Day of the International Forest

The Global Forest Incident occurred worldwide sometime around the 2010s. Trees began popping up all over the world in order to protect the human race from a truly destructive solar flare.

The Incident
Around the 2010s, a global forest, previously the North Forest and Southern Forest, grew overnight all over the world. As the Twelfth Doctor discovered, it was a defence mechanism to protect the planet from the threat of a solar flare (much like the one that had impacted the Bank of Karabraxos) with excess oxygen.

In an attempt to prevent what they perceived as a potential invasion, world governments implemented a plan to destroy the trees with a defoliant, unwittingly dooming the Earth in the process. Luckily, the Twelfth Doctor was able to broadcast a warning to the human race curtesy of Maebh Arden, who could communicate psychically with the trees. Maebh's message was heard all over the world, advising everyone to be calm and that the trees are to be left alone as they serve a benevolent purpose to mankind. With this message, the governments left the trees alone.

Once the solar flare struck and the worldwide forest's work of preservation succeeded, it promptly regressed to its former state. (TV: In the Forest of the Night)

Aftermath
Back at her apartment, Clara and the Doctor watch from the balcony as the trees covering London crumble away. She wonders what will be told about the trees in the future, but the Doctor tells her that the entire event will have been forgotten as it's the human superpower to forget. (TV: In the Forest of the Night)