Walking into a tropical jungle for the first time is overwhelming. While I’ve been lucky to do so in a few countries over the last five-ish years the wonder and amazement never fades. One characteristic that puts a twinkle in my naturalist’s eyes is the abundance of foliage. LUSH green leaves, frons, moss, lichens, of the diverse plant life, appear to crowd out each other. Layers, and layers, and layers! 

During my most recent trip to Costa Rica from December 2021 to Jan 2022, I focused on plant life more than any trip before. Because of this, I pondered many questions during the daily jungle walks. The one I’m tackling today touches on the tiny communities that thrive on leave’s upper surfaces.

It’s easy to notice how thick many of the leaves are, and also how “old” they appear to be, lingering onto their pedicles much longer than those I’m used to. I noticed how moss, lichen, and others were growing on the upper side of the leaves. What allows these non-vascular plants to grow epiphytically on vascular plant leaves? What’s the story here?

Whoa, whoa, whoa, what do some of those terms mean? 

Vascular: having vessels that circulate liquids, like the blood vascular system in humans

Bryophyte: nonvascular plants that include true mosses and liverworts

Epiphyte: a plant that grows off the ground, and on another plant (parasitically or not). It receives its nutrients and water from the air and rain

The epiphytes I’m exploring today are known as epiphylls. These are small epiphytes, typically including mosses, liverworts, lichens, and algae that grow on the surface of leaves, usually the upper surface. Check out the photos above.

The epiphylls I was noticing were most of the time on plants that are at or near the ground. This is where they thrive. But why? Leaves on slower growing plants stay longer, which are the understory trees and shrubs that are more shade-tolerant. Whereas in the canopy, leaves are exposed to the sunlight and fall sooner, therefore fewer epiphylls are present in this layer of the jungle.

“Sometimes impossible to locate the leaves of the host tree.”

-Costa Rican Natural History edited by Daniel H. Janzen

So how many species can be on one leaf? Well, one researcher went looking for that answer, and did so in a park we visited on this trip to Costa Rica, see a previous photo (Lucking, 1997)! Braulio Carrillo National Park was the study site, and within the park, Lucking’s particular site contained “83 [total] epiphyllous bryophytes…the highest species number on a single leaf was 24.” Wow! Other studies in similar tropical locals found 64, 52, and 46 total species. But these species can be tiny, with scattered distributions, or only occasionally found in the understory which may explain the potentially under-represented counts. 

First things first, a little background on epiphylls. They’re important to the ecosystem they live in, providing food, shelter, and other essential ecological roles, such as fixing atmospheric nitrogen (a sneak peek of what’s to come). They can be the primary colonizers, meaning they are some of the first organisms to establish after a disturbance. Depending on where you find yourself, whether on a mountainside or in a tropical rainforest, lichens for example are great indicators of ecological life zones. If for some unfortunate reason a habitat is degrading, or complete loss, lichen’s presence, or disappearance is an indicator of a site’s condition. This is so because lichens, as well as mosses, and liverworts rely on their “hosts” to stay around. 

Well, aren’t these epiphyll-loaded leaves at a disadvantage? How can they possibly photosynthesize? There must be something beneficial for the plant going on here. . . .

Traditionally, yes, it was viewed that epiphylls are evil. . .I mean detrimental to the photosynthetic capabilities of the host plant, even verging on parasitic. But science is ever-changing with new information, and we now know the host plants do receive some benefits! 

First off, the epiphyllous community helps prevent predation of the plant’s leaves! However, it can come at a price. Photosynthesis can be reduced by 20-30% (for a 2-year-old leaf with 55-855 epiphyllous coverage). That’s a lot to me considering normally ground-dwelling plants get only “0.5 to 5% of full sun.” Additionally, epiphylls keep the host leaf surface wet for long periods, which may increase the probability of infection (Gregory, 1971).  Also not great.

A 1970 paper by Edmisten “found that blue-green algae among the epiphylls fix atmospheric nitrogen, suggesting a beneficial input to the host leaf.” Many plants and plants’ relationships with fungi bring nitrogen, a necessary component to growth into the soil, but I’d never thought that was possible on a leaf’s surface. How NEAT! Studies have found that nitrogen fixation is recorded at much higher rates in lab settings than in the wild. In the real world, the levels of nitrogen uptake may depend on the plant’s location and light availability. It’s thought that the quick-growing leaves that require more light, the more nitrogen uptake occurs because these leaves have thinner cuticles. The opposite is true for shaded species.

So what’s the community structure like? How long does it take to form a nice little neighborhood of epiphylls? Or do certain ones become the evil neighbor everyone else is collectively scared of? 

First off, it’s understood that these little communities can develop “within 1 year” (Mezaka et al 2021). How fast! But everything in the jungle goes at hyper-speed compared to North America.

A 1996 paper on epiphyll colonization by Coley and Kursar found that “liverworts appeared to be dominant as they could overgrow lichens. We never saw lichens overgrowing liverworts.”  But those interactions could be the result of microsite variation. “Both light and humidity could contribute to the differential success of liverworts and lichens.” Liverworts are definitely in the running to earn the “overbearing neighbor” award.

On the same theme, one paper by Mezaka et al (2021) tested if communities would differ between forest-gap and closed forest sites, and if the age of the leaf matters. For the first point, algae and lichens changed their spatial associations (random, aggregated, or segregated) over time, these groups were segregated from the others by the end of the study. BUT liverworts and fungi didn’t change overtime at all. They found epiphyll density increased with time in most of the functional groups except fungi.” Their functional groups were algae, lichen, liverwort, and fungi. So the older the leaf, the greater the density, of algae, lichens, and liverworts. Neat!

In terms of young versus older leaves, it’s not what you’d expect. From a 1993 paper by Coley et al. “in species with rapid rates of colonization, leaves are completely covered in 2 yr. This would clearly put longer-lived leaves at a disadvantage and could cause selection for defenses against epiphyll colonization. Longer-lived leaves have both lower rates of colonization as well as lower accumulated cover throughout the entire leaf lifetime.” They gave an example, a 1 yr, short-lived Alseis leaves had 27% cover, whereas long-lived Ouratea leaves had only 2% cover.” So there is definitely some adapted defense mechanism being put forth by the plant. I would have thought there would be low rates of colonization across short and long-term leaves, and any differences would be defined by which species colonized initially and microsite differences in humidity/rainfall etc.. But nope! Longer-lived leaves experiences less epiphyll colonization and accumulate epiphylls less. Sucks to be a short-term leaf, but it makes absolute sense that a longer-lived leaf would evolve defenses. What those are, who knows but “long-lived leaves are well known for being better defended against herbivores and pathogens” says Coley et al.. Go evolution!

Monkeys. One factor that I didn’t consider was the presence or lack of epiphylls due to wildlife highways. A good portion of tropical wildlife typically hangs out in the canopy, or with all limbs on the ground. So it’s quite obvious that epiphytes would have trouble establishing in areas frequented by animals such as monkeys. As noted, these epiphytes need a stable environment, and getting bashed around by howler fingers, toes and tails aren’t the most stable situation. Just like wildlife paths in scrub or forest ecosystems, foliage is dampened where they frequent, so why would the tropical canopy equivalent be any different?! DUH!

Well, there you go, my dive into tropical epiphylls. I could have gone into much greater detail about each group of epiphylls as the papers I was reading definitely did, but I’ll leave that to you, if interested, as this post is LONG enough!

I’ll be continuing to investigate other botanical, and entomological observations I made while in Costa Rica, so stay tuned if that interests you. To get blogs directly to your inbox, add your e-mail to the box at the bottom of this page.

Pura Vida!

Resources:

• Dictionary.com
• Garrido, Ana, et al. “Lichen community structure and richness in three mid-elevation secondary forests in Costa Rica.” Revista de Biología Tropical 69.2 (2021): 688-699.
• Ecology of Foliicolous Lichens at the “Botarrama” Trail (Costa Rica), a Neotropical Rain Forest. III. Phorophyte Ranges and Patterns of Phorophyte Preferences By Robert LUCKINg.
• Janzen, Daniel H., ed. Costa Rican natural history. University of Chicago Press, 2018.
• Job, Joseph & R.S, Reshma. (2019). An Introductory Study on Foliicolous Lichen Found on Common Trees in Kerala, India: Characterization of its Symbiotic Partners. International Journal of Current Advanced Research. 8. 18462-18468. 10.24327/ijcar.2019.18468.3530.
• Coley P.D., Kursar T.A. (1996) Causes and Consequences of Epiphyll Colonization. In: Mulkey S.S., Chazdon R.L., Smith A.P. (eds) Tropical Forest Plant Ecophysiology. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-1163-8_12
• Mežaka, Anna, et al. “Life on a leaf: The development of spatial structure in epiphyll communities.” Journal of Ecology (2021).