Orchids that grow on trees in the forest canopy may access food by connecting to a network of fungal threads
Wild orchids growing on trees often rely on fungal networks to survive
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High in the rainforest canopy, a network of fungal threads links the roots of orchids in a kind of suspended, bark-bound network. This shared community of symbiotic fungi could make life away from the soil a bit more tolerable for orchids that grow on trees, and could also provide a target for conservation efforts.
The plants are reliant on fungi for survival, says Rémi Petrolli at the National Museum of Natural History in Paris, France.
“Orchids are incredibly weird,” says Nicole Hynson at the University of Hawai‘i at Mānoa, who wasn’t involved with the study. They create many hundreds of thousands of minuscule, dust-like seeds that blow on the wind, but have negligible calorie reserves. So, they depend on fungi to feed a growing seedling, she says.
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Much of what is known about these orchid-fungi ties comes from orchids that grow in soil in temperate environments, says Petrolli. But the majority of orchid species live in the tropics and are epiphytic plants, meaning that they live and grow on other, larger plants like trees.
On the Indian Ocean island of Réunion, Petrolli and his colleagues sampled roots from 10 species of rainforest orchids growing on six individual trees, three each from two different tree species. The symbiotic mycorrhizal fungi that invade the orchid’s root tissue also grow into the surrounding bark, so, the team also carved out small bark samples, making 453 bark and root samples in total, 348 from orchids directly.
The team used genetic sequencing and analysis to figure out what kinds of fungi were in the samples and how they were spread among the 10 orchid species found in the trees. By comparing the coordinates of each sample’s location on the tree trunk, they found that orchids were sharing the same fungus by way of their root systems, creating clustered networks across the bark. Sometimes, three or more different orchid species would share the same fungal mass.
Petrolli says he is surprised at the consistency of the network patterns across different trees and several orchid species. “This phenomenon could be very frequent on trees in tropical forests,” he says.
The team also found that young orchid seedlings were associated with a fungus that was linked with neighbouring adult orchids. “Seed germination may be facilitated by the presence of adult orchids, around which symbiotic fungi are concentrated,” says Petrolli.
“[The researchers] did an awesome job of showing that [orchids] can share fungi,” says Melissa McCormick at the Smithsonian Environmental Research Center in the US, but adds that she isn’t necessarily convinced that orchids are facilitating the growth of others, and that the findings could actually show competition between orchids. “If you have only so much fungus available and that fungus can feed 10 orchids, well, it could be 10 of your offspring or it could feed a bunch of different species,” she says.
The team wants to test if nutrients and water flow between orchids via their shared fungal networks. Resource sharing could make epiphytic life easier as water and minerals aren’t as reliable as they are in the soil below.
Many epiphytic orchids are threatened with extinction, says Petrolli, so if the plants share resources across their fungal networks this could aid protection programmes.
“It would be more efficient to sow seeds of threatened orchids on bark in the vicinity of adults rather than at distance or on other trees,” says Petrolli.
Journal reference: Journal of Ecology, DOI: 10.1111/1365-2745.13986
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