In the heart of the mountains of California live some of the largest, oldest and most magnificent creatures on Earth, the giant sequoias (Sequoiadendron giganteum). These timid colossi of wood and leaves have inhabited the Californian lands for millennia, but today they are in serious danger.

The United States has been in the grip of a severe drought for months — the worst in its history — which is bringing both its population and its monumental trees to their knees. The snowpack on the Sierra Nevada, California's mountain range, reached its lowest ever recorded level last April. According to a study conducted by researchers at the Carnegie Institution in Washington, up to 58 million large trees suffered severe water loss between 2011 and the present day, putting their survival at risk.

"The impact of drought on ancient forests remains unknown," the researchers write in the study published in the journal Proceedings of the National Academy of Sciences. According to a report by the US Forest Service released in early 2015, more than twelve million trees are thought to have been killed by the drought in California. The research team, using spectroscopy in combination with other satellite data, sought to establish the impact of the drought on California's forests.

It emerged that approximately 10.6 million hectares of forest, containing up to 888 million monumental trees, have lost a significant amount of water from their canopies. The greatest dehydration, of 30 per cent or more, affected up to 58 million large trees. Numerous sequoias growing along the coast at Big Sur have begun to shed their leaves, and those that remain are characterised by a brown colour.

By monitoring and mapping the water content present in tree canopies, scientists can identify which trees are under water stress and therefore more vulnerable to wildfires and at greater risk of death.

"California relies on its forests for water supply and carbon storage, as well as for timber products and tourism, so they are extremely important from an ecological, economic and cultural standpoint," said Greg Asner, the lead researcher of the team, in a statement.

Aerial monitoring of forests will help to mitigate the negative effects caused by drought; however, a change of course is needed to save California's ancient forests.

"It took more than three thousand years to grow some of the trees of the West, through all the wonderful centuries since the time of Christ, and indeed long before, God has cared for these trees, saving them from drought, disease, avalanches and a thousand storms and floods. But God cannot save the trees from fools," wrote naturalist John Muir — the foremost champion of the Sierra Nevada and the sequoias — in words that proved prophetic long before their time.