Early spring is a great time to take a good look at bark before the trees leaf out. The bark of each variety of shrub and tree is different in texture and color, and changes as the tree grows.

I started noticing bark after a field trip and presentation I attended a couple of years ago given by Tom Wessels, a professor of ecology at Antioch New England and author of Reading the Forested Landscape.

The bark of paper birch is one of my favorites. You’ll find this tree growing way up north in the coldest of temperatures­it can survive to minus 40 degrees F. Of course, its white color allows it to stand out in any clump of trees, but Wessels told us the bark also allows the tree to thrive where other trees might not survive.

First, the white bark allows the tree to reflect winter sunlight. If it couldn’t, the low winter sun, even in January, might heat up the inside of the tree, causing it to expand; then, as the sun sets, the bark would contract faster than wood underneath and split.

Wessels told us the paper birch developed the ability to peel off in those gorgeous layers to discourage the growth of epiphytes (plants that grow on other plants without harming them,) which could expand and darken the bark. Underneath the exfoliating layers, new white bark grows, keeping the tree safe from winter sun.

The oily birch bark also acts as a vapor barrier, keeping water inside the tree during the cold, dry months. Native Americans of the Northeast knew that and used the bark to build canoes, baskets, and watertight boxes.

So how do other trees survive cold winters if they don’t have white bark? Take a good look at a mature oak or a black cherry. The rough, ridged bark of the oak and the scales of the cherry bark act like fins that dissipate heat into the surrounding air rather than conduct it to the wood beneath.

Another bark that offers great winter interest belongs to the Common Ninebark (Physocarpus opulifolius), a shrub native to New Hampshire. Its common name gives you a clue as to why it’s such an interesting plant to have in your garden. The bark exfoliates, layer after layer, peeling off great long strips of pale orange brown bark. Beneath them, you can see the newer layers, which are lighter in color. When I feel that nothing is happening in the winter garden, I trudge over to the Ninebarks and enjoy the texture of their stems, noting the marks where bark has loosened itself to flutter in light breezes.

In the swamp near our home, there are many dead trees, killed by the rising waters behind beaver dams. It’s easy to tell the hemlocks. Hemlocks, pines and spruces all rot from the outside in, but the hemlock retains its bark around the rotting trunk. If I find a tree with just the bark still in place and not much else, I know it was a hemlock. Of course, enough time must have elapsed for the wood to rot away. Wessels said he’s seen hemlocks 35, maybe even 50 years dead.

As bark leaves a dead tree, what’s left behind is fascinating to look at. It’s easy to find where a limb broke off years ago and was then covered with new growth. The tunnels made by wood- eating insects meander up and around. Often you can find large bore holes, rectangular and wide at the surface and narrowing down as they go in. These were made by woodpeckers, removing dead material to get to the insects living inside.

The dead trees that still have bark often have wonderful epiphytes growing on them, curly white and gray-green lichen, crawling up the sides and spreading out like a slowly widening pool of water or looking as if someone had thrown paint onto the trunks.

Many dead branches have strands of plant material that twist in the breeze. At a distance, the trees still look alive with their coating of pale green. It’s only when you get close you notice the lack of leaves, the gaping holes, the slightly peeling bark. Near the base, I’ll often find lovely, soft moss crawling up the trunk. In season, small stalks with little rounded tips appear. The moss works its way into the crevices of the rough bark and I love to run my hand over the smooth surface.

Like our own skin, the bark covers the tree and protects it. When a nick is made in the bark, the tree works to repair the opening, growing from the sides over the wound. In a few years, the gash is covered once more. The bark is whole again and the tree continues its quest for the sky.

By Susan M. Poirier, Master Gardener