I enjoy walking far enough into the woods so there are no sounds of town, highways, dogs, or anything else. Silence.

If I sit still long enough, I begin to hear other sounds: chipmunks rustling in the leaves, the whoosh of air from the down stroke of a raven flying low overhead, the drumming of ruffed grouse, chickadees chattering. On a recent afternoon it was so still I was surprised to realize I was hearing the sound of a leaf detaching from its twig and floating to the ground. It wasn’t much of a sound, but against the backdrop of silence, it was remarkable.

Not long after I’d heard it, another detached and sailed down. I’ve read about this. As trees prepare to lose their leaves, cells between the twig and the end of the leaf stem release enzymes and form an abscission layer that frees the leaf to fall. So why are some hardwood trees still holding fast to their dead, dried leaves while others sport bare branches?

All trees shed their leaves at some time, but there is variation in the timing. Consider the evergreens, which seem to be fully leafed all the time. While conifers lose some of their needles every year, many species retain needles for several growing seasons.

At the other end of the spectrum are deciduous trees which lose all their leaves at the same time every fall. To understand this variation requires a consideration of both the physiology and evolutionary history of trees.

From the fossil record we know that evergreen was once the only kind of tree. As conditions and ranges changed, trees responded by developing new ways of growing and shedding their leaves. Now we still have some evergreens but also the newcomers — birch, maple, cherry, and aspen — with short-lived leaves.

Between those two is a third class whose leaves don’t all fall when they die. Marcescence, or leaf retention, is most common in oak species, American beech, with hazel and hophornbeam.

Scientists believe that evergreens benefit by increasing the time available for photosynthesis and reducing the loss of nutrients caused by dropping leaves. Leaf fall of deciduous trees evolved to help trees in environments with changing seasons to lessen water loss and frost damage during the cold while improving the efficiency of photosynthesis during warmer seasons.

So what might be ecological advantage for being somewhere between these two extremes? Are the leaf retainers just progressing more slowly along the evolutionary track? Most plant physiologists agree that marcescence is a trait of juvenile trees, but that doesn’t explain leaf retention in mature trees.

Some speculate that the adaption helps trees growing in dry, infertile places, and it’s true that one sees beech and oak out-competing other species in these conditions. They argue that keeping leaves until spring delays their eventual decomposition, so more organic material is available when it’s needed most.

Often the retained leaves are only on the lower branches. This suggests a strategy of holding onto leaves to trap snow, resulting in more moisture at the base of the trees when that snow melts in the spring.

Others argue that retained leaves may lessen browsing by deer and other animals. Those dried leaves may hide buds or make it more difficult to bite them from the tree.

Whatever the reason for marcescence, when spring comes the expanding buds will shove those leaves off to make room for new greenery. In the meantime, those brown leaves add a special sound to the winter woods and provide some welcome shelter for overwintering birds.

If you are looking for the perfect holiday present for the naturalist among your family and friends, it exists! Mary Holland has just released her new book Naturally Curious Day by Day. This gorgeous book has photographs and essays on animal and plant species of the Northeast for each day of the year. Nature lovers of all ages will enjoy and appreciate this treasure trove of information and beauty.