The winter landscape can look rather empty and bleak this time of year. And although the animals can seek shelter in burrows, under snow, or other protected places, the trees have no choice but to stay put and tough it out. But unlike most trees native to North Dakota that shed their leaves in the fall the evergreens such as pine, spruce, and juniper retain them.
Trees have a variety of adaptation to survive the winter. Most of these adaptations fall into four general categories: drop their leaves or develop adaptations for keeping them, physiologically acclimate to the winter conditions, resolve water issues, and/or reduce mechanical damage such as the breaking of branches from snow or ice accumulations.
Leaves are a major source of water loss for plants, and as such are difficult to protect during winter so most trees are deciduous. However, conifers have a different strategy, and their needles have some significant adaptations to endure the winter cold and desiccation. For example, there are generally fewer stomates on the needles of conifers and they close much more tightly than deciduous trees which help considerably in the reducing water loss. Plus, the needles of conifers also have a thick waxy covering or cuticle to reduce water loss. And the dense clusters of needles on conifers helps to form and maintain a microclimate around the needles which helps to conserve water. Lastly, most conifers will drop their needles after 2-3 years, but the shedding is not done all at once.
Conifers also have physiological changes that help them through the winter. Conifers as well as other plants become tolerant to subfreezing temperatures by a process called acclimation. The process is complex and not well understood, but it apparently begins in late summer with the translocation of some organic compound or compounds such as a sugar or a hormone, so the plant increasingly becomes more “hardy” or resistant to the potentially damaging effects of below freezing temperatures.
Even the shape of conifers is an adaptation to winter conditions. The thick pyramidal growth form also provides considerable resistance to the wind. It also helps to shed the heavy snow and ice accumulations from breaking branches. Snow accumulation generally just causes the branches to dip a bit and eventually just fall along the outsides of the branches without doing any major damage.
