The little blue flowers that have grown for a century now in France near the graves of the war dead at Douaumont can easily be mistaken for local forget-me-nots.

In fact they are a foreign import, an American flower brought as seeds on the hooves of the US army horses used at Verdun during World War I.

"They call it the blue-eyed grass from Montana," says Patrice Hirbec of the National Forests Office (ONF).

Sisyrinchium montanum, best known simply as American blue-eyed grass, is part of a rich legacy left by the carnage of World War I in France's Forest of Verdun: a unique mix of flora and fauna.

The Battle of Verdun was one of the longest and bloodiest battles of the war.

Launched in February 1916, it lasted 300 days, killing in this region alone more than 300,000 soldiers and making Verdun synonymous with the wanton slaughter that characterised that war.

Jean-Paul Amat, geography professor at the Sorbonne University in Paris, says the fighting caused so much upheaval that the soil went through the equivalent of 10,000 years of natural erosion.

"The flowers and plants there simply adapted to the turmoil," Hirbec says.

In some places the soil was so dry that thyme started to grow, while elsewhere pear trees sprang up, a reminder of the orchards destroyed in the fighting, or perhaps a clue to the soldiers' diet.

On the other side of the battle lines, the Germans made their own contribution, says Stephanie Jacquemot, an archaeologist with the region's cultural affairs section.

That is why gentian, a flower not normally found in the region, grows there. Perhaps the German soldiers used it to make gentian eau-de-vie, a spirit normally found in Alpine regions.

Lurking in the craters left by the shell impacts are crested newts -- as well as yellow-bellied toads, which are even rarer.

And bat colonies inhabit abandoned fortifications.

In addition, "There are 15 species of ferns," Hirbec says as he stoops to pick up a shell fragment, adding that orchids bloom in May and June.

- A forest shroud -

The fort at Douaumont offers a panoramic view of the forest below, studded with pines that were planted after the war.

Some 20,000 hectares (50,000 acres) of the Meuse region had been devastated and needed to be replanted.

The day after the armistice was signed on November 11, 1918, France decided that no one would ever be allowed to live there.

Millions of shells had wreaked havoc on the land, robbing the soil of the humus that makes it fertile.

Thousands of hardy Austrian pine trees were planted, using saplings Vienna gave as part of its war reparations.

Between 1923 and 1931, 36 million trees were planted along the German and French fortifications, around the trenches and around the bodies of the dead.

Even today, some 80,000 soldiers rest among the orchids and the pines.

"This forest is their shroud," Hirbec says.

To visit these sites, as well as caring for them, is to honour the fallen, he adds.

The ONF has joined forces with the local authority and France's Fondation Patrimoine (Heritage Foundation) to raise funds for a network of trails winding through the trenches, ruined villages and what is left of the fortifications, to keep the memory of the war alive.