Why are mountains getting greener if alpine plants are dying off?

By Stephen Beech

Alpine plants are dying off despite mountain summits getting greener, reveals a new study.

European mountaintops may be gaining plant species as the climate warms, but researchers found that apparent boost in biodiversity is masking a growing wave of local extinctions.

The two-decade survey of mountain summit plant communities showed that the losses are greatest among species exposed to stronger warming and can be preceded by declines in abundance.

The research team says their findings, published in the journal Science, suggest that shrinking plant cover could provide an "early warning" of future alpine extinctions.

They said the disappearance of plants from summits can take decades, but has accelerated over the last 20 years.

The researchers looked for species that disappeared between surveys and examined whether local losses were associated with warming, earlier declines in plant abundance or a species occurring near the upper edge of its normal elevation range.

They found that local extinctions increased over time and were more common in mountain regions where warming was stronger.

Species were particularly vulnerable near the lower edges of their elevation ranges and in communities experiencing greater "thermophilization" — the increasing dominance of warm-adapted species.

The research team says that, although upward shifts increased local species richness on mountain summits, they came at the expense of loss of higher-elevation species.

Study first author Johannes Wessely said: "Until a few decades ago, short vegetation periods, low temperature sums and year-round frost risk made alpine landscapes difficult to colonize for many species of the European flora.

"However, accelerating climate warming has relaxed these constraints.

"Species have hence started to expand their ranges upward and mountaintop vegetation has consequently become more species-rich.

"However, the research demonstrates that this apparent success story also has its downside: while total species numbers are rising, loss rates are increasing, too, and the losses primarily struck specialists of high-elevation ecosystems."

The team analyzed the vegetation of 62 European summits, across 16 mountain ranges, which have been surveyed for their species composition four times between 2001 and 2022 as part of the global monitoring initiative GLORIA.

The data shows that the percentage of species not re-detected was increasing with each survey.

Species with a center of distribution above the focal summit were overrepresented in the group of lost species.

Wessely, from the University of Vienna, says the results of the study suggest that climate warming drives those local extinctions.

He added: "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."

GLORIA coordinator Harald Pauli, from the Austrian Academy of Sciences, said: "When species already grow on warmer-than-average sites they become more sensitive to further temperature increase.

"Specialists of high elevations tend to be particularly stress tolerant and long-living.

"Their local extinction can hence lag behind the onset of environmental changes that trigger these extinctions, resulting in the accumulation of an extinction debt in biological communities.

"These lag times were already supposed to be a main reason why colonizations by new species have so far been documented more frequently than extinctions of former residents in alpine vegetation.

"The results of the current study are in line with this interpretation: local extinctions of species are preceded by longer phases of population decline.

"Hence higher current extinction rates could be the response to changes that occurred or at least started already decades in the past."

Project coordinator Stefan Dullinger added: "They could consequently indicate that an extinction debt starts to be paid off.

"However, whether this interpretation holds true needs further investigation."