The changes in the treeline are so gradual, one could easily mistake them for an optical illusion. Step by step, as I climb out of the valley on the famous Grizzly Lake trail in Tombstone Territorial Park, the birches and willows grow smaller around me. The plants shrink from overhead to shoulder height until, when I hit the high point of the trail, they barely rise above my ankles.

These changes represent the world’s longest ecological border, the place where the boreal forest meets the tundra. And that border is moving: slowly, inexorably, the trees are climbing, pushed up the mountain by a warming climate.

Resting at the top, gazing down from the Grizzly Creek lookout, the forest below is dark and dense, the roots of tall coniferous trees knotting the ground under the cover of thick underbrush.

In a 750-metre climb I’ve almost imperceptibly passed through three different ecosystem types, all of which are contorting to meet the new demands and opportunities of a changing climate.

“That’s a problem,” explains botanist Paul Sokoloff of the Canadian Museum of Nature, describing the process as the “shrubification” of the Arctic.

“It’s affecting the whole ecosystem there. There are trickle-down effects to other parts of the ecosystem.”

Plants are getting bigger and bushier. They’re moving up the mountainside and deeper into the tundra. And that’s squeezing what’s already there: blocking the light, changing micro-climates, habitats and food supplies and in some places phasing out local populations entirely in a microcosm of global climate change.

The gradual change masks the planetary scale disruption of which this trail is one small part. The changes cascade down throughout the ecosystem, affecting every living thing. Some of them will keep up with the pace of change, and some will not.

Plants, like animals, find habitats that are suitable to them and make a life there. That habitat can be determined by nutrients, such as nitrogen or phosphates, by sunlight, by water, by microbial communities, by wind, by soil depth — endless factors that can mean the difference between oak, cactus or dust.

Unlike animals, individual plants can’t move around when the water dries up or the food runs low. Over time, however, their communities do move. Plants move up and down mountains, into warmer or cooler climates. They colonize new islands and disappear from others.

The climate-determined places where plants grow have been termed “cliomes.” In the Yukon, a 2016 study in the journal Global Ecology and Conservation predicted that seven of the territory’s 18 cliomes are going to disappear by 2090. One new one will arrive.

In Tombstone Park, more than half of the land will slide into a different cliome in the next 40 years. By the end of the century, more than three-quarters will be completely different from what it is today.

Scrubby trees and hardy lichens that are centuries old will lose their habitat as new colonizers invade the mountainsides, valleys and alpine meadows. Steeper parts of the territory will be affected more, and more quickly: plants that can reproduce quickly enough will be chased up the mountain, higher and higher until there’s nowhere left to grow.

The dense boreal forest of the valley will creep its way up, its spruce and tamarack crowding out the crowberry, grasses, sedges and other diminutive plants of the tundra.

Trekking through the alpine meadows, it’s clear that it’s already happening. Willow trees poke up from beds of grass and sedge, while low birch shrubs encroach on the trail.

This all happens in conjunction with the temperature changes already being observed. The Yukon saw a rise of between one and 2.5 degrees Celsius over the 20th century, causing permafrost to melt, glaciers to recede — famously even causing the Slims river to change course — and forcing changes in plant and animal patterns and behaviour.

This change could challenge the very raisons d’être of some parks. The Yukon Protected Areas Strategy and parks legislation stress the need to establish parks that protect representative samples of the territory’s biodiversity. But then what happens when the plants and animals move away from the park?

“The ecological representation on which protected designations were based may be altered and potentially undermine their conservation values,” write the authors of the paper on cliome changes.

Tombstone Territorial Park is home to more than 1,000 species, according to Bruce Bennett, coordinator of the Yukon Conservation Data Centre, which recently conducted a survey of all the plants and animals in the park. He’s on the lookout for invasive species of plants, new arrivals that can push out the existing plants.

Tombstone is only seeing a few of the invaders — but he says this is just the start.

“Invasive species tend to be more of an issue in the south,” says Bennett. “It’s a newly emerging issue in the north.”

The march of the plants is sounding a drumbeat for the rest of the species inhabiting the tundra margin. As they struggle to cope with the physical changes in their climate, the invertebrates, birds and mammals will also have to adapt to new habitats if they are to survive.

The shattered rocks of a talus slope — pulverized over millennia by the freezing and thawing of water in the cracks of the rocks — make a hellish ascent as I cross the steep Glissade Pass. Catching my breath to the side of one such slow-motion rockslide, I get a glimpse of a tiny fur ball zipping from rock to rock.

The collared pika is a small rabbit-like creature that lives amongst the labyrinths of steep talus slopes, its fur a mottled grey to blend in with the loose rock that covers the mountainside. It’s considered an indicator species for climate change, and a Species of Special Concern, in part because the tiny mammals live in isolated “islands” from which it isn’t always possible to migrate to another.

Like many mountain plants and animals, when the conditions in an area change beyond the limits animals like pikas can survive in, they have nowhere to go. Models show that alpine species are especially sensitive to climate change. Their environment is already so variable along the slope that they would have to migrate much more quickly to keep up with the rate of change.

This principle can be applied to the tundra, too — in the same way as a species can’t move past the peak of a mountain, there’s nowhere to go past the tundra.

Scientists looked at what this means for the tundra in a 2009 study. They found in some parts of the tundra, 90 per cent of the animal species that could survive there would change in the coming century. The animals living there, they wrote, ”will bear little resemblance to those of today.”

The tundra isn’t a monolith either: even within the tundra as a whole there are tiny micro-habitats, entire worlds unto themselves. Any one site drying up or being overgrown with newly arrived shrubs and trees would be devastating to the unique invertebrate communities that live there.

The pika darts in and out of the talus slope, gathering small plants to store in “haypiles” amongst the rocks — closely related American pikas will make about 13 trips per hour to gather enough food for the winter.

Just as squirrels in their rush to bury enough acorns for the winter will incidentally plant oak trees throughout a forest, the pika will collect a supply of food that could last about 350 days, resulting in decomposing plants throughout the talus that could help more soil form while also supporting insects, microorganisms and other plant growth.

But the pikas in Tombstone park are declining at an alarming rate: 15 per cent in four years. Their bodies don’t tolerate heat; American pikas will die from 25-degree temperatures, and there’s no reason to expect their northern relatives to fare any better.

If the pikas disappear from this slope, the entire ecology of the mountainside will change.

“Summer lasted three weeks this year,” an interpreter lamented at the Tombstone welcome centre, nestled into the valley between Fold Mountain and Mount Robert Henderson. Clouds were gathering on both, and reports from Divide Lake were even uglier: snow, ice, rain and hail.

Lying in my tent next to Divide Lake, listening to the heavy drops pummelling the tent a few days later, I admitted to myself she hadn’t been lying. The weather hadn’t let up as my rosy outlook had assumed it would. It was August 22. Summer wasn’t supposed to be over yet, but here I was.

A day later, snow would cover my tent as well as Glissade Pass, making the loose scree and steep rock of that climb even more treacherous than usual.

The effect of climate change isn’t just increased average temperatures. It’s variability and unpredictability. Mountain weather is always hard to forecast, but weather has begun to defy even the established trends of seasons and geography: Snow in Georgia’s peach country. Drought in Vancouver’s rainforest. Ice on my hiking poles in mid-August.

Seasons are thrown out of whack, and this doesn’t just take hikers by surprise; the animals that have for eons set their biological clocks by the reliable signs of changing seasons — shifts in daylight, for example — aren’t able to do so anymore.

A 2008 study of Greenland caribou showed how catastrophic this mismatch can be. The caribou had evolved to time their calving season to when the plants they eat were emerging, which had always been when the daylight was approaching its maximum. But the plants were on another calendar, one tied to temperature. As the temperature increased, and the daylight remained the same, the plants emerged before the caribou were ready to eat them.

“As a consequence,” wrote the authors, “offspring mortality has risen and offspring production has dropped fourfold.”

Caribou across the Northern Hemisphere have also begun starving more and more frequently as their food gets trapped under ice that forms when spring heatwaves melt the snow then refreeze it on top of their food. Other animals, like pikas, experience the same effect.

Meanwhile the expanding shrubs are providing more cover for wolves and other predators — allowing them to close in on an unsuspecting herd without being noticed — and outcompeting the lichens the caribou depend on. Simply put, climate change is hitting the caribou in almost every way imaginable. The tundra, which had once been an almost perfect habitat suitable for herds so dense they shook the ground, is becoming a minefield.

As imperceptibly as the colours are changing on their leaves, from green to yellow to orange to red, the dwarf birches begin rising overhead again as I hike back down out of the alpine valley, following the route through which Grizzly Creek drains Grizzly Lake.

The area underneath the shrubs of willow or birch contains a completely different set of plants and lichens from the area surrounding them on the tundra. The influx of these high woody plants is devastating to the rest of the plant community, according to a 2016 study, “decreasing predicted species richness, amplifying species turnover and increasing the local extinction risk for ambient vegetation.”

The micro-climate under the birch and willow shrubs that are growing across the tundra is warmer than the open plain around them. The shrubs trap heat from the sun and the wind can’t whisk it away, while at the same time the leaves and bark themselves are darker than the lichen and moss of the tundra, attracting even more heat even as they block the sunlight from reaching the ground. The result feeds back on itself, with more shrubs growing because more shrubs are growing, and on they march up the mountains and into the Arctic.

The climate conditions that once held plants like the willow and birch shrubs back are no longer doing so. A new study in Nature Climate Change has found that during the last 30 years the amount of land where the vegetation is limited by cold temperatures has decreased by 16.4 per cent — and that is only accelerating.

“Ecosystem growth is very sensitive to temperature, so as temperatures warm, we expect that plants will grow faster and be able to grow larger,” the study’s lead author, UC Berkeley assistant professor Trevor Keenan, told The Narwhal.

The study also found that by the end of the century, only 20 per cent of the land currently limited by cold will remain that way.

Shrubification is already overgrowing the grasslands in the southern half of the territory.

“It’s one of the largest slow-moving threats to southern Yukon grassland communities,” Bennett says.

Newly arrived plant species like sweet clover and smooth brome are successfully pushing further north than ever before. As the plants get established, they can change soil chemistry, redirect fires, change bird nesting behaviour and choke out their neighbours. Bennett’s recent biodiversity survey found smooth brome in the Grizzly Creek trail parking lot.

“It’s right at the northern end of its range,” he says.

This fundamental change in what the tundra looks like is causing chaos among the animals as well. The stunning arrival of beavers on the Arctic coast is thought to have been caused by the shrubification of the river valleys of northern Yukon. That arrival is having knock-on effects throughout the ecosystem, with lakes drying up, fish-bearing streams blocked off, and the native species left to contend with the changes.

Descending back into the dark forest of the valley feels like re-entering the southern world, with its still, moist air, the fragrance of lichens and Labrador tea replaced with the more familiar scent of mossy soil and pine needles. Those worlds are colliding on the hills above, plants and their dependent animals sparring in all the ways they know how — grazing by caribou, hares, voles and lemmings has been shown to slow the invasion — in a pitched battle that will eventually see this endless boreal forest and its robust trees emerge victorious over the tiny, delicate plants of the neighbouring tundra.

The world’s longest border is moving, and the identity of the North is changing. Like the inevitable dimming of the sky as clouds roll in, the change is too slow to perceive — but once the moment has come it’s impossible to miss.

Hikers walk along the Grizzly Creek Trail in Tombstone Territorial Park. Photo: Jimmy Thomson / The Narwhal