycorrhizal fungi and, consequently, in the abundance of trees associated with these fungi – which correspond to 60% of trees.
Researchers warn that this loss could lead to more CO2 in the atmosphere, because these fungi tend to increase the amount of carbon stored in the soil.
“THE CO2 limits photosynthesis and, in principle, its increase in the atmosphere could have a fertilizing effect. Plant species that grow faster may be able to make better use of this increase in CO availability2 in the atmosphere than those that grow more slowly. This way, we could have a selection of species. But there is still no answer to that question,” said Joly.
Another question that researchers have sought to answer is what would be the impact of the interaction of increased CO availability2 in the atmosphere with the rise in the planet's temperature in the development of plants. As temperatures rise, plants will have to spend more resources on respiration, which will increase more than the rate of photosynthesis. The balance of this balance in vegetation growth is still unclear, say the researchers.
“These questions, which concern tropical forests, are still open. Continuous monitoring of permanent forest plots will help us respond,” said Joly.
The article
The article Climatic controls of decomposition drive the global biogeography of forest-tree symbioses (DOI: 10.1038/s41586-019-1128-0), by BS Steidinger, TW Crowther, J. Liang, ME Van Nuland, GDA Werner, PB Reich, G. Nabuurs, S. de-Miguel, M. Zhou, N. Picard, B. Herault, X. Zhao, C. Zhang, D. Routh, GFBI consortium and KG Peay, can be read in the journal Nature at www.nature.com/articles/s41586-019-1128-0.