On Europe's Mountain Summits, Rising Plant Diversity Hides an Accelerating Wave of Extinctions

Plant life on Europe's mountain summits has become more diverse over the past two decades, as warming temperatures let species from lower, milder slopes push upward onto the peaks. But a 21-year survey of 62 summits across the continent's major ranges finds that this apparent gain conceals a faster and less visible process: the specialist plants that have lived on those same summits for millennia are dying out at an accelerating rate, and the losses now track most closely with how much a given peak has actually warmed.

The study, led by researchers at the University of Vienna and published in Science, is among the largest and longest-running assessments of how alpine plant communities are actually responding to climate change, as opposed to how ecologists have generally assumed they would respond. The headline finding — that rising overall species counts on mountaintops can mask an underlying wave of local extinctions — complicates a widely cited narrative in mountain ecology that has treated increasing summit biodiversity chiefly as a hopeful sign.

The numbers

The research draws on vegetation surveys of 62 summits spread across the Alps, the Pyrenees, the Apennines, the Carpathians and other European ranges, each resurveyed four times between 2001 and 2022 using a standardized protocol developed by the Global Observation Research Initiative in Alpine Environments, or GLORIA, an international monitoring network coordinated by the Austrian Academy of Sciences and the University of Natural Resources and Life Sciences, Vienna (BOKU). At each summit, botanists record every vascular plant species present in fixed plots and track it across successive surveys, making it possible to see not just how many species occupy a peak at any one time but which individual species disappear and when.

Total species counts on the summits have, on balance, continued to climb, consistent with a long-documented pattern in which warmth-associated plants migrate upslope as temperatures rise. But the rate at which existing high-elevation specialists vanish from individual summits has also risen over the 21-year study period, and that rate of local extinction correlates more strongly with the actual magnitude of warming a summit experienced than with the simple passage of time. Species were disproportionately likely to disappear from summits that had warmed the most, and within those summits, the plants growing closest to the lower, warmer edge of their elevational range were the most vulnerable.

How researchers got here

The finding builds directly on earlier GLORIA-linked research, notably a 2018 study in Nature that analyzed 302 European summits and found that the rate at which new plant species were colonizing mountaintops had itself accelerated in step with warming — a result widely read, at the time, as documentary evidence that alpine ecosystems were "greening up" under climate change. That earlier work tracked arrivals. The new analysis is one of the first to track departures with comparable rigor and over a comparable timescale, made possible only because the GLORIA network, launched in 2001, has now accumulated four full resurvey cycles at many of its roughly 130 sites worldwide.

Ecologists have long warned that biodiversity counts which rise on the surface can obscure deeper turnover underneath — a phenomenon sometimes described as compositional homogenization, in which generalist newcomers replace specialist residents without necessarily reducing the total tally of species. The Vienna-led team's contribution is to show that on European summits this turnover is not merely occurring but intensifying, and that its pace is now closely tied to local warming intensity rather than simply following a steady trend over time — evidence, the authors argue, that the extinctions are a direct physiological response to heat rather than a background process that happens to be visible against a warming backdrop.

The plants most exposed are so-called high-elevation specialists — species that are adapted to the cold, short growing seasons and thin soils of the uppermost alpine zone and that, unlike generalist lowland plants, have nowhere higher to retreat to once conditions above their current range become too warm. Because Europe's mountain ranges are already fragmented into isolated massifs, these species cannot easily disperse to cooler refuges elsewhere, leaving summit extinction as a comparatively near-term risk for a meaningful slice of the continent's alpine flora rather than a distant, worst-case scenario.

The pattern also matters beyond Europe. GLORIA's protocol has been adopted at alpine sites on six continents, including roughly two dozen in the United States, run in cooperation with the National Park Service and the U.S. Forest Service, and the Vienna group's method for disentangling arrivals from departures gives those networks a template for detecting the same masked extinction signal wherever long-running resurvey data exist.

• 62 European summits resurveyed four times between 2001 and 2022
• Local extinction rates have risen over the two-decade study period
• Loss rates track the magnitude of local warming more closely than the passage of time
• Species near the lower edge of their elevational range are most likely to disappear

What scientists are saying

Johannes Wessely of the University of Vienna's Department of Botany and Biodiversity Research, the study's first author, said the data point to warming itself, not merely time, as the operative force behind the disappearances.

"The data clearly show that species were more likely to disappear from summits which experienced stronger warming. This correlation was even stronger than the temporal trend," said Johannes Wessely.

Harald Pauli, GLORIA's coordinator at the Austrian Academy of Sciences and BOKU, framed the vulnerability of low-margin populations in physiological terms: plants already living near the warm edge of what they can tolerate have the least buffer left before conditions cross a threshold. Stefan Dullinger, the project's coordinator, raised a further possibility still under investigation — that some of the extinctions being recorded now reflect populations that were already doomed by warming that occurred years or decades earlier, a lagged effect ecologists call extinction debt, and that the observed losses could represent that debt "starting to be paid off," in his phrase, rather than a purely fresh response to current conditions.

What happens next

The authors caution that distinguishing a freshly triggered decline from the delayed payoff of older extinction debt will require further work, including closer study of population trends in the years before a species disappears from a plot rather than treating each resurvey as an isolated snapshot. The team also notes that the correlation between warming and local extinction, while stronger than a simple time trend, still leaves a substantial share of the variation between summits unexplained — meaning factors such as local snow cover, soil moisture, grazing pressure and competition from newly arrived species likely modulate how quickly any given summit loses its specialists.

The study was funded in part by a European Research Council grant under the European Union's Horizon 2020 program. Its authors say continued five-year resurveys across the GLORIA network will be needed to determine whether the accelerating loss of specialist plants seen so far is an early stage of a much larger contraction of Europe's alpine flora, or whether it will level off once the most exposed low-margin populations have already been lost. A summary of the findings prepared for a general audience, including further comment from the authors, is available via Phys.org.