The end of the great northern forests? The tiny tree-killing beetle wreaking havoc on our ancient giants

Forests across Europe, the US and Canada have been hard hit by drought, fires and bark beetles. Now scientists fear the northern hemisphere's greatest carbon sink is nearing a tipping point

The giant sequoia is so enormous that it was once believed to be indestructible. High in California's southern Sierra Nevada mountains, the oldest trees - known as monarchs - have stood for more than 2,000 years.

Today, however, in Sequoia national park, huge trunks lie sprawled on the forest floor, like blue whale carcasses stranded on a beach. Many of these trees were felled by a combination of drought and fire. But among the factors responsible for the rising toll is a tiny new suspect: the bark beetle.