What’s Up with Treelines? Mountain Researchers Gather to Tackle an Age-Old Question
Treeline in the Wasatch Mountains, Utah. Photo: J. Varner

Treeline in the Wasatch Mountains, Utah. Photo: J. Varner

If you love to spend time in the mountains, the experience of emerging out of a sub-alpine forest into alpine tundra or grassland may seem commonplace. But have you ever stopped to consider why the trees stop growing exactly where they do?

I have to confess that I always marvel at the beauty and diversity of the plants above the forest, but despite being an ecologist myself, I had never given much thought to this question.

It turns out, of course, that many other mountain researchers have spent decades trying to understand just exactly why treelines exist. And whether they might change in the future.

Last weekend, 17 alpine ecologists gathered in a countryside mansion in Scotland to spend four days tackling these big questions. We put our collective heads together to synthesize data from a global experiment and to review the literature.

Some tree lines may be limited by climate, while others are limited by the availability of soils in the rocky alpine zone. Photo: J. Varner.

Some tree lines may be limited by climate, while others are limited by the availability of soils in the rocky alpine zone. Photo: J. Varner.

So, why does treeline exist anyway?

Perhaps the most classic theory about alpine treelines is that they are ultimately set by growing season temperature: above a certain altitude, there are simply too few days that are warm enough for trees to photosynthesize and grow.

And if these boundaries are set by temperature, then we might predict that global warming will allow trees to move upwards.

But that’s not what seems to be happening. In fact, in a recent analysis of treeline research around the world, just over half of treelines are advancing as predicted.

It turns out that the mechanisms that set and maintain treelines are complicated and involve many interacting biotic and abiotic factors. These include climate as well as disturbances like fire, soil type, wind, and microsite conditions.

Treelines in the cloud forest of the Peruvian Andes. Photo: J. Varner

Treelines in the cloud forest of the Peruvian Andes. Photo: J. Varner

For example, maybe the climate beyond the current treeline is suitable, but it’s simply too windy for upright tree growth. Or the soil is simply too rocky for seeds to take root. In some places, it seems tree seeds never even get down to the soil because the mosses or grasses in the tundra form a dense and impenetrable mat. Furthermore, different processes may be happening in tropical versus temperate mountains.

While many studies have demonstrated these processes at a single site, gaining an understanding of how treelines are maintained and what the future brings requires a global perspective.

And that’s just what this group set out to do.

The Germination of GTREE

In 2009, a group of researchers headed by Carissa Brown at Memorial University of Newfoundland set out to tackle some of these questions. Together, they established a Global Treeline Range Expansion Experiment, known as G-TREE. Dozens of scientists from many countries followed common protocols at treeline sites from Alaska to Australia, South America and Europe.

Current GTREE sites. Map source & learn more at: https://carissa-brown.squarespace.com/people/

Current GTREE sites. Learn more at: https://carissa-brown.squarespace.com/people/

Their goal was to disentangle the factors that limit whether new trees can establish above treeline.

At each site, the researchers carried out the same experiments – planting seeds and seedlings in the forest and above treeline in the tundra. They also created small artificial disturbances to see whether removing the existing tundra vegetation affected whether tree seeds could establish and grow.

What happens when you lock 17 alpine ecologists in a country mansion for 4 days?

Now, a few years after the first experiments were conducted, a dozen GTREE researchers gathered for a 4-day workshop in the Scottish countryside, following the Mountains of Our Future Earth conference in Perth. This group set about collating, analyzing and synthesizing data collected at each GTREE site.

The goal was to see if the same things were happening around the world. Of course, this kind of work included lots of coffee-powered spreadsheet-wrangling, fastidious data cleaning, fine-combing R code for typos, and comparing protocol implementations in the field.

Brainstorming disturbance processes and climate change impacts at treeline.

Brainstorming disturbance processes and climate change impacts at treeline.

At the workshop, a second group of GTREE researchers and other treeline enthusiasts sifted through the published literature. Our goal was to determine how disturbances like fire, grazing animals, or insect outbreaks might interact with climate change at treeline.

We scanned over 1300 paper abstracts to gather evidence of how disturbances affect treelines, identify patterns and knowledge gaps in the literature, and propose experiments moving forward.

Both teams are up to our necks in analyses and data-wrangling, so at this time, we don’t have any big findings to present… yet. But don’t worry: both teams plan to publish their results in the coming year, so stay tuned!

The GTREE and literature synthesis group, in the gardens of the Eastfield House. Photo: J. Varner.

The GTREE and literature synthesis group, in the gardens of the Eastfield House. Photo: J. Varner.

Of course, we extend a big thanks to the Mountain Research Institute and International Arctic Science Committee for funding the workshop!