The man, “most likely a journalist” according to the police, died on Wednesday after falling from bridge that was suspended between two of the treehouses.
“We simply cannot continue as if nothing has happened,” North Rhine-Westphalia interior minister Herbert Reul told a press conference.
A website of the forest activists said that “a friend who has accompanied us as a journalist for a long time in the forest, fell today from a suspension bridge over 20 meters high in Beechtown and died”.
They also tweeted that they were “deeply shaken” and went on to call on the police and energy giant RWE, which owns the site, to “leave the forest immediately and stop this dangerous operation”.
“No more human lives should be endangered. What is needed now is a moment of rest.”
Police officers have faced fierce resistance since the middle of last week while attempting to clear the demonstrators from Hambach Forest near the border with Belgium and the Netherlands.
State interior minister of North Rhine Westphalia, Herbert Reul, at a press conference on Wednesday. Photo: DPA
On Sunday, nine environmental activists were injured and 34 detained as part of the operation to remove dozens of treehouses, some as high as 25 metres (80 feet) off the ground.
The occupation began in 2012 and had until now been quietly tolerated. But local authorities have now ordered the woods to be cleared immediately, citing fire hazards.
The death of the journalist was a “tragic accident” and not “related to the ongoing evacuation work,” said the police.
1/4 15:45 Ein Freund, der uns seit längerer Zeit im Wald journalistisch begleitet, ist von einer über 20 m hohen Hängebrücke in Beechtown gefallen. Zu dem Zeitpunkt wurde von Polizei und RWE versucht das Baumhausdorf zu räumen. Das SEK war gerade dabei einen Aktivisten ..
— Hambacher Forst (@HambiBleibt) September 19, 2018
Hambach Forest activist group posts a notice on Twitter about the man's fall. They said: “A friend who has accompanied us as a journalist for a long time in the forest, fell today from a suspension bridge over 20 metres high in Beechtown. At the time police and RWE were trying to clear the treehouse village”.
The activists, who are protesting the expansion of RWE's open-pit lignite mine, one of Europe's largest, had called for a mass mobilisation by supporters to prevent the forest from being cleared.
A police van and rescue helicopter near the forest site on Wednesday. Photo: DPA
The battle has intensified since the German energy company RWE announced plans to clear half of the forest's remaining 200 hectares (500 acres) from mid-October.
RWE owns the forest and is legally allowed to cut down trees to access the brown coal, or lignite, in the ground during the annual logging season.
It says the clearing is necessary to ensure energy supply, including of nearby power plants in Germany's most populous state, North Rhine-Westphalia.
Activists oppose the use of the cheap but polluting fuel, and say the forest is home to protected species such as Bechstein's bat and century-old beech and oak trees.