Nalini Nadkarni is sitting with her notebook and tape measure near the top of a strangler fig in Costa Rica’s Monteverde forest, on a branch 35 metres above the ground that is almost broad enough to walk along.

Attached to the rope she has climbed to reach her perch, she squats among mosses, orchids and other plants that grow thickly on the bark, giving the canopy the impression of a hanging garden.

Known as epiphytes – plants that grow on other plants – they are Nadkarni’s special field of scientific research, omnipresent in the unique ecosystem of Costa Rica’s montane cloud forest. They thrive between 1,500 and 1,800 metres and depend on their ability to take water and nutrients directly from the swirling mists that should cloak these slopes.

But the continued existence of the cloud forest in Monteverde, familiar to the millions of tourists who have visited it, is under threat from global warming.

Only occurring in a narrow altitude band whose upper limit is defined by where the mountain tops run out and by the altitude at which the trees can grow, it is being squeezed in a vice from below by warmer and drier weather patterns eradicating the enfolding cloud.

Amphibians that once lived here have died back, to the point of species extinction in some cases.

The breeding patterns of the area’s emblematic resplendent quetzals – the colourful birds that have long drawn tourists to the area – are being disrupted. Lowland species of bats and birds, including toucans, have begun moving into the cloud forest.

One of the Costa Rica cloud forest’s most persistent and high-profile champions, Nadkarni has become increasingly concerned about the forest’s future.

A 64-year-old American scientist who has spent almost 40 years climbing ropes to study the forest canopy, she has seen profound changes in the decades she has spent studying the Monteverde forest.

“The tropical cloud forest is a very particular type,” Nadkarni says, reflecting on the fact that only about 1% of the planet’s woodlands are cloud forest.

“Its most important aspect is the fact that it gets a lot of mist and cloud during most of the year. What’s fabulous about it is that it really nurtures this incredibly diverse and abundant group of plants that live in the canopy, which obtain their water and their nutrients from this mist and cloud.

“When I climb into the forest, when I get away from the dark, damp, windless forest floor – well, you really enter kind of a different world.”

Nadkarni identifies the montane cloud forest as one of the world’s three most sensitive along with the threatened coral reefs and ice caps.

“It is one of the canaries in the coal mine, more sensitive to climate change than others. All are indicative of a world that is experiencing climate change and that’s happening now.”

Nadkarni is not wrong. Once inside, the forest is a dark and dripping place at ground level, its perspectives shifting spookily with the passage of the mists across the rise of the continental divide.

Punctuating the green curtain are flashes of colour.

Subtle mauve bells of drymonia compete with the spiky balls of yellow justicia and the blades of vibrant red that attract butterflies and other insects.

There are sloths and monkeys in the branches, while coatis and agoutis scamper in the undergrowth.

But it is the trees that are most striking, not least the towering strangler figs with trunks like church buttresses.

The sheer scale of the diversity is explained by another facet of this ecosystem: the compression of multiple “life zones” into the narrow band of the cloud forest supports huge numbers of species, many still to be identified.

Following Nadkarni and her colleague Cameron Williams 30m up a free hanging rope into the branches, it is hard to credit that her passion for the forest tops almost killed her three years ago. Then, climbing in a forest in the American Pacific north-west, a rope severed, precipitating a 15-metre ground fall.

“You know I’ve climbed for 39 years and never had a problem. I always felt that if we take care with our equipment – with our process or protocols – that trees actually are one of the safest places to be.

“But three years ago, in the temperate rainforest of the Olympic Peninsula, my rope failed. My graduate students said that I didn’t utter a scream. I just fell like a silent sack of sand. I was knocked out for 10 minutes and I was on the forest floor for about six hours before the helicopter came.”

The injuries would have stopped most people of 61.

Five thoracic vertebrae were “exploded”. She lost her spleen as well, and her pelvis was broken in three places; she suffered nine broken ribs, lacerated lungs and a broken fibula. “I was in bad shape.”

And Nadkarni sees her accident as a metaphor for the threat to the cloud forest.

“I think that fall from a tree made me realise how fragile the human body is. By extension it relates to what I’m trying to do in understanding and communicating the importance of forests.

“You can walk into a forest that seems so robust and so healthy and so diverse and so connected and so strong and so useful and so beautiful. Just like I was – I was robust, I was vibrant, I could climb to the top of any tree.”

Because one of the paradoxes of the cloud forest, even under threat, is that it still looks vibrant even as new species move in, undermining its uniqueness.

In the research centre at the Monteverde cloud forest reserve, Nadkarni’s fellow scientist Alan Pounds, one of the first people to warn of the threat to the cloud forest from the lifting cloud base in the 1990s, explains that it is not simply a question of the cloud rising.

Instead the weather appears to be in the midst of a complex, wholesale change that has seen dry days in the forest quadruple in the last four decades. That has also been accompanied by more extreme rainfall, including flash-flooding.

“While it’s hard to quantify exactly, judging from some of the shifts in cloud forest organisms, I would say [the cloud has] shifted upward, so maybe a couple hundred metres over these last 40 years or so. What you can see looking over the years is an increasing number of dry days, in fact quadrupled since the 70s. It’s averaged over 100 days [a year] since 2011.

“What we’re teasing out in our analysis is the fact that it’s getting wetter and drier at the same time,” he says, explaining how the persistent mist has given way to drier periods, punctuated by heavy episodes of rain.

“Actually it’s becoming more like a rainforest. We just have more contrast between wet and dry periods.”

Among species challenged by these changes, Pounds – like many the Guardian talks to – mentions the resplendent quetzal which comes to breed in the cloud forest, feeding on a kind of small avocado.

Male resplendent quetzal, which breeds in the cloud forest

“It’s a good example of what we are talking about when we say it is getting squeezed because there’s no place for it to go upslope.

Pounds mentions a nearby area of woods. “I used to hear them singing every day during breeding season and now they’ve moved out of that area and they’ve become more restricted, moving higher up the mountain here.”

What is doubly ironic, given the threat, is that the community in Monteverde has long been ahead of the curve in terms of conservation efforts, in part the result of a historical coincidence.

One key element was the arrival in the 1950s of US Quakers – pacifists – who emigrated to the area fleeing the draft for the Korean war.

Daily life in Santa Elena, which borders the Monteverde cloud forest

While these new residents cleared forest for pastures for dairy farming, they protected parts of the forest to conserve the watershed. In the 1970s, as recognition of the uniqueness of the infrastructure grew, Quakers, incoming scientists and local Costa Ricans formed an informal coalition that has continued to this day as ecotourism emerged as an economic mainstay.

But if there is a catch, it is that on an isthmus bounded by warming oceans on two sides – the Atlantic on the Caribbean side and the Pacific on the other – it has been impossible to insulate the cloud forest from what is a worldwide phenomenon.

And the impact of global warming has been noticed not only in the forest but in the coffee plantations at its edges.

On his plantation, Guillermo Vargas, who doubles as the head of the local tourism council, has seen the mountains transform in his 58 years.

“It’s been over the last 30 that we have noticed the change in the precipitation,” he says.

“It used to rain – like drizzle – for many months. It used to be very cloudy most of the year. It used to be very windy with a lot of drizzle and now – like today [in November, at the end of the rainy season], it’s almost like the dry season.

“Now it is raining differently. It rains very hard so it impacts the soil, causing more erosion.

“Also, the fact that the wet and dry seasons are now not very well defined means that the coffee blooms in January through June. In the past the bloom was concentrated in a period of maybe four or five weeks in the beginning of May, June.”

That, says Vargas, has caused economic issues both for farmers and the pickers, including those who come from Nicaragua, who need to come for longer but with fewer coffee beans per day to pick, as the period of the harvest has become much more spread out.

Another issue, he adds, is that pests once just seen in Costa Rica’s lowlands have begun appearing at ever higher elevations.

It is not only the impact on coffee he is worried about but ultimately on the ecotourism that sustains many of the jobs in the local community.

“I know that for more than 40 years we have said we are the most famous cloud forest in the world. That this is unique in Costa Rica.

“We have been selling Monteverde as a cloud forest and it won’t be a cloud forest in a few decades. It will be like a rainforest. So what visitors can see here, in 2040 they will be able to see [elsewhere].”

All of which means, says Vargas, that the ecotourism industry – which the community has built from a base of 15,000 visitors a year to almost 200,000 – will also have to adapt to climate change.

“The fact that the cloud forest is changing means we have to adapt in terms of the story we tell, to be able to understand the process, and then explain to visitors about the process and still be able to demonstrate that Monteverde is a unique destination not just because of the cloud forest but because of our capacity to teach about climate change.

Down from the tops of the swaying strangler figs and on solid ground again, Nalini Nadkarni reflects on how the threat of climate change to the place where she has worked for four decades has transformed her in recent years from a scientific researcher into something more activist.

“[Climate change] is actually happening faster than we thought it was even 10 years ago. And there are certain points that might turn what’s going on in Monteverde right now to change even faster.

“You know when I first came here in 1979 as a graduate student and I looked up into the canopy of this Monteverde cloud forest and I saw these plants and animals high above the forest floor, I was just curious about what is going on. I wanted to find out about what’s going on with these canopy plants.

“But just 15 years ago I was up in a tree and I heard the sound of a chainsaw just outside the boundary of the Monteverde cloud forest, and I realised in an almost metaphorical way that in fact deforestation, human activities and their negative effects were literally right outside the border of this beautiful little reserve.”

It is an issue, she concedes, that has become even more pressing in the Trump era, where climate change denial has been embraced by the White House.

“As an American and as a scientist I am horrified. I am ashamed. I actually feel like going into denial about my own country in terms of its response to global climate change.

“All of us scientists, not just in America but around the world, know that climate change is being exacerbated. Being caused by human activities, by overconsumption, by use of fossil fuels. And for our leadership to take exactly the wrong turn, to remove ourselves from the Paris treaty, to encourage coal mining …

“What I feel I need to do is to bring my science, bring my understanding of what’s going on in the tropical cloud forest and other ecosystems to the people, to policymakers.

“I think that scientists are becoming more political. We have become less afraid to speak out against the political regimes that are making these wrong decisions. In the past, even ten years ago, my fellow scientists would not be making these statements.”

Nadkarni reflects on the change. “You know each species that moves or disappears has repercussions in terms of the ecosystem as a whole. Now the plants are a little bit harder to see. But I know when I climb in the forest, that compared with when I started here 39 years ago, the canopy dwelling plants – the mosses, the filmy ferns – they were much more abundant, much more plush, much more … just wet, than they are now.”