Why Be Evergreen?

Advantages and disadvantages of trees losing leaves in winter

January-February 2025

by Steve Carroll, Contributing Columnist

As I write this column, most of our trees have dropped their leaves and will remain leaf-free until spring. Most trees, though not all. Obvious exceptions are conifers, such as pines and hemlocks, which are green year-round.

Why do some plants (deciduous species) lose all their leaves each year, while others are evergreen? Not surprisingly, there are advantages and disadvantages to each strategy.

The most obvious advantage of being evergreen is that these plants — though they do shed a portion of their leaves each year — needn’t replace their entire leaf canopy every spring. This represents a significant energy savings, energy that can instead be used for growth, reproduction, and defense against herbivores and pathogens.

Staying green also means that when temperatures are sufficiently warm and soils are not frozen, these plants can use sunlight’s energy to photosynthesize (combine carbon dioxide and water to produce sugars and oxygen). It has been argued that the conical shape of many conifers not only helps shed winter snow, but also increases their capture of the low-angle sunlight characteristic of winter, especially at high latitudes.

We should not be surprised that being evergreen also has disadvantages, for, as Kermit the Frog crooned, “It’s not easy being green.” For example, maintaining leaves year-round requires protecting the tissue from low temperatures (especially farther north) and the drying effects of winter winds.

Given the environmental challenges of winter, how do evergreens survive, and even photosynthesize? Much of this ability is due to their leaves. Whether pointed and needle-like, as in pines and spruces, or scale-like, as in juniper and Eastern white cedar, conifer leaves provide better protection from desiccation than do the relatively thin, flat leaves of most deciduous trees.

Conifer needles have a thick, waxy layer over their epidermis, which decreases water loss. They also have a low surface-area to internal-tissue ratio. Plants take up carbon dioxide and lose water through pores called stomates, which are typically sunken or embedded in the leaf epidermis.

Conifers are the dominant trees in many nutrient-poor environments, including the far north and high elevations, bogs, poorly drained habitats, and sandy soils. Given this wide range of habitats, conifers should not be expected to have identical characteristics. For example, the seed cones of jack pine and loblolly pine are serotinous; they open in response to fire and release seeds after the fire has eliminated or weakened the competition, creating a suitable, mineral seed bed. Similarly, longleaf pine in the Southeast U.S. has evolved a set of characteristics that allows it to thrive in fire-prone environments.

As always, there are exceptions to the patterns described above. For example, deciduous conifers, such as bald cypress and larch, and evergreen flowering plants, such as holly and southern magnolia. But that’s a discussion for another day.

Evergreens’ defining characteristic — staying, well, evergreen — provides many benefits when planted in our yards and gardens: year-round visual interest, cover for wildlife, privacy, a windbreak, and a means of hiding your neighbor’s ugly shed and clutter. Perhaps there is an evergreen just right for your property.

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Steve Carroll is a botanist and ecologist who speaks and writes about trees, gardening and the world of plants. He is the co-author of “Ecology for Gardeners,” published by Timber Press.