A fern thought to grow on trees still keeps a root on the ground

The Aspleniaceae is a family of ferns, some of which are terrestrial and some of which are epiphytes. A new study has found a rarity, a hemiepiphyte.

Plants grow in soil. Like most simple statements about plants, this is wrong. Some plants, epiphytes, can grow on other plants. Mosses are an obvious example, but another would be ferns. The Aspleniaceae is a family of ferns that has members that can grow either in the soil or as epiphytes. It’s not been clear how a species might move from one way of living to another, but the discovery of a hemiepiphyte might be a bridge between the two modes.

Hymenasplenium volubile. Photo James Watkins.

Jacob Watts, a co-author on the paper Hymenasplenium volubile: documentation of its gametophytes and the first record of a hemiepiphyte in the Aspleniaceae, and sophomore student, explained what a hemiepiphyte is. “The definition of a hemiepiphyte is a plant that lives half of its lifecycle upon another plant, usually the bark of a tree, and the other half of its lifecycle rooted in the soil. Hemiepiphytic ferns start as epiphytes, low on the trunk of a tree. They then grow a singular root down to the soil. When the root reaches the soil, the plant becomes terrestrially rooted, but it continues to grow up the tree like a vine until it is mature.”

Each habitat has different demands. Watts said that it’s the difference in challenges between two forms that makes hemiepiphytes so interesting to study. “If a terrestrial plant were uprooted and strapped to the side of a tree, it would not survive for too long. Likewise, if an epiphyte were stripped from the bark of a tree and planted in the soil, it too would not be very happy. These two habitats have inherently different abiotic (non-living) conditions to which specific plant species are specially adapted. There are almost no reported cases of plants that can survive in both the epiphytic habitat and the terrestrial habitat. In other words, it takes a very special plant to live in both habitats in one lifetime. Hemiepiphytes are an interesting exception to the rule that plants are specially adapted to survival in one specific habitat type.”

Watts noted that the difference between the two habitats means the same plant must be comfortable dealing with both poverty and an excess of nutrients. “The bark of a tree has lower water availability than soil because soil can store water for weeks after rain, whereas the bark of a tree is bone dry a few hours after rain. Therefore, a hemiepiphyte must be drought-tolerant, but also be able to utilize the abundant water stored in the rainforest soil. Similarly, with the availability of nutrients, the bark of a tree has far fewer of the essential nutrients that all plants need to survive than the soil. So, when a hemiepiphyte is in its epiphytic stage, it must be efficient at nutrient uptake, but when its roots reach the soil, it can draw nutrients from the soil to its leaves up the trunk.”

“In general, farther up into the forest canopy the environmental conditions are brighter, hotter, and less humid, all of which can be stressful for plants. These harsh conditions limit the dispersal of plants to the rainforest floor, not allowing most plant species to occupy the vast niche space available in the rainforest canopy.”

While a fern that can live as a hemiepiphyte might well have advantages of drawing from two habitats, it also has challenges. Watts said there are fewer than 15 known hemiepiphytic species out of the 10,000 known fern species in the world. “They can only germinate within the first meter and a half above the soil on the side of a tree in a tropical rainforest, because their roots must be able to reach the soil before the plant is too stressed out by the harsher conditions in the epiphytic habitat. Because of their low diversity and highly specific habitat, it has been hypothesized that hemiepiphytes are not evolutionarily stable and instead merely act as a bridging species between two habitats. Hemiepiphytes are buffered from having to the survive their entire lifecycle as an epiphyte, but they are also allowed to accumulate adaptations to the epiphytic niche so that their ancestors may survive entirely in the canopy, disconnected from the soil.”

The bug for hemiepiphyte hunting struck Watts almost straight away on his arrival at college. “As a first-year undergraduate student at Colgate University fascinated by plants, I joined Professor Eddie Watkins’ fern lab, where I was given the freedom to conduct my own experiments and explore fern ecology and physiology. Professor Watkins and I travelled to Costa Rica together to collect fern spores at the beginning of the first summer that I worked for him. The trip was a two-week whirlwind trip to every biome in Costa Rica. The tropical rainforest took my breath away. I was hooked on tropical plants immediately. We aimed to collect spores from as many fern species as possible. We drove a rental car from research station to research station, collecting ferns all day and preparing “spore bags” all night. When I went to Costa Rica, I did not know a single fern species. When I left two weeks later, I could name 30 to 40 genera and a few common species. The diversity that we saw was astounding and beautiful.”

“One day, we were wandering down the trail at Las Cruces Biological Station in search of new ferns for our spore collection when we came across a wet river valley containing a very unusual fern. This fern had the taxonomic characteristics of Hymenasplenium volubile, but Eddie was confused, because it was exhibiting an interesting growth habit, one that had not been previously reported for this species. Previously, people had thought H. volubile to be an epiphyte, Eddie explained. And as it turns out we documented its growth habit and found it to be a hemiepiphyte, the first known in a huge family of ferns.”

“The rush of this initial discovery and its implications have kept me interested in hemiepiphytes for more than a year now. I have read countless scientific papers on the ecology of hemiepiphytes and scoured books on epiphytic plants in search of every scrap of information that is known about hemiepiphytes to help write the paper that documents this new hemiepiphytic species.”

“I am also interested in epiphytes, because of their astounding ability to survive in the harsh epiphytic niche. Epiphytic ferns are specifically interesting, because ferns are generally thought of as limited to damp, dark environments, yet they are one of the most diverse lineages in the tropical rainforest canopy. My research has focused on the exceptional organisms that exhibit extreme phenotypic plasticity (organisms that can survive in both the epiphytic niche and the terrestrial niche). A limited number of ferns can occupy both niches because of how inherently different they are. Professor Watkins and I think that it is the exceptions that can tell us something meaningful about how epiphytes evolved from terrestrial species.”

Finding a fern in the Costa Rica rainforest is not that difficult. Finding the right fern at the right stage in its lifecycle is a very different matter. “Ferns are generally not in the most inaccessible parts of the forest,” Watts said, “but their small, independent gametophytes (the sexual stage of the fern lifecycle) are usually very hard to find. It takes getting down on your hands and knees and using a hand lens to see them. I have put my face in many dirt piles and root crevices to try to find fern gametophytes.”

“Fieldwork can be difficult, but I find it very enjoyable to spend a day out in the forest, collecting and measuring plants. The days are long, hot, and humid. The insects and snakes in the rainforest can be a hazard, but the beautiful plants and thinking about how those plants interact with their environment makes it all worth it. I have never once regretted a day of fieldwork.”

It wasn’t just the state of the fern that told Watts he had found something special. He also observed a change in the behaviour of one of his colleagues. “I could tell that this was a big discovery when Eddie kept frantically walking around with his high-powered camera speaking fast about how unusual this fern looked.”

They went on to find more juvenile plants that were growing as epiphytes and mature plants that were connected to the soil via a long, thin root from the base of the rhizome to the earth. “We rushed back to the lab,” Watts said, “to take further pictures of every stage of the fern’s lifecycle under the microscope and pressed the plant for reference for future researchers. We also called Robbin Moran, a co-author on the paper and one of the world’s experts on Central American ferns and asked him about the plant. He was shocked that we found that plant at Las Cruces because he had been looking for that fern for ten years.”

Spotting the same fern at different stages showed how the plant grew down to the ground and not up from it. This meant it could not be a root-climbing species. “Root-climbers (and rhizome-climbers) are plants that use their roots (or rhizomes) to climb up a tree or over the foliage of other shrubs to gather more sunlight in the dark rainforest understory..” Watts said. “This way they can outcompete other plants without having to support their own weight with energetically expensive materials such as cellulose and lignin, which make plant stems stronger and able to grow higher. In this way, hemiepiphytes and root-climbers are similar, as they both use trees for support and both climb higher than the understory plants around them. They germinate in the soil and grow their entire lives without disconnecting from the soil. They do have certain traits that may pre-adapt them to the epiphytic niche. For example, much like epiphytes, the roots of root-climbers grow out of the soil and may function like and epiphytic root. However, no phylogenetic study has demonstrated root-climbers in an ancestral position relative to any epiphytic group.”

“The discovery of the first hemiepiphyte in the Aspleniaceae, a family with 700 species has large implications for the evolution of epiphytism in this group. Some researchers are skeptical about the hypothesis that hemiepiphytes act as an evolutionary bridge between the forest floor and the canopy, but every new discovery of a hemiepiphyte can help to provide a better picture how epiphytes evolved.”

Watts said that this discovery highlights how much there is to find in rain forests. Yet it is a habitat in danger. “It is important to acknowledge that rainforests are threated ecosystems, by fire, climate change, and deforestation. Yet, we are still describing the diversity within these hyper-diverse forests. New species are being discovered every day, and Hymenasplenium volubile being classified as a hemiepiphyte is just a drop in the ocean compared to the diversity that is yet to be understood. Epiphytes play an important role in maintaining proper rainforest health, and almost nothing is known about how epiphyte diversity and abundance is going to change as the climate changes. As we continue to sharpen our understanding of epiphyte evolution, we can also better understand their nuanced limitations and how those limits will be pushed as epiphytic species inevitably begin to experience increasingly stressful environments.”

Further reading

Watts, J. L., Moran, R. C., & Watkins, J. E. (2019). Hymenasplenium volubile: documentation of its gametophytes and the first record of a hemiepiphyte in the Aspleniaceae. Annals of Botany. https://doi.org/10.1093/aob/mcz124