Mountain greening threatens genetic diversity of iconic plants

Walk through a Mediterranean mountain meadow, and you’re likely to spot a patch of Sideritis – a hardy, aromatic plant that has long been used for making Greek mountain tea. For centuries, this herb has been prized by locals for its healing properties; it is believed to soothe everything from colds to stomach issues.

But in recent decades, something has been shifting quietly in these high-altitude ecosystems. The grassy slopes where Sideritis once thrived are being taken over slowly by shrubs and trees. This transformation is not just about aesthetics. It’s a sign of how climate change and land use are altering the very makeup of mountain biodiversity.

Researchers from the Senckenberg Biodiversity and Climate Research Center, the National Observatory of Athens, and the University of Copenhagen have been studying the genetic consequences of these changes, with a focus on Sideritis. Their findings are unsettling.

Plants are losing genetic diversity

Mountain regions are known for their rich biodiversity – they support countless unique and specialized species. However, over the past 50 years, many of these habitats have seen changes.

Warmer temperatures and reduced grazing have triggered what’s known as “mountain greening” – a steady spread of woody plants into areas that were formerly open grasslands.

Sideritis is among the species feeling the pressure. Once common across the Mediterranean’s montane grasslands, it now faces a shrinking habitat. But it’s not just the plant’s numbers that are declining. It’s losing genetic diversity as well, which may compromise its natural resilience to stress and change.

“We examined populations in 11 Greek mountain ranges and combined satellite data from several decades with genetic analyses of herbarium specimens from the 1970s and present-day plant samples,” explained study leader Spyros Theodoridis.

The team’s approach was unusual: they paired old herbarium specimens with modern genetic testing and decades of satellite imagery. This allowed them to see how Sideritis populations had changed genetically over time and how those changes tracked with the increase in denser vegetation.

The toll of inbreeding and isolation

“The results show that in eight of the 11 mountain regions we studied, genetic diversity declined significantly during this period. In particularly affected regions, up to 20 percent of the genome of individual plants is now subject to inbreeding – an indication of declining population sizes,” explained Theodoridis.

The numbers point to a worrying trend. As woody plants crowd out the open spaces, Sideritis populations become smaller and more isolated. This leads to inbreeding, which weakens the species’ ability to handle diseases, droughts, and other stresses.

“The speed at which shrubs and trees are spreading in previously open grasslands can be directly linked to the decline in genetic diversity in Sideritis populations,” noted study co-author David Nogués-Bravo.

“The genetic diversity of a species is crucial for its ability to adapt to environmental changes. If this diversity dwindles, resistance to disease, drought, or other stress factors decreases, which can lead to extinction in the long term.”

Watching plant changes from space

One of the most impressive parts of the study is how it links satellite technology with genetics. For years, scientists believed it was impossible to track genetic diversity using remote sensing. This study proves otherwise.

“This combination opens up new possibilities for biodiversity monitoring,” emphasized Theodoridis. “It allows us to use satellite images to identify indications of genetic changes in mountain ecosystems without having to genetically examine each individual population on site.”

This method could make it easier to monitor hard-to-reach mountain regions or areas where on-the-ground data is limited.

“However, our results show that the extent of genetic erosion can be predicted with surprisingly high accuracy, based solely on the increase in vegetation density,” said study co-author Thomas Hickler.

“This renders our method particularly attractive for use in mountainous regions that are difficult to access or in areas where genetic monitoring has hardly been possible to date.”

Historical specimens hold the genetic key

This research was made possible, in part, by historical plant specimens that have been stored in herbaria – archives of pressed and preserved plants.

“Without the historical plant specimens in herbaria, the direct comparison over a period of 50 years would not have been possible,” explained co-author Marco Thines. “These archives of nature are invaluable for biodiversity research.”

These old samples provided a genetic snapshot of the past, which was essential for comparing with modern populations.

Threats to the genetic diversity of plants

This isn’t just a local problem. Mountain greening is happening all over the world, as climate warms and traditional land-use practices decline.

You can see the shift clearly in satellite images – meadows turn darker as shrubs and trees move in. That’s why the researchers believe that now is the time to act.

“There is an urgent need for comparable studies with other species and in other regions,” concluded Theodoridis. “This will allow us to gain a comprehensive picture of how environmental changes are affecting the genetic basis of biodiversity – and how we can effectively counter this development.”

Protecting plants like Sideritis means protecting more than just a tea ingredient. It means defending the integrity and resilience of entire ecosystems – before they vanish under a wave of greenery.

The full study was published in the journal Current Biology.

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