It is understood proposals for a £12m project at Fairy Glen would mean half an acre of wood is destroyed, Coed Cadw has claimed.
It has asked Snowdonia National Park Authority to reject RWE Innogy's plans.
The scheme would be capable of generating up to 13,000 megawatt hours of electricity each year, enough to supply around 2,700 households.
RWE Innogy revealed the plans three years ago and the authority has been carrying out a consultation.
It would involve getting water from the river above Penmachno bridge and diverting it through more than half a mile (1km) of pipeline around Fairy Glen, before returning it to the river near the River Lledr junction.
Rory Francis from Coed Cadw said: "When woodlands such as this, which are centuries old, are lost they cannot be replaced.
"We urge the Snowdonia National Park planning committee to say no to the  proposal and for the company involved to think again."
RWE Innogy said a consultation had taken place, and that the project had been designed to to minimise any environmental impact.
A spokesman said: "Let's not forget that one of the biggest threats to the environment and wildlife is climate change itself, and so it is important that we are able to deliver projects like this which make an important contribution towards tackling climate change."
Woodland in Conwy is "under threat" if a hydro-electric scheme goes ahead, a charity has said.