18 Feb 2013

Changing thymes: plants adapt to climate warming

Before ending up on a dinner plate, thyme plants use ingenious ways to survive extreme cold and severe drought. By sniffing around in the field, scientists discover that wild thyme adapts remarkably quickly to climate change.

In the early 1970s, a research team from the Centre National de la Recherche Scientifique led by Philippe Vernet made a very odd finding. While collecting samples of wild thyme (Thymus vulgaris) from a small basin tucked between hills on the north of Montpellier, in France, the scientists discovered that the plants growing on the hill slopes smelled different from the ones growing inside the basin. At the time, little was know about these different types of thyme, so the team spent the next few years trying to understand how a single thyme species produced plants with different odours.

Wild thyme (Credit: A. Renaux)

There are six types of wild thyme, each with a modification in the genetic chain producing the oil that gives the plant its characteristic smell. This scented oil can have up to six phenol chemical groups attached to it (depending on the genetic modification), which makes slightly different molecules that have, however, very distinct smells, ranging from ‘non-phenolic’ to ‘phenolic’.

Vernet had found that non-phenolic plants grew only inside the basin, while phenolic plants thrived in the surrounding rocky hills, but it wasn’t understood why there was this sharp separation of thyme genetic types over just a few kilometers. In a new study published in PNAS, John Thompson and colleagues from the Centre for Functional and Evolutionary Ecology in Montpellier show that wild thyme adapts to climate change by shifting the frequency of these genetic populations.

Thompson had always suspected that the split distribution of thyme genetic types in the St-Martin de Lourdes basin, where Vernet's study was conducted, was related to climate. In the Mediterranean region of the south of France, temperatures often fall as low as -10˚C, typically in early winter. But in the basin temperatures can drop spectacularly, with historic record lows of around -30˚C, because cold air is trapped by the hills. This creates the weird effect of ‘temperature inversion’, when temperature declines as you go down the hills, rather than when going up. “It can be one to two degrees in the hills, and down in the basin it will be -5˚C, and the colder it is, the bigger the difference” says Thompson.

About ten years ago, as he was setting up some experiments with thyme in this area, Thompson literally smelled something wasn’t right. “We were smelling plants, and I couldn’t find any non-phenolics,” he says “I looked at the map from the original study in 1970, I knew I was in the right place”. The researchers collected thyme samples and sent them to the lab to be analysed. These results confirmed Thompson’s sniffing suspicions: the phenolic plants were now growing inside the basin, where they were completely excluded from 30 years before.

St-Martin de Lourdes basin (Credit: J. Thompson)

Could this be due to a change in climate? Previous studies by the team showed that non-phenolic plants cope well with extreme cold but don’t survive severe drought, so they prefer the deep moister soil of the basin. In contrast, phenolic plants thrive in the typical Mediterranean rocky dry landscape of the hill slopes, but can’t tolerate freezing temperatures.

The researchers dug out old temperature charts from a weather station in St-Martin de Lourdes and looked at the lowest temperatures measured in the past six decades. Even though extreme freezing events below -12˚C had always been sporadic, temperatures in the basin haven’t dropped below that level for over 20 years. Recent milder winters favoured the thyme genetic types sensitive to freezing, and this is why phenolic plants invaded the basin. “Mean values in ecology don’t mean a lot, what is important is the extreme values […] because you get extreme mortality events, and that is what causes natural selection” Thompson explains.

“This study reports compelling evidence that the genetic composition of wild thyme populations is being altered by current climate change” says S├ębastien Lavergne, an evolutionary ecologist from the University of Grenoble who was not involved in the study. “More studies like this one are needed to document how strongly human activities are reshaping life on earth”.

Thompson cautiously remarks that local climate changes, such as the rise in winter freezing temperatures in the St-Martin de Lourdes basin, do not necessary reflect a change in climate on the planetary scale, but he suspects this might be the case. “The prediction is the warming up, and one part of warming is that you have less extremes” he says.

This study shows that even long-lived perennial plant species like wild thyme can adapt very rapidly to climate change, which is good news. However, if this adaptation occurs by migration, for instance by spreading of seeds or pollen, then even resilient species may be vulnerable to local extinction. “If humans induce fragmentation of landscape, to the extent that species and their genes can’t move, then they are likely to stop responding to climate change” says Thompson.

Thompson, J., Charpentier, A., Bouguet, G., Charmasson, F., Roset, S., Buatois, B., Vernet, P., & Gouyon, P. (2013). Evolution of a genetic polymorphism with climate change in a Mediterranean landscape Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1215833110

This article was published in Lab Times on 18-02-13. You can read it here.

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