There are two times of year I particularly love being among trees. The first is at the height of spring, when new leaves and woodland plants are at their most intensely green. The second is the point in autumn when the summer hues shift overnight and the woods take on a whole new coat. In Japanese there is a wonderful term for both, mikkaminumanosakura, meaning a moment of sudden, dramatic change. As we move deeper into autumn, we’re heading for one of these moments, and I’ll be out in the woods trying to catch it.

I have spent much of the past two years travelling the length of the UK and Ireland exploring our relationship with trees and woodlands for a new book. I wandered in stunning beech groves in Oxford, pinewoods in the Highlands, ancient oaks in Wicklow and Atlantic rainforest in County Clare.

The journey started among the trees my father planted 11 years ago on his smallholding near Oswestry on the border between England and Wales. Listening to him discuss the trees he knows so well with my son, I realised how little I really understand them. I could name only a handful of tree species from their leaves – and far fewer in winter. The realisation sent me spinning off to explore why woodlands hold such allure for us, what they do for our health and wellbeing, and what our relationship with precious fragments of ancient woodland looks like now.

One of the first woodlands I visited is also one of the most heavily studied in the world. Wytham Woods in Oxford is owned and managed by Oxford University, where walkers are as likely to encounter research teams as to spot deer in the undergrowth or birds in the canopy. Trees across the 405-hectare (1,000-acre) site are festooned with all manner of gadgets that are wonders in themselves – signs of the populations of scientists who study bats, birds and beetles, or the spread of diseases such as ash dieback.

To walk at Wytham (entry is free but requires a permit) is to walk through not one woodland but many, from medieval coppice woods and glorious beech groves planted to best frame a view of a country house, to trees around which first world war training trenches snake, and which become bright with bluebells in spring.

On Wenlock Edge in Shropshire, I wandered beneath towering ash trees as mellow autumn sunlight filtered through the leaves on to the limestone escarpment, glimpsing long views across the county between the trunks. Walking in Wenlock is a peaceful affair: hiking trails follow the line of the Edge, an 18-mile limestone ridge, so it would be difficult to get lost. To get a different view, though, I spent a night here, in reputedly the most haunted woodland in the country, trying to scare myself with ghost stories, and succeeding. Among the many stories is that of robber knight Ippikin, whose ghost is said to roam the Edge at night, pushing unwary walkers off the escarpment. As darkness fell, walking deep in a wood that had been familiar and friendly in daylight, I experienced some of what writer Sara Maitland calls “forest fear”, and retreated to my campervan.

Over and again as I travelled, I came across “keep out” signs, so it was uplifting to take part in an organised trespass into a privately owned wood on Dartmoor with writer and land access campaigner Guy Shrubsole and the South Devon Right to Roam group. This transgressive walk ended among the ancient oaks and beeches of the lush – and entirely legally accessed – Dendles Wood. This is a wonderful example of temperate rainforest, home to rare beetles. Mosses and lichens draped from the branches form a kind of green veil, inspiring a hushed awe. I marvelled at the depth of colours and the complexity of the lichens, which seemed whole worlds in themselves.

Often small, tucked away and under threat, patches of temperate rainforest can be found across the UK’s western fringes, from the ancient oaks at Coed Crafnant in Gwynedd to the stunning hazels at Ballachuan, on the island of Seil south of Oban. To walk here is to be reminded of the natural wonders these islands offer, and I defy you not to feel protective of them after visiting one. As I wander through such ancient groves, I imagine the time when forests like these covered much of our islands. I hope we will see our way to giving more land over to these precious habitats.

In Scotland, where access rights mean one can roam on nearly all land and inland water, my hike took me from the towering conifers and rushing burns of Reelig Glen, through farmland and private woods to one of Scotland’s first community-owned woodlands, Abriachan Forest Trust. It covers 163 hectares, so it’s easy to find your own slice of wilderness, and it is worth visiting for the variety of woodland birds alone.

Set in a hanging valley a stone’s throw from Loch Ness, and with miles of bike trails and paths, community events and a forest school, Abriachan is a model of what a community can do when it makes a woodland work for everyone, from children to pine martens. Nearby, Lodges on Loch Ness offers forest accommodation up close with the trees (from £280 a night for a lodge sleeping eight).

Across the Irish Sea, I slowed my pace with forest bathing guide Cat de Wit in one of the few remaining old-growth oak woods on Ireland’s east coast, at Tomnafinnoge, Wicklow. Paying close attention to the wood’s textures, smells and sounds, under Cat’s instruction, made a walk in the woods a meditative experience in which I began to spot layers of detail, from tiny fungi in the leaf litter to deer watching us between the trees.

Taking the ancient woodlands at a crawl rather than my usual fast stride, listening to the woods carefully and hugging the odd giant trunk, I found a whole new appreciation for the woods and emerged refreshed. This was not entirely unexpected, as some of the proven benefits of forest bathing include stress reduction, reduced blood pressure and improved sleep and mood. (A two-hour forest bathing walk in Wicklow costs €35pp.)

From Wicklow, I headed west to County Clare to join a community tree-planting day at the headquarters of the charity Hometree in Ennistymon, which works to protect and expand the few remaining fragments of Atlantic rainforest in one of Europe’s least-forested countries. Just over 11% of Ireland is wooded, compared with an average of 35% in the rest of Europe.

As we planted willow whips, crab apples, field maples and alders, I listened to people talking about the need to feel they are making a difference in the face of climate change, doing something positive instead of being overwhelmed by eco-anxiety. Tree planting at Hometree takes place on the first and third Saturdays of every month, and there are similar events across the UK and Ireland. Charity Plant One, for example, is holding a Celtic rainforest restoration day at Cabilla in Cornwall on 30 November, while non-profit Forests Without Frontiers runs tree-planting in Sussex too. Check out the Woodland Trust, the Tree Council, and the National Forest for other listings.

Towards the end of my journey, I returned to the trees my father planted, to see them with new eyes and new appreciation, and to experience mikkaminumanosakura, the moment of change in which they shift from summer finery to their more austere winter selves. As autumn approaches, now is the time to get out into the woods, to breathe in the rich scents of fallen leaves and reconnect with our glorious woodlands, to get to know the trees around us better and, in doing so, perhaps learn to protect them better too.

• The Heart of the Woods by Wyl Menmuir is published by Aurum Press, £16.99. To support the Guardian and Observer order a copy at guardianbookshop.com. Delivery charges may apply