More than half of Europe’s endemic trees are threatened with extinction as invasive diseases, pests, pollution and urban development take a growing toll on the landscape, according to a study.

Ash, elm and rowan trees are among those in decline, says the assessment of the continent’s biodiversity, which could complicate efforts to tackle the climate crisis through reforestation.

“It is a threat. It is not just the naturally occurring trees and woodlands, it is also some of the big commercial conifers that are threatened by invasive species,” said one of the authors of the report, David Allen of the International Union for Conservation of Nature, who produced the study.

He warned that countries such as the UK were keen to import more saplings to draw carbon out of the atmosphere, but said young trees needed to be carefully screened to avoid diseases and pests entering the country and depleting existing forests.

“We are encouraged to plant more trees, quite rightly, but we have to be very careful to ensure they don’t come with pest species. We need to be very careful about biosecurity,” he said.

Invasive species – spread through the trade of plants or untreated timber – are the largest threat to native trees that are found only in Europe, sometimes only in one valley or region.

The IUCN’s European red list of trees found 58% of these endemic trees are threatened and 15% (66 species) were classified as critically endangered.

Many of those at greatest risk are in the Sorbus genus. This includes rowan, mountain ash and Ley’s whitebeam, of which there are only nine plants left – all in Merthyr Tydfil, Wales. Scientists say this particular tree is a relatively recent hybrid and there was only ever a very small population in a restricted geographical area, so the knock-on effects are likely to be minuscule.

Of greater concern is the demise of more common trees. Tim Rich, another of the contributors to the study, said he was alarmed by the loss of ash trees due to an invasive fungus.

“I’ve been keeping an eye on it over the past five years. Last year, I began to get quite worried. This year, huge areas are experiencing a dieback and it’s not just affecting saplings like it was before. Now it’s whole big trees. I drove in some parts of Pembrokeshire recently, and every five or 10 metres there was an ash tree dead or dying. This is a major problem – way worse than I expected it to be.”

The horse-chestnut – beloved by generations that played conkers as children – has been classified as vulnerable due to the spread of an invasive leaf-miner moth that damages its leaves. This adds to existing pressures from forest fires, tourist resort expansion and logging. Other species are negatively affected by excessive nitrogen depositions from air pollution, housing estates and pig farms.

The study of trees is part of a wider European red list that examines the status of overlooked species in order to determine priorities for conservation. It found 20-50% of terrestrial molluscs, shrubs and bryophytes, such as moss and liverworts, are threatened with extinction due to a loss of wild areas, expanded agriculture and climate change. Although these species are unglamorous and rarely attract attention, they play a vital role in food production and other natural life support systems through oxygen production, nutrient recycling and soil regeneration.

“The high extinction threats revealed by the European red list are very alarming, given that 92% of the mollusc species native to Europe are endemic to the continent. Thus, once these species are lost from Europe, they are gone for ever,” Eike Neubert, an IUCN mollusc specialist, said. “In order to restore terrestrial mollusc numbers in Europe, essential changes will be needed in policies relating to land use, along with careful control of urbanisation and sustainable management of semi-natural areas.”

Rich, who has studied trees for several decades, said he was increasingly worried by the broad trend.

“We are seeing our natural environment being eaten away,” he said. It’s such a wide scale problem, rather like climate, that it needs major policy change but what we should be doing seems impractical so it is only when things get really dire that we will take action. We should be looking after this more strategically. There really is no Planet B. When I think at how this place will be in the next 50 years, it is extremely worrying.”