Of all the known trees found solely Europe, more than half are at risk of dying out, the International Union for Conservation of Nature (IUCN) reports.

Some of these ancient natives have existed here since before the last ice age, but as the sixth mass extinction unfolds, Europe's perennial woods are more threatened than its reptiles, mammals, birds, bees, and butterflies.

In fact, trees are one of the most highly endangered groups yet assessed for the IUCN's European Red List. The risk of extinction for this group of species is exceeded only by freshwater molluscs and leafy plants.

The worst part is we've only just noticed. In an effort to study some of the world's most overlooked species, the IUCN's newly-published Red List of Trees makes visible the scale of our unmistakable destruction.

Evaluating all 454 tree species native to the European continent, analysts found 42 percent of its indigenous species were regionally threatened with extinction.

For the endemic trees, which exist only in Europe, well over half were at a high risk of dying out, while 15 percent were deemed critically endangered - a step away from extinction.

Even among the trees that were okay, a dozen species were on the brink of being threatened themselves, and the authors admit a full 13 percent lacked enough data to assign them a conservation status at all.

"The impact of human-led activities is resulting in population declines and a heightened risk of extinction of important species across Europe," says Luc Bas, the director of the IUCN's European Office.

"This report has shown how dire the situation is for many overlooked, undervalued species that form the backbone of Europe's ecosystems and contribute to a healthy planet." 

Recently, it's become clear that many plants and trees on our planet are facing an uncertain future. In the past few years alone, we've quadrupled the number of known plant extinctions catalogued since the 18th century.

A study published in June of this year found that since 1900, an average of three plant species have disappeared each year. That's an extinction rate at least 500 times faster than is naturally expected, and twice the number of amphibian, mammal, and bird extinctions combined.

Awareness is slowly growing around the crucial role of leafy and woody plants, as is our knowledge of the main threats they face. According to the IUCN's new report, trees in Europe face the most danger from invasive species, impacting 38 percent of the species examined.

This threat was then followed by deforestation, wood harvesting and urban development, as well as issues like climate change, livestock farming, land management and fires.

The impact of these pressures, however, does not strike evenly across all tree species. In the analysis, a whole three-quarters of tree species in the Sorbus genus, such as Mountain-ash, were assessed as threatened, and about one-third of those were critically endangered. A further 22 species had such patchy information that they were unable to even be assessed.

Tim Rich, a taxonomist who worked on the study, told The Guardian he's been keeping an eye on ash trees for the past five years, and in that time he's been shocked by the plant's rapid decline.

"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," he said.

"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 only encouraging finding is that nearly 80 percent of these native tree species are found in at least one protected area, and many are contained in botanic gardens and arboreta. But while they might continue on in a few areas, their presence will be sorely missed elsewhere.

"Trees are essential for life on earth, and European trees in all their diversity are a source of food and shelter for countless animal species such as birds and squirrels, and play a key economic role," says Craig Hilton-Taylor, the head of the IUCN Red List Unit.

If they were to disappear, our world would look very different than it does today.

The European Red List of trees is an IUCN publication and can be found here.