Tough As Pine Needles??

The forests of New England provide lush, dense greenery in spring and summer, but something happens in the fall that always results in the landscape looking like a monotonous mash up of grays and browns. Trees cut off the nutrient source to their leaves and in a matter of weeks they go from green, to bright red, orange and yellow, to completely bare. Except for, of course, the pine trees!

​Many of you may have noticed over the years that the pine trees sharing the forest with broad-leaf, deciduous trees do not go through the same cyclical shedding of extra weight in preparation for the winter. This is because pine trees have adapted over time to withstand much harsher weather conditions than we have in Massachusetts. The lucky pine trees that have found their way to the temperate forests of the east coast of the United States are on vacation compared to some of their relatives a little further north!

​The most common pine tree in our neck of the woods is the Eastern White Pine. There are other firs that grow in Massachusetts, but Pinus strobus is by far the most abundant. Eastern White Pines are identifiable by their long, soft, flexible needle clusters that come in groups of five. One for each letter of the word W-H-I-T-E.

​Broad-Leaved Tree: ​Any tree with flat, wide leaves
Deciduous Tree: Trees that shed their leaves after one growing season
Adaptation: A behavior or characteristic of a plant or animal that makes it better able to survive in its environment
Temperate Forest: A forest with a moderate climate that tends to have four distinct seasons: summer, winter, spring and fall

White pine branch with multiple needle clusters

As mentioned, Massachusetts has a mix of both pine and deciduous trees, but as you travel more northward in North America, pine trees begin to define the landscape. This is because pine trees have more adaptations that allow them to be successful in cold, dry, nutrient deficient conditions year-round, whereas deciduous trees are better adapted for climates with distinct seasons such as those in New England. Looking at ecosystems further north, such as into Canada and northern parts of Europe and Russia, deciduous trees become non-existent.

 
The biome that is dominated by pines is known as the Taiga. The Taiga does not have distinct growing seasons and ranges between 23-41 degrees Fahrenheit all year round. It has long, cold winters, short summers, low precipitation and nutrient poor soil. So, how are pine trees able to survive these extreme conditions?

​Biome: An ecological community of plants and animals that covers a large portion of the Earth

First, their behavior. Pine trees are also known as evergreen trees and this is because of their appearance of being green throughout the entire year. Unlike deciduous trees, evergreen leaves (yes, needles are leaves) live for more than one growing season. Because of this, they appear to keep their leaves forever, but we know by looking at the forest floor under pine trees that they do, in fact, lose their needles at some point. This is true. The White Pines of Massachusetts will typically drop needles every two years. Because new needles are grown each-year, not all of the needles fall off at once making it appear as though they never lose their needles. The advantage of dropping needles so infrequently is that pine trees do not have to use as much energy in the springtime to produce a whole new set of leaves. This is less important in Massachusetts, but extremely important when we think about the Taiga and lack of distinct seasons. For Massachusetts, spring is a time of regrowth when plants get more sunlight, water and nutrients, but springtime in the boreal forests, is not much different than the rest of the year and there is no influx in resources to support an entire biome’s worth of regrowth.

Green shading indicates where the Taiga biome covers on earth

Boreal Forests: Northernmost forests with freezing temperatures for six to eight months throughout the year. The taiga biome is home to boreal forests.

Outlined in yellow is the boreal forest found in North America

Alaskan boreal forest

Second, their structure. A key characteristic of pine trees are their needle leaf structure. Needles serve the exact same purpose for pine trees as broad leaves serve for other trees. They use chlorophyll, which is why they are primarily green in color, to convert the sun’s light energy into sugar. A process also known as photosynthesis. You may be wondering, if they serve the same purpose, what are the benefits of the distinct needle-shape? Well, a pine needle is essentially a broad leaf that has been rolled up tightly around the center vein of the leaf. Just as fat keeps animals warm in the winter, these rolled up structures keep the internal workings of the leaf from getting too cold, allowing nutrients to continue to flow freely between the needles and the tree trunk.

Another important adaptation of pine needles is the waxy coating that surrounds each individual needle. This coating is to ensure that moisture does not leave the needles when the air outside is dry. If we think about when the air is driest, it is usually during the winter time. Cold air holds less moisture than warmer air and humans are a great example of an organism that gets dried out as the air gets colder – just think about your hands and lips in the winter! The waxy coating of the needles is not the only adaptation of pine trees to conserve moisture. The bark is also extremely important for pine trees when storing water. Next time you are in a mixed forest, compare the bark of an evergreen to that of a deciduous tree. What do you notice? Hopefully, you will observe that the pine tree bark is much thicker, which is just another adaptation that keeps the trunk of the tree warm and prevents moisture loss to the cold, dry air of the Taiga.

Thick, waxy needles of a balsam fir tree

Pine tree trunk

Silhouette of a pine tree that shows conical shape

 The last point I will mention about structure, although there are many more, is the overall shape of most evergreens. When we think evergreen, we typically picture the triangular-like tiers of a Christmas tree. The top of the tree is very small and gradually gets wider towards the base of the tree. This shape is imperative to the survival of evergreen pine trees during times of excessive snowfall. If the tops of pine trees were as wide as the bottoms, or wider like in the case of many deciduous trees, the snow build-up would be too much weight for the tree to handle and it would most-likely bend or snap with the added pressure. Because the trees have a conical shape, there is minimal snow build up at the tops of the trees, and it is the bottom branches that are burdened with most of the weight from excessive snowfall. The branches at the base of pine trees also happen to be the oldest and sturdiest of all the branches meaning that they are most-able to handle the extra weight!

And finally, pine trees have an exceptionally cool way of dispersing, or spreading, their seeds to ensure the survival of future generations. Each scale of a pine cone is home to one seed of a pine tree (also known as a pine nut) and together, the many scales create a protective structure known as the cone. When conditions are dry, pinecones will open up to drop their seeds, but if conditions are wet and not favorable, the pinecones will close so as not to drop the seeds at times when they may not survive. This is something we can observe when looking at pine cones in Massachusetts. If you stumble across a cone bearing pine tree during dry weather, you will notice the individual scales will be spread out so that seeds can drop with ease. On the contrary, if you come across a pine tree on a rainy day, you will notice that the cones are tightly closed so as not to drop the seeds when they might drown from too much water!

Example of an open pine cone ready to drop its seeds

Example of a closed pine cone not ready to drop its seeds

Evergreen pine trees are tough and everything about them is to ensure their survival in areas where many other plants and animals struggle to thrive. Can you think of any other adaptations that pine trees may have to help them combat harsh weather? Next time you are walking in a mixed forest, try to find a pine tree and a deciduous tree to compare with each other! Do you notice any adaptations about the deciduous tree that makes it better able to survive the changing seasons?