Running up the hill – Climate change is forcing plants to move upwards

Imagine yourself going for a hike in the mountains on a sunny day. While enjoying the fresh air, you will pass by beautiful grasslands – full of flowers adding specks of white, red, yellow, and blue amongst the greenery.

If you decide to take a break in the middle of this landscape, you will be able to observe insects visiting flowers, such as bees and butterflies, or birds feeding on the insects, berries, and seeds.

Mountains are important recreational areas for us humans, where we can relax or challenge ourselves on bikes or skis. But mountains and their grasslands play a crucial role for nature in many other aspects.

Carbon storage in the mountain soil

Interestingly, many biodiversity hotspots are found in mountains. One potentially surprising fact is that not only tropical rainforests store large amounts of carbon, which can help to regulate the climate, but also the mountain soil in our region does.

We therefore don’t have to travel across the globe to observe all these important functions and processes, we can simply find it right in front of our door. However, this idyllic scenario is jeopardised by climate change.

Illuminating the mystery of moving plants

The Norwegian climate is getting warmer, which can have implications for the vegetation. Warming is forcing plants to move higher up in elevation to keep up with their optimal temperature range, their comfort zone. We can call them 'runners.'

One prominent example you might have noticed of the upwards movement of plants in the mountains is the treeline. Trees are now reaching higher elevations, where they previously could not establish themselves. All the smaller plants, like herbs and grasses, are facing the same challenge.

The escape strategy works if there is enough space. Plants on the mountain peaks are meanwhile restricted by the problem that they can’t move higher but must stay and cope with the novel conditions. These are the 'fighters.'

This creates a dual challenge: Being exposed to warmer temperatures and novel plant neighbours that are climbing up to escape their fate in the lower elevations. Science shows, however, that there are lags in the upslope movement. An explanation for these lags is biotic interactions with other plants.

Are mountain plant communities more resistant than expected?

A good way to investigate processes happening with climate change is through experiments. Mountains are an amazing system for studying climate change because temperature changes rapidly with elevation. This makes it easy to mimic future conditions for plants by planting them higher.

With the RangeX project, we want to understand how plants move and what that means for the whole ecosystem. Here, we accelerate the upslope movement of plants by transplanting seedlings of local lowland plants into the mountains.

What is new in our project is that we disentangle the direct effects of warming from the indirect effects of interactions with neighbouring plants.

For this, we placed a mini greenhouse on top of the plants to simulate warming and/or removed the vegetation in some parts. It's like weeding the garden, where we only keep the plants we want. Examples of Norwegian species are oxeye daisy, red clover, and spotted St. John's-wort.

We expected that temperature is the main driver of upwards movements. However, our findings are intriguing because we observe that it matters more to have plant neighbours than to be exposed to warmer temperatures.

It is therefore highly important to study these range expansion processes from two perspectives: the 'runners' and the 'fighters.' When we understand the drivers behind the whole process, we can predict the winners and explain who is able to thrive in a changing future.

Having plant neighbours outweighs the effects of warmer temperatures

The main finding of our research is the importance of interactions with neighbouring plants. Surprisingly, these outweigh the effects of warmer temperatures alone. That means, it matters a lot for plants which other plants grow next to them.

At warmer sites, in their natural range, plants become bigger to outcompete the others in the competition for light. Plants therefore need to compete for light and space to be able to perform photosynthesis and grow.

Consequently, fewer resources can be invested into reproduction. At higher elevations, this changes. Temperatures are lower, and there is less time to grow, due to later snowmelt, for instance. These conditions are therefore harsher. This leads to an interesting phenomenon called 'facilitation' where plants are competing less but rather indirectly help each other out to thrive. The question now is how we can detect these hidden interactions.

As ecologists, we are equipped with a toolbox full of different methods to investigate and explore the mechanisms behind range expansion processes in detail. We can, for example, measure the characteristics of every plant (so-called plant functional traits).

This helps us to understand how well a plant is growing and reproducing. We observe that plants surrounded by neighbours produce fewer leaves and flowers and can be clearly separated from those without neighbours.

Warming, however, does not influence the plants appearance. Another approach is to take a closer look at the reproduction itself by studying flowering phenology (the timing of flowering events).

It is important to track the onset of different phenological stages, like budding, flowering, and fruiting to observe potential shifts with changes in temperature. Here, we find that plants start to flower earlier when they are warmed, both with and without neighbours.

Hope for the mountain biodiversity

Overall, the results of our experiment indicate bad news for the moving species, but good news for the mountain biodiversity, because the high elevation vegetation ('fighters') can resist against the new lowland plants ('runners') to a certain extent.

In a hypothetical scenario where these plants are already able to establish themselves at higher elevations, their performance is worse when neighbouring plants are around, compared to when these were removed. This also means that we have a chance to buffer the impacts of climate change by managing grasslands.

When we understand what is happening in the near future with climate change, we are better equipped to preserve the mountain vegetation and can continue to enjoy our next hikes.

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