Deadwood-decomposing fungi feed germinating orchids, providing the carbon their tiny seeds don't have. The Kobe University finding not only closes a gap in our understanding of wild orchid ecology but also uncovers an important carbon flux in the ecosystem.
Orchid seeds are as small as dust and do not provide any nutrients for the young plant to grow. The adult plants are known to rely on a certain type of fungi that develop structures within the plant's roots, but whether these same fungi also help with germination has not been established. "Studying orchid germination in nature is notoriously difficult. In particular, the painstaking methods required for recovering their seedlings from soil explain why most earlier studies focused only on adult roots, where fungi are easier to sample," explains Kobe University plant evolutionary ecologist Kenji Suetsugu.
During fieldwork, Suetsugu's team noticed a strange pattern. He says: "We repeatedly found seedlings and adults with juvenile root structures near decaying logs, not scattered randomly in the forest. That recurring pattern inspired us to test whether deadwood fungi fuel orchid beginnings." These juvenile root structures are coral-shaped rhizomes and have been interpreted as a seedling's organ retained into adulthood -- and they are often associated with wood-decaying fungi rather than with those found in adult orchids without these structures. Being experts in orchid ecology and evolution, Suetsugu's team took on the challenge to find out who feeds the young orchids.
In the journal Functional Ecology, the Kobe University team report that amongst seeds of four model orchid species they buried in various forest locations, they observed germination only near decaying logs, and that the seedlings virtually exclusively associated with wood-decaying fungi. "We were struck by how exclusive and consistent these fungal partnerships were. There is an almost perfect match in the fungi that seedlings of a given orchid species associate with and the fungi on adult plants with coral-shaped rhizomes of the same species. We think that the plants without coral-shaped rhizomes shift to other fungi as their nutritional needs change during growth and the carbon source offered by rotting logs dries out," says Suetsugu.
Among the relatives of the orchids the Kobe University team studied, there are many species that have independently evolved full mycoheterotrophy, that is, they have abandoned photosynthesis and instead feed on fungi throughout their lives. "The propensity of these orchids to maintain their association with wood-decaying fungi into adult life probably facilitated their evolution of full mycoheterotrophy," Suetsugu says.
In the paper, the team writes, "As woody debris represents a major carbon source in forests, associations with wood-decaying fungi may enhance carbon acquisition, especially in warm, humid habitats." Suetsugu adds: "For conservation, our results mean that protecting orchids in the wild is inseparable from protecting deadwood and its fungi. For ecological sciences, they reveal a hidden carbon route from deadwood to green plants, explaining how seedlings can establish themselves on dark forest floors. And they show that deadwood is not dead -- it is a cradle of new life."
This research was funded by the Japan Science and Technology Agency (grants JPMJPR21D6, JPMJFR2339), the Japan Society for the Promotion of Science (grant JP25H00944) and the Research Institute for Humanity and Nature.
