Millions of monarch butterflies winter in the cloud forests of Central Mexico, seeking sanctuary in the humidity. Singing nightingale thrushes, meanwhile, settle in the treetops of Costa Rica’s Monteverde cloud forest, much like the elfin woods warbler makes its home in Puerto Rico’s misty El Yunque. But these cloud forests are threatened, research published Wednesday in PLOS One shows, and the creatures that live there will soon be in considerable danger.
Cloud forests are unique tropical ecosystems characterized by clouds that persistently engulf tropical mountains at elevations cool enough for the clouds to form. These habitats contain isolated pockets of unique biodiversity, reliant on the misty and humid environment. Climate change is a major threat to cloud forests, scientists report in the new paper.
First author Eileen Helmer, Ph.D., a research ecologist with the US Forest Service, has long suspected that climate change would affect cloud forests, but she has lacked the quantitative data required to make any actual predictions about their fate. Helmer, who has studied cloud forests for the last two decades, tells Inverse she came to a sudden realization about how to model their future.
“Finally after years it came to my mind how to do it while I was working on something else,” she laughs. What she discovered did not bode well for the diversity-rich habitats.
Helmer and her colleagues used a new approach that incorporates mapping, historical data, and future greenhouse gas emission scenarios. Mapping cloud forest areas across the western hemisphere and collecting historical data on the ecology of the mountains, they noted the change in elevation of clouds over time. Historical data allowed them to construct a computer model that predicted how cloud density would change under several possible climate change scenarios.
"Many species are going to be lost."
This process revealed a startling outcome: In 25 years, climate change could shrink and dry 60 to 80 percent of all cloud forests in the western hemisphere. And these ill effects weren’t just the consequence of extreme scenarios: If greenhouse gas emissions continue to increase at the current rate, 90 percent of western hemisphere cloud forests would lose cloud density as early as 2060. Scientists estimate density using cloud immersion, a measure of the clouds surrounding the canopy of a forest.
Climate change affects cloud immersion in two major ways, says Helmer. First, it causes ambient air temperatures to become warmer, in turn forcing clouds to form at higher, cooler elevations. Second, it reduces humidity, which causes thinner and less frequent clouds — meaning that when clouds do condense at higher elevations, there will be less of them.
When this happens, Helmer says, “many species are going to be lost.”
The unique species that live in cloud forest mountain tops have evolved in isolation and adapted to exist in high humidity. For example, many trees experience something similar to drought when there isn’t enough water vapor in the air, and the reproduction of cloud forest amphibians is heavily dependent on the air being very humid.
“Species, ranging from lichens and mosses and trees to mammals and amphibians, are all at stake,” Helmer says.
What’s also at stake is water: Some of the places where cloud forests occur are pretty dry, except at the top of the mountain where the clouds are. Cloud water vapor can supply up to 75 percent of stream water in drier places — water that humans use as well.
Helmer hopes this study is helpful for future biodiversity reports as well as studies that attempt to project the success of tropical species. Scientists are racing to determine how individual species will react to climate change: This study paints an unfortunate picture of a crucial habitat that’s home to thousands.
Clouds persistently engulf many tropical mountains at elevations cool enough for clouds to form, creating isolated areas with frequent fog and mist. Under these isolated conditions, thousands of unique species have evolved in what are known as tropical montane cloud forests (TMCF) and páramo. Páramo comprises a set of alpine ecosystems that occur above TMCF from about 11° N to 9° S along the Americas continental divide. TMCF occur on all continents and island chains with tropical climates and mountains and are increasingly threatened by climate and land-use change. Climate change could impact a primary feature distinguishing these ecosystems, cloud immersion. But where and in what direction cloud immersion of TMCF and páramo will change with climate are fundamental unknowns. Prior studies at a few TMCF sites suggest that cloud immersion will increase in some places while declining in others. Other unknowns include the extent of deforestation in protected and unprotected cloud forest climatic zones, and deforestation extent compared with projected climate change. Here we use a new empirical approach combining relative humidity, frost, and novel application of maximum watershed elevation to project change in TMCF and páramo for Representative greenhouse gas emissions Concentration Pathways (RCPs) 4.5 and 8.5. Results suggest that in <25–45 yr, 70–86% of páramo will dry or be subject to tree invasion, and cloud immersion declines will shrink or dry 57–80% of Neotropical TMCF, including 100% of TMCF across Mexico, Central America, the Caribbean, much of Northern South America, and parts of Southeast Brazil. These estimates rise to 86% of Neotropical TMCF and 98% of páramo in <45–65 yr if greenhouse gas emissions continue rising throughout the 21st century. We also find that TMCF zones are largely forested, but some of the most deforested areas will undergo the least climate change. We project that cloud immersion will increase for only about 1% of all TMCF and in only a few places. Declines in cloud immersion dominate TMCF change across the Neotropics.