Q: While looking for our Christmas tree, my daughter asked why some trees have needles and some don't. She also wondered why the needles don't fall off. I don't think she thought my answer was very good, so I am asking you.
A: Your daughter is brilliant. Thinking about how plants work is an excellent start to investigating the natural world. I hope you encourage her to ask more questions.
We can start by asking her if she knows where most of the needle-leaved trees like pines, spruces, hemlocks, junipers and other similar trees grow. They are mostly native in northern areas around the world, and they grow on the higher places on mountains. These two places are cold in the winter, and many of those places are also cool during the summer.
Many people don't realize that even though needle-leaved trees are called evergreens, the needles do fall off. They often stay on the tree for three or more years, but then they fall off. Some evergreen species lose the needles all at once (during a week or two) in the fall, and in some species, the needles fall off at other times of the year.
The needles that fall off in the fall cover the ground to help protect the tree's root system. If they fall on lawn areas, they need to be raked up to prevent them from killing the grass. Many trees used for Christmas trees lose their needles in the fall, and they often remain stuck in the tree, so the trees need to be cleaned out before you bring them home.
It is hard for plants living in northern areas and cold, mountainous areas to produce enough food from photosynthesis during the short, cold summers. Needles that remain all year can still get some sun and do some photosynthesis in the early spring and late fall, when deciduous trees don't have any leaves. They can even do some in the winter, if temperatures are right and the snow isn't covering the leaves.
Speaking of snow, the needle shape helps to hold snow on the branch during the coldest weather to protect the tree from cold and winds. The snow will then melt next to the tree, where the roots can get it. Many mountainous soils are very rocky and don't hold much water, so needles catch the snow before it blows away. The needle's shape and arrangement on the branch, pointing in every direction, also help to receive more sunlight than a flat leaf only pointing in one direction.
Evergreen needle leaves are smaller and have less surface area than deciduous leaves, so they survive cold temperatures better. The stomata openings for gas exchange are all on the bottom of the needle, so snow can't clog the openings. The needles and the rest of the tree are filled with chemicals called terpenes that prevent the water in the tree from freezing, and these chemicals give us the fresh pine scent.
Some of the products we get from the terpenes in the needle-leaved trees are pine oil and turpentine. Both are very flammable, which is one of the reasons Christmas trees burn so easily.
There is another thing we should consider about the natural range of the needle-leaved evergreens. Many of them grow in areas that have cool summers and rocky, fast-draining soil. If you plant a spruce tree in a warm summer area or in a clay soil area, it might survive, but it won't thrive. Then, if there is a very warm summer or an especially wet season, the tree may drop down into the slow-death range of plant health. Just because a plant will survive in your landscape, doesn't make it the best choice.
Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.
Photo credit: jplenio at Pixabay