Trunk call: A red oak in Anna Pavord's garden is set to get a serious trim

(Alamy)

We inherited a red oak (Quercus rubra) when we moved into the garden I still think of as "new". For an oak it grows quite fast, and though it is nowhere near its potential height (25m), it's an important feature in the garden. The leaves are bigger than those of our native oak and in the right sort of autumn, they turn rich, tawny shades of brown, yellow and red.

Unfortunately, it is already beginning to die back in the crown. Whenever there's a gale (and there seem to be more of those than there used to be), bits of lichen-covered branch crash down on to the ferns and cyclamen growing below. They probably mind less than I do. A gardener's instinct is to tidy up, keep order. Perhaps too much. There are plenty of creatures who will do the tidying up for us, but their time scale is more leisurely than ours. We aren't prepared to wait.

At this time of the year, I'm generally cruising round the garden, making a list for the tree man who comes every winter to do jobs for us. There are plenty of cowboys roaming around in their trucks, with chainsaws to hand, but for tree work, you need someone who is properly trained and who is not going to land a heavy branch on your garage roof. For help in finding the right person, go to the Arboricultural Association's 2010 directory (01242 522152, or trees.org.uk).

Our list is different each season, though it always includes work on the clumps of hazel scattered through the garden. Hazel is constantly throwing up new growth from the base, so that an established clump contains stems of different ages and sizes. Each year we take out a couple of the oldest stems in a clump. They make excellent firewood – the most sustainable source of all. Working on a 12-year rotation, you can get all the winter wood you need from a couple of clumps of hazel. And the catkins are gorgeous.

There's usually a tree in the garden that needs limbing up. This is entirely for my benefit. Some shrubs enjoy growing in the dappled shade provided by a tree, but I don't want the canopy actually sitting on their heads. The big willow in the garden is drooping into the path of a pear (Pyrus calleryana 'Chanticleer') that we planted six years ago. I'd always favour the pear over the willow, so a couple of branches need to come off the willow to give the pear growing room. Only one branch is actually in the way, but we'll be taking another off the other side, so the tree does not become lopsided.

And the magnolia (M. x wieseneri) I planted in the semi shade of a native oak has finally decided it is not going to die and has put on an impressive 75cm this year. After five years of doing nothing, it was about time, but I'm glad I can now dare dream of its huge, cream, June-July flowers. Meanwhile, an oak branch has spread too close to the miffy magnolia. It will have to go.

The red oak was on my list too, as I thought we would get the dead wood cut out of the centre of the crown. I've since changed my mind. Why? I blame the most recent Ancient Tree Guide, The Special Wildlife of Trees. The guides are published by the Woodland Trust, or more specifically, the Trust's Ancient Tree Forum. They've brought out other titles – Trees and Climate Change, Trees and Development – but this one (their sixth) spoke of a particular world so cleverly integrated, so harmonious, so balanced, it made me feel ashamed that I could plan to crash into it, just because the dead wood, to my eyes, looked out of place in a garden.

The red oak isn't ancient. Not by any means. I'd guess it was planted in the early Seventies. But the dead wood I've been clearing away from the ferns and cyclamen is thickly encrusted with lichen. Something, at least, is making good use of it. And the larvae of certain moths, in turn, use the lichens, grazing on them quietly. Lichens prefer bark that is alkaline rather than acid, but have suffered because of acid rain which leaches out some of the vital ingredients, making it less attractive to a lichen spore looking for a home.

Lichens are only one part of the complex eco-system that's explained in The Special Wildlife of Trees. The bats and birds that use old trees already have plenty of people fighting on their behalf. But let's hear it for the bark beetle, the brown tree ant, the cobweb beetle – all of which depend in one way or another on dead or decaying wood. All of the things that I, as a gardener, would think bad in a tree, turn out to be good for some other living thing: deadwood in the crown, lightning strikes, broken and shattered branch stubs, holes that fill up with rainwater, flaking loose bark, decaying heartwood. All have potential tenants eager to move in.

So that's why the oak job got struck off the list. It's a minuscule gift to a few of the 2,000 insects and other invertebrates which need dead and decaying wood for their homes and food. Generally, I'm unmoved by the wave of opinion telling us how to garden to attract wildlife. Much of it is ill-informed and impractical. But the world of the tree and its dependents (2,000 species of lichen in the UK alone) is a wonder that thanks to the Ancient Tree Forum, catches the imagination.

Between now and Christmas, I'll be planting new trees, too, most of them cousins of the trees that stand around us in the Dorset landscape: hollies, thorns, wild cherries, wild crab apples. And another magnolia, M. denudata. WJ Bean, whose multi-volume Trees and Shrubs is my bible, describes it as "one of the most beautiful and distinctive of all flowering trees" but also warns that it is "an occasional victim to the inclemency of an English spring". All the magnolias I've planted so far have been summer flowering, precisely to avoid this problem. But there we are. I can't resist.

'Ancient Tree Guide No 6: The Special Wildlife of Trees' is available free from Lorna Templeton at The Woodland Trust, Autumn Park, Grantham, Lincolnshire NG31 6LL, 08452 935682, ancient-tree-forum.org.uk