Hard times for hemiepiphytes: Aroids have trouble making a comeback in second-growth forests

Estefania Fernandez Barrancos is a PhD student and Christensen Fellow at the University of Missouri St. Louis, where she is affiliated with the Harris World Ecology Center and the Center for Conservation and Sustainable Development at the Missouri Botanical Gardens. Estefania has previously written about how to restore bromeliad populations. Here she describes a recent study asking how well hemiepiphytic aroids recover in secondary forests in Panama.

Most people know aroids as the familiar swiss cheese plants found growing in hotels and shopping malls. But few people realize that the aroid family (Araceae) is the fifth most diverse plant family on Earth. These plants provide essential food and refuge for birds, bats, insects, and primates in tropical forests throughout the world.

Like many other plants, aroid populations are dropping because the rainforests where they live are being converted into farms. My new research shows that aroids are also slow to recolonize new forests that become available.

City Aroid Country Aroid

City aroid (left, Monstera deliciosa in a building), country aroid (right, Monstera sp. in a Colombian forest). Photo sources: Left Maja Dumat CCBY 2.0; Right – Thomas Croat via Tropicos.

Before I describe that research though, here is some botanical jargon for the uninitiated. Epiphytes (a.k.a. air plants) are plants that grow on other plants (but not as parasites). Hemiepiphytes are plants that grow on other plants but only for part of their lives. Many aroids are hemiepiphtyes because they start life in the soil of the forest understory and grow until they find a tree. Then they climb up the tree and live above the ground, but they always keep a connection to solid earth.

To study their recovery, I surveyed hemiepiphytic aroids in native tree plantations (9-years old), natural secondary forests (8-14-years old), and mature forests (>100-years old) near the Panama Canal. These forests are part of Agua Salud – a tropical forest restoration experiment led by the Smithsonian Tropical Research Institute. In the dense forest, I found aroids by looking for their stems coming down from the trees, then I followed the stem with binoculars until I found their leaves, which helped me identify the species. In all, I surveyed 1479 trees this way.

Estefania surveying mature forest tree Panama

Estefania Fernandez (below) and field assistant Carlos Diaz (above) look for aroids in a mature forest tree in Panama.

I found out that there were virtually no aroids in secondary forests or plantations. I recorded more than 2000 aroids from at least ten species growing on trees in mature forest, but in secondary forests and plantations I found less than 1% as many aroids and only three species.

Why do aroids have recovery troubles?

One reason for the lack of aroids could be that seeds from adult aroids in mature forests can’t reach the new forests. This seems unlikely because all of the secondary forests and plantations in my study were close to mature forests full of aroids, less than one kilometer away. Also, birds that are present in secondary forests are known to eat aroid fruits and disperse their seeds.

Another reason could be that the young forest canopy is too open for aroid seeds to germinate and grow. Unlike most plants, some aroids start out life growing away from light and towards darkness. (This has another great word: skototropism). It seems counterintuitive since most plants need light. But it is actually a good strategy. By growing away from light, aroid seedlings are more likely to run into a tree, which they need to climb up into the canopy and get to the light that they need to photosynthesize. So it is possible that there is too much light in the young forests and it keeps the aroid seedlings from finding a host tree.

Whether dispersal or establishment limits aroids in secondary forests, it is likely that more time will help. As forests become older and darker and birds bring in more seeds, aroid populations should eventually begin to recover. My research suggests that there is a considerable lag time required for aroids to recolonize disturbed habitats such as secondary forests and plantations.

More importantly, my study highlights how important it is to hold onto old forests. Forest restoration is a poor substitute for mature forest conservation. To the extent that we can prevent older forests from being cut down, it will help preserve many species of aroids as well as other plant and animal species that are threatened by habitat loss.

Aroid being pollinated by scarab beetles at Barro Colorado Island, Panama. Source: www.aroid.org.

You can read more about Estefania’s research in her new open-access paper in Tropical Conservation Science, or on other posts from Natural History of Ecological Restoration (here and here).

El Niño’d: tropical field research when climate won’t sit still

Steve Roels is a PhD candidate in the Department of Integrative Biology at Michigan State University. His research asks how trophic cascades interact with tropical forest restoration. When not in Panama, he enjoys documenting biodiversity and restoring native vegetation on his own 6.2 acres of Michigan.

Tropical field biology has a lot of uncertainty built into it. The scientific community is still barely scratching the surface of tropical biodiversity and the immense complexity of biotic interactions (relationships between organisms). Biologists, myself included, often get lulled into thinking of the tropical climate as a stable abiotic backdrop that lies behind the great drama of biotic interactions. But what happens when those abiotic conditions change abruptly and dramatically?

The current El Niño event in the Pacific is now regarded by meteorologists as one of, if not the, strongest El Niño events ever recorded. The North American media understandably focuses on how El Niños affect our continent; usually wetter West Coast winters and dryer, warmer Midwest winters. What many North Americans don’t realize is that El Niño events have their most profound effects on Pacific countries in the tropics.

El Niños are one extreme of a much larger climate pattern, the Southern Oscillation. The El Niño-Southern Oscillation (ENSO) is an erratic seesaw of Pacific surface water temperatures from warm to cool (La Niña events) and back again. Temperature swings from one extreme to the other occur every few years (on average about 5) and “the switch” is often flipped very abruptly, shifting ocean currents, air pressure, and precipitation throughout the eastern Pacific. It is important to keep in mind that ENSO events are not “bad” per se, just different, and that creates biological winners and losers.

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Strong El Niño conditions in the eastern Pacific during my field season. Image from: www.ncdc.noaa.gov.

In central Panama, where I research bird communities in forest restorations, El Niño conditions generally bring warm coastal waters and drought. I say “generally” because each El Niño is like a snowflake—there are some basic patterns, but every event is unique. This El Niño is sticking to the pattern: central Panama is currently experiencing a severe drought and creating headaches for many of my colleagues at the Smithsonian Tropical Research Institute (STRI). The drought has played havoc with the frog biologists, who are waiting for mating frogs, who have, in turn, often been waiting for rain. Coral researchers are scrambling as abnormally warm waters cause coral bleaching. However, some scientists view this El Niño as an opportunity because it could be considered a proxy for future climate. The El Niño is compounding the warming effects of global climate change, putting 2015 on track to be the warmest year on record. A friend of mine who studies tree physiology and water use in forest restorations says she is getting great data. After all, a key challenge for restoration ecology is deciding what we restore to. An ecosystem that tries to match what was formerly present? Or one that will continue to thrive in an uncertain future?

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The Agua Salud restoration site. The blocks of vegetation in the landscape are different experimental tree planting treatments. Lake Gatun, part of the Panama Canal, lies in the haze on the horizon. Lake levels are anticipated to drop to record lows this dry season.

The effects of current El Niño on my own research are difficult to assess. I study trophic cascades (basically, ripples in food webs) at STRI’s Agua Salud forest restoration project, especially focusing on birds, insects, and trees. I conducted an experiment this past July-August, which is normally the heart of the wet season, but was instead a historic drought. How this drought effected tree growth, insect populations, and bird behavior—all components of my study—is hard to say. Prior research on ENSO effects on trophic relationships is limited (it’s hard to plan research around an unpredictable and irregular event!) but some long-term studies have found large ENSO effects on food webs in Panama and Chile.

When I returned to the United States after my field season and talked with my research advisor about the uncertainty the El Niño brought to my study, she said, “You’re going to hate me for saying it…” I replied, “I already know what you’re going to say.” Maybe I need to do the experiment again next year.