The geothermally heated soils that Jan-Niklas studies are special as they are heated all year round for hundreds, probably thousands of years. Grasses grow there on soils reaching up to 90 ° C 10 cm below the soil surface, whereas the surrounding areas experience harsh winters and cold summers.  This sharp contrast, stable over at least several generations, offers the opportunity to study how plants react to drastic changes in temperature conditions. Just a very limited set of species are able to grow on these soils, grasses being one of the most abundant families. Much research has been conducted to understand, how they are able to cope with the extreme heat. For example, an earlier study found a fascinating 3-way symbiosis between a grass, a fungus and a virus, necessary for the grass to be able to grow at the hottest sites.

Lush meadows on geothermally heated soils in Hengill Iceland, one of five heated areas studied as part of Jan-Niklas Nuppenau’s project. Photo: Jan-Niklas Nuppenau.

However, these conditions are also interesting for studying what happens when a stress factor, in this case cold and frost, is no longer present. Much less research has been done on that aspect. “This is of special interest as cold, especially the occurrence of frost, is thought to be one of the most important factors limiting plant species distributions on Earth. In particular, because the patterns of frost and cold exposure are being altered with climate change.” emphasizes Jan-Niklas.  

Freezing temperatures and long, cold winters challenge the survival of grasses and other plants. Cold and frost, can cause dehydration and damage cell membranes leading to cell death, wilting and, ultimately, death of the whole plant.  Survival at higher latitudes and altitudes requires endurance of subzero temperatures which requires physiological adjustments, such as the production of ‘antifreeze’ solutions (mainly containing sugars and proteins) that lower the freezing point, stabilizing the cell walls to prevent cells collapsing.  These adaptations demand a high energy investment from the plant, making it less competitive under warm conditions.

Furthermore, a precise regulation of the plants metabolism, shutting it down during cold periods and starting again when those are over, is needed to meet seasonal changes between cold times and the short favorable growing periods in between.  However, only certain plants, including several grasses, are able to do all this effectively.  For the rest, cold can quickly become dangerous.

“For most perennial plants at mid to high latitudes, like southern Sweden and central Europe, the cold is not too much of a problem. They are used to it and have adjusted their life cycle and metabolism accordingly. Their leaves wilt and drop in autumn and they store energy in their stems and roots. However, even for them, resuming growth too early in spring as a response to warm temperatures can become problematic: Once they have started growing and altered their metabolism, many plants are vulnerable to late, severe cold spells. Warm spring temperatures followed by unpredicted cold spells are expected to become more common with climate change”, says Jan-Niklas.

Damaged Corn after a late cold spell in spring. Photo:

“Corn, originating from Central America and a member of the mainly tropical Panicoideae grass lineage, for example, is not very cold tolerant and needs warm temperatures to thrive. That’s why it’s safer to grow corn further south and it can’t be seen much in Swedish fields”, remarks Jan-Niklas.  Studying the evolution of cold tolerance can help us understand which plants are able to grow where and why.

Part of Jan-Niklas’s project is to see what happens to grasses from naturally heated geothermal areas, when they experience cold. The general questions is whether grasses of the same species show adaptations specific to the very different conditions they are exposed to in the heated and non-heated areas:

“The hypothesis is that these grasses might not be able to deal so well with cold anymore as they are no longer exposed to cold. Since cold adaptations are energy intensive it is possible that they are lost when they are no longer needed. To test this, my supervisor Aelys Humphreys and I have planted grasses from heated and non-heated areas of Iceland, as well as from northern and southern Sweden (Stockholm and Umeå) in a shared garden. They are growing side by side in the same climate, in both Stockholm and Umeå, so that we can compare how each of the populations responds to the shared winter conditions they experience. We are interested in their overall survival rates and, for those that do survive, how they do during the following summer, e.g. in their timing and overall amount of flowering”, explains Jan-Niklas.

Jan-Niklas with the plants for the experiment in the Bergius Botanical Garden, Stockholm. Photo: Aelys Humphreys
Master's student Elsa Höglund helping with setting up the experiment in Umeå. Photo: Jan-Niklas Nuppenau

A further aspect of the project is to investigate if the grasses in heated and non-heated areas still exchange genes with one another.

In Yellowstone National Park (USA), for example, a previous study found that grasses growing on heated soils flowered much earlier than the surrounding non-heated grasses. This resulted in the heated and non-heated populations flowering at completely different times. Different flowering times prevents cross pollination and the populations became reproductively isolated from each other, making populations from heated areas more genetically similar to each other than to those from non-heated areas close by. Interestingly, grasses from as far away as heated soils in Kamchatka (Russia), thousands of kilometers away, were also genetically more similar to the heated than non-heated populations in Yellowstone. This opens the intriguing possibility of a special geothermal lineage that colonized heated areas once and is now spreading to other heated areas worldwide.

Grasses and mosses on heated soils during winter in Yellowstone National Park. Photo: Mike Tercek

“The heated areas in Iceland are less extreme in their temperature gradients and are thought to be younger and less stable over time than the ones in Yellowstone, for which constant thermal activity for at least 10,000 years has been reported. However, the Icelandic geothermal areas still have much warmer conditions than the surrounding areas. Temperatures can be up to 90 °C below ground and above 50 °C at the soil surface at extreme sites. Here, only mosses, thyme and the genus Agrostis grasses can grow.

Agrostis vinealis, brown bentgrass, growing at a geothermally heated site in Iceland. Various mosses and thyme, Thymus praecox, can be seen in the background. The thermometer shows the temperature measured 10cm below the soil surface. Photo: Jan-Niklas Nuppenau

However, most of the heated areas have soil temperatures between 20 and 40 °C and are covered in lush meadows with a variety of grasses and other plants. Interestingly, some plants, such as the fern Ophioglossum azoricum, can only be found in heated areas in Iceland.  

Ophioglossum azoricum, adder’s-tongue. A fern growing in Iceland only at geothermally heated sites. Photo: Jan-Niklas Nuppenau.

“Populations of the grasses I study have colonized the heated soils at some point after the last Ice age, which ended around 10000 years ago. Which means that they experienced for hundreds, maybe thousands of years, much different conditions than surrounding populations. This includes the reduction of the stressful impact of cold and a longer growing season and makes these areas to unique natural laboratories for studying the speed and mode of evolution under contrasting climatic conditions in general and of cold tolerance in particular.” says Jan-Niklas.

Hundreds to thousands of years are from an evolutionary perspective, a very short timeframe, especially compared to temperature changes over millions of years during Earth’s history. However, it is still much longer than any human experiment investigating the effect of temperature change. Therefore are these soils also utilized by many researchers to study short-term evolutionary responses to increased temperatures, which are relevant for understanding how organisms might respond to ongoing climate change.  One of these researchers is Johan Ehrlén at our department, who looks at phenological shifts in a number of plants growing in these heated areas.  However, Jan-Niklas is the first to study grass species growing on heated areas in Iceland and having a special focus on cold tolerance.

 “By comparing Icelandic grass populations from heated areas all over Iceland with populations from non-heated Icelandic and Swedish populations, we hope to learn more about how grasses, and thereby plants in general, react to changing climates. If they lost the ability to tolerate cold already after such a short time, this would speak for high costs involved in keeping the mechanism to tolerate cold. We are also looking at genetic differences among the populations. First, we are testing if the heated and non-heated grasses are genetically different from each other. If this is the case, it would speak for a possibly very quick progression of evolution and speciation in these grasses and temperature being a very important driver of it. This is basic research but our results will increase understanding of speciation and species distributions. If populations from heated areas differ strongly in their heat or cold tolerance and are genetically distinct, it would motivate future research to disentangle which genes are responsible for these differences. Such information is relevant for physiologists trying to understand mechanisms of cold and heat tolerance, as well as for breeders trying to improve crops’ resistance to stressful climatic conditions. ”, explains Jan-Niklas.