Eastern Hemlock leaves: The needles of most conifers remain on the tree for two or three years. Their shape and structure lend themselves to winter survival. (Photo by Mary Holland)

Winter poses challenges for every living thing, two of the most crucial challenges being to obtain enough water and nutrients to stay alive and to not freeze. Humans have it easy; we build ourselves houses, wear layers of clothes, bury our water pipes and, thanks to freezers and grocery stores, have relatively fresh produce year round. Other animals and plants, so often considered "lower" forms of life, have come up with equally, if not more, impressive strategies.

Trees, unlike animals, cannot move in order to avoid extreme winter conditions, and unlike many other plants, they don't have the option to overwinter as seeds or rhizomes. So over the years they have adapted in ways that allow them to survive standing in place. We know that hardwood (deciduous) trees, due to the shortage of available water in winter, lose their leaves in order to minimize transpiration of water vapor into the air. Their broad leaves are a major source of water loss. But conifers—all but the larches—manage to survive and keep most of their leaves. (Conifers do shed their leaves. They just do it gradually, not all at once like broad-leaved trees.)

How do they do this?

The structure of a conifer leaf, or needle, is superbly adapted for retaining moisture. To begin with, the leaves of conifers (needles) have proportionately less surface area than deciduous leaves due to their needle-like shape. Hence, less water is lost. They also have a waxy, water-repellent coating called cutin, which acts as a barrier, preventing excessive loss of water.

Leaves have little mouth-like openings, or pores, called stomata ("stoma" means "mouth" in Greek), through which gases such as carbon dioxide, water vapor, and oxygen move rapidly in and out. The stomata of conifer needles close tighter than those of deciduous leaves, contributing to reduced water loss.

Interestingly, sunny winter days are more stressful for conifers than cloudy days, as far as water retention. Because most needles are dark colored, they readily absorb heat. The temperature of the needles rises considerably above the temperature of the air, causing greater water loss. On a cloudy, windy day, the clouds block warming solar energy, and wind removes heat from the needles, reducing water loss.

When it comes to freezing, ice crystals inside plant cells are usually fatal, but ice between cells is not. Conifers exhibit a protective behavior known as extracellular freezing. By altering the lipid concentration within their cells, conifers induce water to migrate out of their cells. Water outside cell walls freezes first. As this water changes from liquid to solid, small amounts of heat are released, and this heat helps prevent the cellular water from freezing.

The plumbing of conifers differs considerably from that of broad-leaved trees. Given an adequate amount of snow, soil does not freeze, which means some water is available to trees. Broad-leaved trees lose most of their ability to move water during the winter. Capillary action is broken after the first freeze, and they need to re-grow these tissues in the spring. Conifers, however, transport water in tubes that have special valves to allow resumption of water movement during the winter, should conditions be just right. Conifer cell walls are also stronger than the cell walls of hardwood trees and can better withstand ice expansion.

Even though the cone shape of most conifers allows snow to more easily slide off their branches, conifers do have higher leaf densities than hardwoods. This means snow can quickly accumulate enough to break branches. To offset this, conifer branches grow at more obtuse angles to the main stem. This allows branches to reach snow-shedding angles with less bending. Longer wood fibers also generally provide more flexibility.

There are many factors that contribute to the ability of conifers to survive in a cold, snowy climate. (Not only do they survive, but on sunny days, their leaves may even photosynthesize.) Their adaptability allows conifers to manage a fine line between water loss, replenishing leaf moisture and handling the weight of snow.

Mary Holland is the author of "Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England." She has a natural history blog, which can be found at www.naturallycuriouswithmaryholland.wordpress.com.