Climate Change Is Drying Out Canopy Plants—That Could Mean Less Water for the Entire Rainforest

Ecophysiologists have started to uncover the outsized role of epiphytes in moving water through tropical forests, even as these orchids and ferns dry up.

It’s a late-May morning in the misty mountain town of Monteverde, Costa Rica, and plant ecophysiologist Sybil Gotsch is climbing a very tall tree. Suited up in a red helmet and harness, she uses a rope wrench and ascenders to walk her way up a dangling line about seven stories high. “I am just below the canopy at the moment,” she says by walkie-talkie, before throwing her leg over a sturdy branch, and disappearing into the crown’s thicket of green.

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Tucking into the canopy, Gotsch, who’s based at the University of Kentucky in Lexington, enters a different world. Every branch is carpeted in orchids, ferns, bromeliads, and other epiphytes. On this morning, most look healthy. The epiphytes are pert and plump, despite a rare week without rain. But one, off to the left, catches Gotsch’s eye. It’s a shrubby species, closely related to blueberry, with long twiggy stems and thin little leaves. Today, it’s looking sick and patchy. The shrub has dropped about half its leaves, Gotsch notes, a sure sign of drought stress.

Climate change’s effects are already on display in the high rainforest canopy. Delicate, unassuming epiphytes are a “canary in the coal mine,” Gotsch says. With less water, the epiphytes begin to die, setting off a cascade of changes to water collection and distribution throughout the forest. Fieldwork and greenhouse experiments suggest impacts for not only Costa Rica, but also forests around the world. The little treetop plants offer a window, and a warning, of just how fragile some ecosystems may be—and how subtle climatic changes can, over time, have big impacts not only on flora, but also on the people and animals that depend on them.

Pitchers Made of Petals

The epiphytes’ skyscraper lifestyle is what makes them so vulnerable to drying out. They grow from mats of organic debris that flop over branches like shag carpeting. If the mats dry out, the epiphytes do too.

Under normal circumstances, plants in Monteverde shouldn’t have to worry about moisture. Situated in the middle of the country, along the spine of the Tilarán Mountain Range, Monteverde is historically very wet, with more than 100 inches of rain per year. The air is so cool and damp that low-lying clouds coalesce and slink through the trees to create a cloud forest ecosystem, one type of high-altitude rainforest.

But since the 1970s, Monteverde has grown steadily hotter, and the number of dry days has quadrupled, from 25 to more than 110 (1). The clouds are lifting higher on warm currents of air and drifting out of reach of the epiphytes, caused in part by climate, and in part by land use change. Adding insult to injury, thunderstorms, when they do arrive, are more intense. Sudden pounding rains quickly fill epiphytes and soils to their storage capacity. Much of the rain doesn’t soak into anything, instead running overland, leading to flooding and erosion.

Fieldwork and experiments over the last 10 years show that epiphytes in rainforests worldwide will be the first plants to die in a hotter, less-predictable climate—the very same plants that play an outsized role in keeping the rainforest wet (For short film, see Movie S1). In Monteverde, the epiphyte canopy absorbs rain, fog, mist, and dew, then slowly drips that water down to the forest floor, helping the ecosystem retain crucial moisture. Losing these plants could spell the beginning of the end for cloud forests, shifting the region to a drier habitat.

“The very attributes that have enabled epiphytes to thrive in forest canopies in places like the Monteverde cloud forest now make them vulnerable,” says Nalini Nadkarni, a forest ecologist at the University of Utah in Salt Lake City. The physiological ability to use nutrients dissolved in rain, mist, and clouds allowed epiphytes to thrive in the canopy. But as climate change brings longer dry seasons and reduced moisture, those same traits make the plants more vulnerable to water and nutrient stress than terrestrially rooted species, Nadkarni says.

Building on a decade of work, Gotsch, as principal investigator (lead PI), and a team of co-PIs including Nadkarni are now climbing into 20 trees around Monteverde to learn how epiphyte losses will change the landscape, both forest and pasture. Sensors affixed to these trees measure how humidity, temperature, sunlight, and other factors vary in canopies with and without healthy epiphyte communities. It’s all part of an overarching goal: Predict future impacts by incorporating these sky-scraping gardens into models of water cycling in Monteverde—models that could potentially be applied to other rainforests as well.

Links in a Great Chain

For most of the last century, ecologists didn’t think epiphytes did much. Tropical orchids, ferns, and mosses aren’t harmful to the host trees, nor do they have obvious benefits. For many, conventional wisdom held that these plants provided habitats for a variety of birds, insects, and amphibians, but were not particularly important beyond the canopy.

Those ideas “just never made sense to me,” Gotsch says. As she talks, she’s examining the leaves of a Clusia epiphyte, a big plant with paddle-shaped foliage that looks like a cross between a fig tree and a cactus. It’s one of hundreds of epiphytes, growing in long wooden bays on a Monteverde hillside in a shade house—similar to a greenhouse, but with a roof and walls made of netting.

Since 2012, Gotsch has led a team from the United States and Costa Rica to learn about the basic biology of Monteverde’s epiphytes, especially how vulnerable they are to drought. Their work includes more than a dozen studies. For example, the team has exposed the shade house to severe, month-long drought and found that shrubby epiphytes lose their leaves, while succulents like Clusia drain their water stores (2). They’ve also found, reassuringly, that most epiphytes bounce back from short droughts within a few weeks of rewatering.

In the wild, however, there is no such return to normalcy. Hotter, drier conditions have knock-on consequences, including fewer low-lying clouds in forests from Mexico through Argentina (3, 4). Epiphytes can absorb fog through their leaves even when their moss mats dry up, so losing rain and clouds is a double whammy. Indeed, in a study published in 2022, the team droughted epiphytes in two shade houses in Monteverde, one immersed in clouds and one at a lower elevation (5). The plants immersed in clouds “fared much better,” says lead author Briana Ferguson, an undergrad research assistant in Gotsch’s lab at the time. Epiphytes in the lower shade house shut their stomata (leaf pores that control gas exchange) to conserve water. By the end of the 10-week experiment, most were dying—”touch them and they crumbled,” Ferguson says. By contrast, epiphytes in the cloudy shade house kept their stomata open and continued absorbing water, remaining plump and hydrated. The findings suggest that while losing rain is bad, losing mist might be worse.

The combination of drought and especially cloud loss could explain why epiphytes are already dropping leaves in the local canopy. The findings have been so foreboding that ecologists started wondering what losing epiphytes would mean for the rest of the forest, particularly for the flow of water through the system. Epiphytes can swell up to 3,000% of their dry weight when wet, so they’re potentially holding a lot of moisture (6).

Pouring Sky to Soil

For hydrologists, the water cycle is akin to a series of tipping pitchers, each one filling and emptying as it pours into the next—from clouds, to tree leaves, to bark, to the soil. As recently as a few years ago, the widespread assumption, based on just a few studies, was that wild epiphytes were always soaking wet. If this were true, then new inputs of rain, fog, dew, and mist would basically flow right over them, down to the soil. Hydrologists needn’t include epiphytes as an important bucket.

Gotsch and ecohydrologist John Van Stan, at Cleveland State University in Ohio, didn’t buy it. Dry epiphyte mats were common enough that Gotsch climbed with goggles to keep loose fluff from stinging her eyes. In a 2019 paper, she, Van Stan, and coauthors published data from moisture sensors mounted in canopy mats in Monteverde, demonstrating that epiphytes frequently dried out in the few days after a rain or heavy mist (7).

The study included a dynamic vegetation model called LiBry (pronounced “library”). It estimates epiphyte biomass in a given location using climate data, natural disaster frequency, and habitat data, such as wooded area and leaf area. Then, inputting estimated epiphyte water storage capacity, weight, and known evaporation rates for the region, the model simulates how often the plants should be soaked and how often they should be dry (6). “Every hour, we get an estimate of how full the epiphyte bucket is,” Van Stan says. Consistent with Gotsch’s Monteverde field data, LiBry suggested that most epiphyte communities in humid zones spend about 15% of their time near saturation and about a third of it dry. Water fills the mats and pours from them, “which means it’s feeding other parts of the forest,” Van Stan says.

Sowing Seeds of Change

If epiphytes are “the connector between sky and earth,” as Gotsch puts it, then the water cycle is bound to change as these plants disappear. In 2021, Gotsch and three coleaders were awarded a four-year NSF grant to find out how.

But how to study the undoing of a canopy ecosystem? One strategy: intentionally unravel a little of it and then record the consequences, explains co-PI Todd Dawson, a tree physiologist at the University of California, Berkeley, as he stands between two fig trees in a Monteverde pasture. One fig is carpeted in epiphytes. The other has been totally stripped of them. Both trees are strung with cables and instrumented with moisture detectors, anemometers, and other sensors. Every 15 minutes, these various instruments record temperature, humidity, wind speed, sunlight penetration (solar radiation), and wetness of the tree’s leaves. They capture the hyper-local conditions in that tree’s canopy.

Last summer, a team of arborists and researchers climbed into this pair of trees, and nine other pairs around Monteverde, as part of the largest epiphyte removal experiment ever. One tree in each pair was totally stripped of epiphytes, its bark scrubbed clean with a boot brush. The other tree was left intact as a control. Every two weeks, a crew of field techs returns to each tree to upload the data from a logger box mounted to its trunk. The crews will continue monitoring through September before switching to data analysis full time to compare conditions in canopies with and without epiphytes.

Though the project doesn’t yet have published results, some differences are already apparent in the data. The wind speed through the trees, their leaf wetness, and sunlight penetration into the canopy significantly differ between experimental trees and controls, Gotsch says. Standing on the ground, looking up at the two figs, you can see it. The control tree, covered in epiphytes, is dense, imposing, and wet. Hardly any sunlight slips through. Across the pasture, the stripped tree looks like it belongs in a city park, its bark bare and visible.

Soaring Over Treetops

Ideally, researchers would harvest data from far more than 10 pairs of trees. But stripping experiments are time intensive and destructive. So, Dawson uses a drone to help estimate how large swaths of trees are faring across the forest.

Outfitted with a thermal sensor and several cameras, the drone captures temperature, as well as the visible light spectrum and five bands of reflected radiation bouncing off the trees. Zig-zagging over several hundred trees at three research sites, the drone captures the spectral reflectance of the canopy and measures all the wavelengths of light that the canopy does not absorb.

On his laptop in a sunlit cabin in Monteverde, Dawson analyzes the data to calculate, for instance, the water content in the crown of the tree. The key: quantifying the amount of light from the red edge spectrum, around 780 nanometers that’s absorbed or reflected by each tree’s canopy. Water absorbs red edge radiation particularly strongly, so Dawson can extrapolate the crown’s water content. It’s just one metric to assess how well the entire canopy is doing. Combined with data from the 20 stripped and experimental trees, the drone can paint a broader picture of how forests may perform under climate change.

All of these data are building toward a hydrologic model simulating water flow from the sky, through the forest, and down to the ground. “We’re really interested in the role that plants play in altering how much water makes it down into the soils, and then how much gets released back into the atmosphere,” says co-PI Lauren Lowman, an ecohydrologist at Wake Forest University in Winston-Salem, NC. Epiphytes haven’t really been represented before in hydrologic models.

The goal is to represent the epiphyte community as a bucket in the forest that stores and pours water. One set of inputs fills the bucket, and another set of outputs drains it. Water can enter the epiphytes through rain, mist, fog, and dew, Lowman notes. Water exits via evaporation, transpiration, or absorption by the host tree. The first goal is to represent the process of filling and emptying of the epiphyte mat, Lowman says. Next, likely starting this fall, they’ll model the interaction of the tree with the epiphyte mat. And then, finally, using the drone data in 2025, she and collaborators hope to model many trees interacting with many mats, to create regional scale hydrology models.

Eventually, Lowman and her team hope to model the entire water cycle for cloud forests in Monteverde, and ultimately any ecosystem where epiphytes grow. Describing the epiphyte bucket is just the first step, Lowman says.

Rivers in the Sky

While the work in Costa Rica is the most extensive of its kind, a variety of smaller studies from around the world, including the Pacific Northwest and Taiwan, also hint that epiphytes have a major water storage role in humid environments (8, 9).

Along the Chilean and Peruvian coasts where the desert meets the sea, rainstorms are a once-in-a-decade event. Cacti, shrubs, and trees survive on sea fog and are covered in lichens and air plants, nonvascular epiphytes. Working in 2010, ecosystem ecologist Daniel Stanton stripped epiphytes here from a handful of plants at several sites, leaving untouched cacti and trees as controls (10). Stanton, who’s based at the University of Minnesota in Saint Paul, then measured humidity and temperature at the surface of the host plants, as well as soil moisture in the week following a rare rainstorm. Stripped plants were drier, hotter, and thirstier in the week after the rain. They sucked up soil moisture at close to double the rate of water loss under controls.

Three years ago, Stanton published work estimating the biomass of mosses and lichens at field sites in Minnesota. He then converted that biomass into an estimate of water storage among nonvascular epiphytes. They could store 5 to 10% of a typical rain event, he found (11). “It’s on the edge of enough to probably matter hydrologically,” Stanton says. If the epiphytes were to decline in Minnesota—for example, in response to climate change—it would likely make the forests “flashier,” he adds, meaning that water would rush straight into the soil and leave the system quickly, just as is happening in Monteverde. All this is to say that rainforests aren’t the only places likely to change if epiphytes disappear.

Flashier, drier forests have serious implications for the people who live nearby. More intense thunderstorms cause flooding. Less water slowly soaks into the groundwater table and the aquifers. Surrounding communities feel the squeeze on their water supply.

What can be done? One simple answer: more native trees. Their roots stabilize the soil, slowing erosion, and their leaves help trap some moisture in the forest. Already, the nonprofit Costa Rican Conservation Foundation gives native trees to farmers at no cost. “Our main mission is to replace forests as close to their natural state as possible,” says conservation biologist Debra Hamilton, the organization’s co-founder. Over the past 26 years, it’s given away 300,000 trees, all along the Pacific Slope of Costa Rica. Forest technicians, all local and self-trained, collect the native seeds and tend the baby trees, including some endangered species, in nurseries. Then farm owners come collect them, often for windbreaks on cattle farms or to reforest abandoned pasture.

While replacing habitat is the main objective, planting trees may also help cope with epiphyte loss. New tree roots help stabilize hillsides and slow the gush of water running off the mountains. “We need these tall emerging trees to be catching any of the moisture that they possibly can from the air going by,” Hamilton says.

Studying a system that is already climate-impacted “really forces me to think about ways we can contribute to solutions,” Gotsch says. She hopes that documenting epiphyte losses is a first step to dealing with climate change’s pernicious effects. If her research helps the community in Monteverde anticipate, say, a 10% loss of water storage in the region, that gives at least a baseline for mitigation efforts, she says. This latest grant will run through 2025.

It’s the evening after her May climb, and Gotsch is standing in the driveway of her Monteverde house. At long last, it begins to rain. Droplets fall on eaves and gravel, and in the distance, misty clouds coalesce through the mountain peaks. Water is in everything here. Questions about what will become of it are especially obvious, Gotsch says, “in the hyper-green, hyper-mossy, hyper-wet” tropical montane cloud forests. But those same questions apply all over the world. “The issues,” she says, “are the same really everywhere, I think.”