The Woodland Trust wants to buy the land at Llennyrch woodland.
Natural Resources Wales (NRW) has given £50,000 but a further £750,000 is needed and a campaign will be launched on Tuesday at the National Eisteddfod in Meifod, Powys.
The charity said the area has been called a "Celtic rainforest" and it wants to improve wildlife on the site.
NRW chief executive Dr Emyr Roberts said: "This is a fantastic opportunity to bring the whole woodland area under conservation management."
The total cost of the project is £1.5m and the rest of the cost will be met by money left to the Woodland Trust.

It will be one of the main topics under discussion at a global conference of lawyers getting under way in Belfast this week.
More than 400 lawyers from around the world are taking part.
The Law Society and International Bar's Norville Connolly said Brexit was a huge issue for the profession.
"All UK lawyers at the moment can either establish or go into Europe to provide legal services," Mr Connolly said.
"Between England, Scotland, Wales and Northern Ireland there are a huge amount of lawyers who conduct business and create jobs and all that follows from that.
"If there's a hard Brexit, to use that expression, what will happen is that ability to travel abroad, to establish firms abroad, may well be lost.
"That's the fear that the lawyers who practise in Europe, or practise in some country other than their own country, that's their huge fear .
"So there's a lot riding on the hard Brexit v the soft Brexit."

Helen Hinks, of Talybont, and her son William, aged three, discovered the boat while walking along Borth beach.
It had been launched by youngsters at St Andrew's School of Math and Science in Charleston, South Carolina, in May 2015 as part of a school project.
Ms Hinks said it was "magical" to find the boat nine months later, while the school said it was "over the moon".
The boat, named the Carolina Dreamer, had been fitted with a GPS tracker and had sailed for less than a week before washing up on Bermuda.
There, it was relaunched and although initially heading towards Ireland, it made a detour towards the west Wales coast - a journey of about 4,000 miles.
Pupils at St Andrew's had contacted Welsh harbour masters and marinas in the hope it had been sighted and rescued.
Explaining the moment they found the boat, Ms Hinks said: "I was waiting to pick my two daughters up from school and normally take my little boy to the beach if the weather's nice.
"We were looking across the waves when it literally sailed right to us. It was magical."
Ms Hinks said she did not realise the significance of the boat until seeing media reports a week later.
"When we contacted the school they were really excited. My children's school is hoping to relaunch it back to America," she said.
On hearing the news, St Andrew's pupil Paris said: "When I heard somebody had found our boat...I jumped up and down" while Dhyeya said: "Thank you for saving our boat, we appreciate it".
Fourth grade teacher Amy McMahon said: "We cannot tell you how excited and elated we are to hear that our beloved boat has been recovered and has found its way to a nearby school.
"We are over the moon."
A conservation charity is trying to raise £800,000 to purchase 550 acres of land in Snowdonia National Park.
[NEXT_CONCEPT]
A hard Brexit could have drastic consequences for the legal profession in Northern Ireland, according to the International Bar Association.
[NEXT_CONCEPT]
A toy boat set sail by American school pupils has been found thousands of miles away by a family in Ceredigion.