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First indication of carbon uptake via fungi in young woody plants in the tropics

Researchers at the Bayreuth Center for Ecology and Environmental Research (BayCEER) at the University of Bayreuth have found the first indication that young plants of certain tropical tree species obtain part of their carbon through fungi. This mechanism could enable these plants to compensate for the low carbon uptake through photosynthesis in the shaded understory, giving them a growth advantage over other plants. The researchers have their findings in the journal Functional Ecology.

Networks of fungi and plants—so-called mycorrhizal networks, also known as the Wood Wide Web—are highly complex and the subject of ongoing debate. In recent years, popular claims have attributed to the Wood Wide Web a role as a vast communication and nutrient exchange system between trees, suggesting that plants can transfer nutrients and signals to one another.

In particular, young plants are thought to benefit from nutrients and signals provided by mature trees. However, a better understanding of mycorrhizal networks is necessary to assess these claims critically. More data, particularly from natural forest ecosystems, are urgently needed to fully grasp the complexity of these networks.

Little research has been conducted on whether, how, and to what extent young tropical trees obtain carbon not only through photosynthesis but also via mycorrhizal networks. In the recent study, Dr. Franziska Zahn and colleagues from BayCEER at the University of Bayreuth have, for the first time, investigated the carbon uptake of young plants in tropical forests through mycorrhizal symbiosis.

This study was made possible through a collaboration between the BayCEER Laboratory for Isotope Biogeochemistry and the Chair of Functional and Tropical Plant Ecology at the University of Bayreuth.

Chemical elements such as carbon occur in different forms known as isotopes. These isotopes differ in the number of neutrons in the atomic nucleus, resulting in heavier isotopes with more neutrons and lighter isotopes with fewer neutrons.

Plants prefer to metabolize lighter carbon isotopes through photosynthesis and can pass them on to mycorrhizal fungi. The fungi, in turn, metabolize this carbon and can transfer isotopically enriched (heavier) carbon back to the plants. By analyzing the isotopic composition (isotopic signature) in plants, researchers can infer the origin of the carbon.

Using mass spectrometry, the Bayreuth researchers analyzed the isotopic signatures of 41 plant species in the understory of tropical lowland forests in Panama. Their findings provide the first indication that young tropical trees may receive additional carbon from mycorrhizal fungi, which could promote their growth.

The dense canopy of mature rainforest trees allows very little light to reach the understory—yet light is essential for photosynthesis and, consequently, for carbon acquisition. Additional carbon uptake via mycorrhizal symbiosis could help compensate for the low carbon assimilation caused by light limitation.

The carbon taken up by young plants from fungi likely originates from mature trees within the mycorrhizal network. "If this is confirmed, our findings could have far-reaching implications for understanding forest regeneration mechanisms—how young plants grow and establish themselves," says Zahn.

"I firmly believe that further field and laboratory studies are needed to substantiate our initial findings and to deepen our understanding of the complexity of mycorrhizal networks using various research methods."