Many forest species are in deep trouble because of the removal of the dead and dying trees they need, campaigners say.
WWF, the global environment group, says insects, plants, birds and mammals are all suffering because of an increasing tendency to remove decaying timber.
It says old and dead trees mean forests are often in much better shape and more able to resist pests and other perils.
WWF wants landowners to increase the amount of dead wood they leave in their forests to help to sustain wildlife.
A WWF report, Deadwood - Living Forests, says a third of forest-dwelling species rely on dead or dying trees, logs, and branches for their survival.
It says: "The removal of decaying timber and old trees from Europe's forests has led to a drastic decline in species such as insects, beetles, fungi, and lichens.
"Woodpeckers, bats, and squirrels which nest in hollow trees have also lost their natural habitat. Species relying on dead wood for food and/or shelter make up the single biggest group of threatened species in Europe."
WWF says dead wood is "critically" low, mainly because forest managers do not recognise how important it is for biodiversity, and through poor management.
It says: "In western Europe forests have on average less than 5% of the dead wood expected in natural conditions."
Old and tough
Daniel Vallauri of WWF said: "Europe's forests should be allowed to grow old gracefully.
"By stripping a forest of its decaying timber and old trees we are performing a strange and unnecessary cosmetic surgery on a natural ecosystem which threatens much of its biodiversity."
The report says forests with old and dead trees are often much healthier and more resistant to disease, pests, and climate change than tidy young forests.
Dead wood keeps forests productive by providing organic matter and nutrients for trees, preventing soil erosion, and providing long-term storage for carbon, tempering some of the impacts of climate change.
WWF wants European governments, forest owners, and industry to increase the amount of dead wood in managed forests, by up to 20-30 cubic metres - about one truckload - per hectare by 2030.
It also wants an end to what it calls perverse subsidies which require the removal of dead wood. The French government, for example, pays up to $2,230 (£1,220) per hectare for salvage felling without any minimum guidance for dead wood.
Daniel Vallauri said: "We need to debunk the myths that dead wood and veteran trees mean a sick forest. In most cases they mean a healthy forest with a long life cycle and a very high diversity of habitats for species."
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