On a hunt for mountain plants


Valerian flower in a Norwegian valley

We are at the height of our 2017 resurvey of the vegetation along Norwegian mountain roads, and the fieldwork has been highly successfull. It has been great revisiting the plots and discovering the changes – and often the highly interesting lack of it – in the last 5 years.


Harebell flowers on the side of a fjord

The fieldwork brought some annoying bits of rain, yet mostly plenty of sun. It included beautiful flowers and breathtaking views, but also tons of sample bags and hours bending over in roadsides. We climbed rocks, jumped rivers and swam in an Arctic fjord, yet also spent hours in the lab, weighing leaves and sifting roots. An intense ten days, collecting data that can easily keep us busy for a few more years.


Cottongrass in a species-poor mountain marsh

And the first results indeed look very promising, even though a lot of data still has to come in before we can get to any conclusions. Luckily, I can again count on an awesome fieldwork team, this year with three highly dedicated master students who use this extremely interesting study system for their master theses. With their help, the data will soon reveal all its secrets.


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A story of hotspots and stepping stones

Predicting the faith of exotic plant species in cold-climate mountains: our new paper is out now in early view! You can find it here.

Abisko, a small village north of the polar circle in Swedish Lapland. The origin of several mountain trails, winding through the pristine subarctic vegetation towards the breathtaking views at the top. A vegetation mostly consisting of slow-growing mosses and dwarf shrubs that seem to have been there forever. Yet during the last few years or decades, changes in this vegetation increasingly start to become apparent: several new species that are traditionally not a part of the subarctic vegetation are popping up along the trails. Clovers, common yarrow, sweetgrass or annual meadow grass, species that are typical residents of the milder parts of Europe, are now getting a foothold even here, in the high north. They border the trails, grow in the roadsides, line the buildings at the ski resorts and seem to follow humans with every step they take.


A typical subarctic mountain trail, winding through a blueberry field (Vaccinium myrtillus).

The higher up you go in the mountains, the less common these new species become. At some point, often close to the treeline, they disappear entirely. Why do they stop there? Are these elevations finally too cold for exotic species to survive, or is it just a matter of time till they find their way to higher elevations? What drives their distribution and, most importantly, where will we find them in the future?


The subarctic mountain vegetation is dominated by sturdy sedges, tiny mosses and small flowers, like this mountain avens (Dryas octopetala).

To find out what the chances of these non-native species are in the coldest and highest mountains, an international team of ecologists set up an experiment high above the current elevational limit of these species. This was done at 400 meter above the local treeline, in an area where spring and autumn are almost back to back, and the full growing season lasts only a little more than two months. In this harsh environment, dominated by sturdy sedges and tiny mosses, they tested the potential of six non-native species, which have their origins in the mild climate of Western Europe. They simulated different human stressors (e.g., disturbance and soil enrichment) and planted seeds of the six focal species along a temperature gradient. Expectations were low: for a species used to the mild winters and relatively long summers of Western Europe, the high alpine zone of subarctic Lapland was unlikely to be hospitable.


Experimental plot in the mountains of Swedish Lapland

Yet the results were unsettling. Under ideal circumstances (e.g. warm, south-facing slopes and disturbed plots), the seeded plants managed to survive two growing seasons and a winter and they also produced significantly more biomass than in the control treatments. In undisturbed natural vegetation, on the other hand, success of the sown exotic species was negligible. Thus, only plots in which anthropogenic influences, such as fertilization and removal of the vegetationere combined with a sufficiently warm microclimate, resulted in establishment success for the exotic species. If, and only if, those two factors were present, the possibilities for exotic species to establish increased drastically, even at elevations far above where we find them now.


Sweetgrass (Anthoxanthum odoratum) beating the odds in a disturbed plot on a south-facing slope.

Yet where to find warmth in these environments, you might ask? Surprisingly, the mountain climate is often drastically less harsh than one might expect when considering environmental heterogeneity occurring at very small spatial scales, even at high elevations. This is for a large part due to the complex topography of mountains: soils on south-facing slopes for example – which get a lot of direct sunlight all day long – can easily be 7 °C warmer than their surroundings. Yet humans themselves are creating such warm spots as well, by removing the alpine vegetation along trails, for example. The vegetation normally provides a buffer for extreme heat production at soil level during the summer, whereas removing it will increase the temperature. For tiny seedlings, these extra degrees might be lifesaving. In winter, on the other hand, a protective snow cover shelters the small plants from below-zero temperatures.

The surprisingly high success of exotic plants in warmer and more disturbed plots suggests that spread of exotic species into cold mountain regions is likely to increase significantly in the future, if climate keeps warming and anthropogenic disturbance of pristine alpine regions is not halted. But even now already, exotic species can make use of the warm spots in the landscape, either natural or augmented by humans, as stepping stones to reach higher elevations. The potential for exotic species invasion in cold environments is thus most likely heavily underestimated.


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Live from the field

We are currently on a ten day fieldtrip to the beautiful Lapland, where we are monitoring the movement of plants along mountain roads.


A job with a view, plenty of beautiful alpine and arctic plants, and a ton of great and interesting data coming in. More pictures and stories will follow, but now the field calls!

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Impatiens - 2

Time for a plant portrait of one of my favourite Belgian study species: Impatiens glandulifera, a tall herb with its origin on Himalayan mountain slopes, yet introduced all over the world by humans who fell in love with it.

Impatiens - 3

And what’s not to love about the Himalayan balsam? The flowers are beautifully original and shaped like a weird pink hat, earning the plant its common English names ‘Policeman’s Helmet’, ‘Bobby Tops’ and even ‘Gnome’s Hatstand’. 

Impatiens - 5

Yet when the delicate pink flowers disappear, the fun is all but over: the genus name Impatiens refers to the plant’s method of seed dispersal. When you touch the ripe seed pots, they ‘impatiently’ explode, scattering the seeds in all directions. I bet you this provided hours of fun as a child in my parent’s garden!

Impatiens - 1

Yet it is exactly this attractive but aggressive method of seed dispersal, coupled with a high nectar production that attracts countless pollinators, that makes the plant so successful. In many regions in Europe and North America, the plant easily outcompetes the native vegetation, especially along rivers.

Impatiens - 4

It is in that last environment that it provides the most trouble as well. Similar to that other invader, Japanese knotweed (Fallopia japonica), the Himalayan balsam dies off in winter dies off in winter, leaving the river banks bare and unprotected. Such an environment is much less protected against erosion by winter floods than a river bank covered with native reed (Phragmites australis) for example.

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Science with doormats


No, I am not investing in a soccer field, nor am I building an indoor garden for my cat (although the latter loves the idea).


No, we are planning to dive deep into the study of the movement of plant species along mountain roads. Do I need a bunch of weird-looking doormats for that? Yes, as it is the ideal material to trap seeds falling on them.


We will install these mats on strategic spots in in the landscape and then just wait. After the growing season, the mats will be taken to the lab and all seeds trapped in the hairs will be carefully collected and identified.


Even more than just looking where the plants currently are, it will reveal their potential spread in the future. Hopefully more on that later.

And, oh yeah, do not laugh with my mats, they are a proven, published and peer-reviewed tool!


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Aliens and their way to the top

5 years later, we are getting ready for a re-survey of our longterm observational plots along the roads in the Norwegian mountains. The perfect moment to summarize what we learned from our first trip. This post was published first in a series on this summer’s field trip on the INTERACT blog.

Remember my story about how lowland roadsides are flooded with species that do not  belong in the natural system? Remember how these new species could profit from the lower competition when the natural vegetation got destroyed by the process of road building? A lot of these species are well known to Northern Scandinavia, but some of them are not. And this last group deserves our special attention.

A group of true culture-followers. The real roadside species. The ultimate weeds. They followed human development up to the north at one point in time, some decennia or centuries ago. We call them aliens: visitors (and sometimes invaders) from another ‘world’. (But do not let them fool you, because they are just species like our regular white clover!)

Trifolium repens

What is really curious is how almost all these aliens share the same story. Their invasion always starts in the lowlands, where they got introduced, after which they closely follow roads and human structures up into the mountains. Not too many of them really reach the top, however. We see a progressive drop-out of species on the way, victims to the cold alpine climate (but keep in mind this is a dynamic process, they could still be on their way!). This progressive loss of alien species with elevation got the fancy term ‘directional ecological filtering’ and it also seems to be happening in our subarctic ecosystem. The mountain acts as a filter, only allowing a select group of aliens to the highest elevations, while the weaker ones are filtered out (check the clear decrease in roadside alien richness with elevation as visualized by the black line on the graph).

Alien species richness with increasing elevation in the roadsides (black) and the natural vegetation (grey).


The question is which skills are needed to sneak through this filter to reach the highest elevations in the mountains. It turns out that all winners of the race to the top follow a similar strategy: they are all generalists, which means they can thrive in a wide range of environments. That makes them different from the vast majority of plants that got adapted for one particular situation. It also makes them incredibly suited for mountain invasion. Mountain invaders have to overcome both lowland and alpine conditions. Strong competitors loom in the lowlands, where conditions are good and fast and efficient growing are the keys. In the highlands, the harsh climate demands stress-tolerant traits to survive the cold: growing slow, staying close to the ground and using resources to fight the harsh conditions.

Summer snow

That is the reason why pure competitive alien species are stuck in the lowlands, while the generalists can follow the road all the way up to the alpine zone. While both know how to handle the intense competition in the lowlands, only the generalists can change their strategy to deal with the totally different alpine conditions from the highlands. And as soon as these generalists reach the top, they might become problematic and start escaping the roadsides, yet that’s a story for another post to tell.

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So dry


It is dry in Western Europe. Extremely dry. Dry enough for me to to write a blogpost about it for www.eoswetenschap.eu, our local popular science journal. They asked me if the current drought could serve as proof that the climate warming.


Unfortunately, the blog is in Dutch, yet I’ll hint the answer for you: it’s not. We should always be wary about the difference between one weather event – no matter how extreme – and the global climate.


Potato flowering in a hot evening sun. Especially agriculture is suffering from the drought.

Yet the drought does fit in neatly in the observed ànd predicted increase in extreme weather events that come with climate change: more droughts, more heat waves, more and heavier storms… We are up for some years in which one weather record after the other will be broken, as climate gets more and more unpredictable.

And I think it is crucial that we are all aware of that fact.

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