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Killer Vines Strangling the Rainforest

A large staghorn fern

A large staghorn fern (Platycerium superbum) in north Queensland rainforest. The fertile brown “patches” are spores ready to disperse.

By Mason Campbell, Ainhoa Magrach & William Laurance

Woody vines are proliferating in Australia’s fragmented tropical rainforests and threatening the existence of ferns.

If you think the story of a gigantic beanstalk holding aloft an isolated kingdom is confined to the pages of Jack and the Beanstalk, think again: real-life analogues of these green islands in the sky actually exist. And much like the mythical land atop the beanstalk, their existence is threatened.

Stroll into an Australian tropical rainforest and one of the first things you will notice are the spectacular ferns attached to the trunks of enormous rainforest trees. These ferns are epiphytes – they use the trees for support but do not parasitise them.

Epiphytic ferns were already present in the rainforest when the final vestiges of the supercontinent Gondwana were breaking up around 40 million years ago. Today they are isolated green communities suspended high in the rainforest canopy.

These ferns contain a diverse and abundant community of insects and other invertebrates. In fact, one large fern can contain as much invertebrate biomass as the rest of the tree crown. And it’s not just “creepy crawlies” that live here. One study found that nearly half of all sightings of Australia’s largest snake, the amethystine python, were in epiphytic vegetation, with 80% of them in basket ferns.

Epiphytic ferns can become immense, weighing more than a tonne and often encircling the entire stem of a large rainforest tree. Yet these often-massive ferns begin life as a microscopic airborne spore, wafted aloft until they reach a host tree. There, the ferns attach themselves to the tree via small, non-parasitic roots known as rhizomes.

The ferns then grow two different types of fronds. The first of these specialised leaves forms a basket that captures falling tree litter, which the fern uses to create its own humus and soil far above the ground. The basket fronds also form a nest that traps flowing water and protects the rhizomes from dehydration, making them self-sufficient.

The second type of frond acts both as a solar panel to capture the sun’s energy and as a reproductive organ to produce spores.

However, the days in which epiphytic fern communities can live the high life may be numbered. Our research suggests that as the rainforest becomes increasingly fragmented by human land uses, its epiphytic ferns are coming under attack.

Their enemies are woody vines known as lianas.

Lianas and Forest Fragmentation

Across the tropics, rainforests are being cleared and fragmented through human activities such as agriculture, road expansion and logging. Inside these isolated rainforest fragments, the environmental conditions drastically change. The abrupt new edges of the rainforest fragment allow in more light and wind, leading to a loss of moisture from the fragment’s interior. Stressed by such changes, many trees die from desiccation near the forest edge or are toppled by winds striking the edge.

Such changes favour an entirely new suite of plant species. Lianas are a widespread feature of healthy rainforests, but they can become abnormally abundant when the forest is fragmented. They love high-light conditions near forest edges, and use their ability to withstand long periods of drought to proliferate wildly. As a result, they often become one of the major villains in fragmented forests. They become major structural parasites of trees, suppressing their growth and reproduction and even killing them.

Although the detrimental impact of lianas on trees is relatively well-known, until recently lianas were thought to have little impact on other plants. However, our findings suggest this isn’t the case. We carefully recorded the abundance and location of lianas and epiphytic ferns in 10,000 square metres of rainforest within five fragments ranging from 18 to 9500 hectares in size. After mapping out everything, we discovered that lianas are threatening epiphytic ferns by heavily competing with them for tree hosts.

This competition clearly threatens the ferns, robbing them of their specialised microhabitats. We believe that the capability of lianas to “move” around the forest by rapidly growing both vertically and laterally – they often grow for tens of metres sidewards before heading up towards the canopy – may provide them with a competitive edge. Additionally, lianas can clone themselves wherever their stems touch the forest floor, re-rooting and re-shooting to create new individuals.

We think both of these traits are helping lianas win the contest for tree hosts in the competitive rainforest real estate market. All in all, lianas are formidable opponents, and the epiphytic ferns and their small zoos of dependent animals seem to be losing the war.

Further Study

If we are to understand how lianas impact epiphytic ferns in rainforest fragments, we must first understand the lianas. At present, liana ecology remains highly understudied in Australia. We are aiming to help fill this void by examining the response of lianas to both local-scale and landscape-scale disturbance, ranging from individual treefalls to large forest clearings.

Once these broader patterns are understood we hope to discover which liana species most impact on trees and epiphytes. Are they mostly native species that have run amuck, or do introduced species also create problems? We would also like to know if lianas proliferate steadily in fragments or increase in pulses following disturbances such as cyclones.

Answers to such questions will help us to better understand and manage lianas in fragmented forests. For instance, researchers in undisturbed Bornean rainforest recently discovered an interesting three-way relationship among Crematogaster ants, epiphytic staghorn ferns and lianas. The staghorn ferns house the ants, which in return protect their hosts by pruning any liana tips that grow close to the tree. This behaviour shields the ferns from liana competition. Although this phenomenon has not been recorded in Australia, it could potentially happen here too, as Crematogaster ants occur in our rainforests as well.

Conclusion

If we are to prevent the loss of the ancient, iconic epiphytic ferns from Australia’s rainforests, we need to protect forests from fragmentation and learn more about the ecology of the ferns and their liana competitors. Otherwise, just as the giant felled Jack’s beanstalk, these unique islands of life in the sky might be lost from our forests forever.

Mason Campbell is a PhD student at James Cook University. Ainhoa Magrach is a postdoctoral fellow at ETH Zurich. William Laurance is a Distinguished Research Professor and Australian Laureate at James Cook University.