An extraordinary exhibition of sculptures hewn, carved and manipulated out of wood is opening in Cardiff to celebrate the 50 years – or 200 seasons – that the artist David Nash has spent beavering away in and around a former Methodist chapel in north Wales.

The biggest and most ambitious show of Nash’s work in his adoptive homeland ranges from the vast – a towering spire of cork bark – to the modest, such as nine balls chopped out of a felled tree that cracked when they were left on the floor of the chapel and appear to be grinning cheekily.

There are also moving films and drawings that tell the rollercoaster stories of two of his most famous “free-range” pieces, Ash Dome, a circle of trees planted near his studio/home that has been struck by disease, and Wooden Boulder, which washed down to the sea and has vanished.

Facebook Twitter Pinterest Nash’s towering spire of cork bark. Photograph: Dimitris Legakis/The Guardian

Nash said he hoped the bittersweet exhibition of 120 works at the National Museum Cardiff would make people feel joy but also give pause for thought. “I’d like people who visit to think they have been engaged with something, taken out of their normal chain of thought and feeling. I’d like to think people will find it joyful,” he said.

Speaking to the Guardian before the opening, Nash said he had come around to the idea that human beings were “parasites” and that nature would get on better without us. He explained that when he looked at the landscape around the chapel, Capel Rhiw in Blaenau Ffestiniog, it did not seem as bright as it once did.

“There’s a certain dullness I can feel. I don’t know if it’s me or if it’s actually there. I can feel a lack of vibrancy in the land. When I look at the Moelwyn mountains, which I’ve looked at since I was four years old, they don’t seem to have that dynamic they used to have. There’s something in the texture maybe. It’s a feeling, not a fact.”

He despairs of Donald Trump and Brexit. “We are killing ourselves. There are too many of us. I think there will be some huge plague or pestilence.”

Many visitors will focus on the stories of the free-range pieces. Ash Dome is a circle of 22 trees that Nash planted on a hillside in the Ffestiniog valley in 1977 and shaped into a sculpture over the following decades. He began the piece at a time of austerity and concerns about nuclear war were prevalent. “People were saying we wouldn’t see the 21st century. This was planted as a space for the 21st century.”

The dome in its current form is doomed, having contracted ash dieback. Advice poured in about how he could combat the disease but Nash decided he could not fight it. “I had to accept it was a natural force. The fungus is a natural force.”

He enlisted his son, his main helper and their children to help him plant a circle of oak saplings around the dome. As the oaks grow they will be shaped into a new dome. After Nash dies he hopes his helpers will continue to maintain the new trees. “I can’t make them look after it but that is what’s hoped for,” he said.

Also vividly told in the show is the twisty tale of Wooden Boulder. In 1978, Nash hacked a great ball out of the base of a felled oak tree on the other side of the valley to Ash Dome. As he tried to get the weighty object to the chapel it plunged into a stream, and over the years it had tumbled slowly down into a river and then out into the estuary.

Facebook Twitter Pinterest A sketch of Ash Dome, a circle of 22 trees planted in 1977. Photograph: Dimitris Legakis/The Guardian

Wooden Boulder vanished in 2003 and reappeared in 2013. It has not been spotted since 2015 but Nash is still looking for it. “Lots of people in the area are keeping an eye out for it. If it turns up I will get a call,” he said.

Nash said he had realised that wood was either “coming” when it grew as a tree or “going” when it was dead. Ash Dome used to be a “coming” sculpture, until it was hit by disease; Wooden Boulder always was a “going” one.

Now 73, Nash continues to work. His latest piece is also displayed, not made of wood but a series of five blocks of colour that represent the shades an oak leaf takes on the first 12-16 days of its existence.

“As they get bigger they lighten in colour and the chlorophyll starts to form and they go green. I observed this a couple of years ago. If you work from observation, there is a sense of truth.”