In many places, fall is the most beautiful time of year. Cool outside, the days are getting shorter and the light softer, with the changing leaves creating a riot of color. In New England, for example, fall foliage festivals mark the busiest tourist season of the year, world-renowned for the stunning scenery and incredible variety of landscape. People come from all over to enjoy this annual phenomenon. In fact, most northern areas that experience true winter – as in cold, snowy winter – also see the leaves change color and eventually drop to the ground as a signal that summer is over and winter is on its way. But have you ever wondered why this is? Why do leaves change color? And why do they eventually fall off trees each winter? Here are a few facts about leaves.
Why Are Leaves Green?
There are two main types of trees – evergreen and deciduous. While almost all evergreens retain their leaves year-round, the leaves of most deciduous trees change color and eventually drop. The way it works is that in the spring and summer, the leaves are charged with creating food to enable the growth of the tree. The cells in the leaves contain something called “chlorophyll”, which gives leaves their green color. Chlorophyll absorbs sunlight and water, and uses these along with carbon dioxide to generate nutrients in the form of carbohydrates, such as sugars and starches. This process is known as “photosynthesis.”
Along with chlorophyll, the leaves contain other elements such as carotene and xanthophyll. These cells are typically orange or yellow but are overpowered by the much greater level of chlorophyll that is present.
Why Do Leaves Change Color?
As summer begins to fade, the temperature drops and the days get shorter, triggering the leaves to stop generating food in preparation for winter. The tree, for its part, notices these changes in the weather as well, and sets about sealing up the hole that connects the leaves to its branches. Since the leaves will no longer be providing sustenance, the tree eliminates the opening in order to retain as much moisture as possible.
Meanwhile, the chlorophyll, no longer needed, begins to break down, allowing the already present oranges and yellows to become the majority and slowly change the overall color of the leaf. There are a large number of factors that determine the exact color. Different trees have different chemical makeups, meaning they experience different color patterns as the leaves slowly die. For example, trees that change to deep reds and purples normally contain larger quantities of red anthocyanin pigments.
Why Do Leaves Fall Off Trees?
Once deciduous leaves stop producing food, their usefulness essentially ends. As part of the natural cycle, a new set of cells grows at the base of the stem that eventually eliminates the supporting tissues that keep the leaf connected to the tree. As the link to the tree weakens, the leaf is either blown off by the wind, or eventually its weight just helps it fall to the ground.
The vast majority of broadleaf trees in northern climates lose their leaves each fall. Some trees, however, have leaves that simply die but remain on the tree until spring, when they are eventually replaced by new leaves.
How Are Evergreen Trees Different?
Evergreens are trees that never lose their leaves, such as pine, spruce, fir, and cedar. Of course, there are exceptions to this but, for the most part, the leaves remain green and in place all year-round. The reason for this is in the structure of the leaves themselves. Most evergreens have small, tightly-wrapped leaves with a waxy coating (i.e. pine needles). This makes them resistant to inclement weather and, more importantly, allows them to retain water much longer than a regular leaf. This is particularly useful to the trees in winter, as the leaves can continue to provide water after it has become too cold to draw moisture from the soil.
Autumn treats us to many variations around the world due to the wide range of weather patterns. Temperature, how quickly it changes, along with how much sunlight and rain occur, will determine the exact pattern of the changing of the leaves. In general, cooler, cloudy weather with more rain will result in more intense colors. That’s unless there is an early freeze, which speeds up the process and eliminates certain steps. In large part, it is this unpredictability which makes fall foliage festivals so popular, since no two are ever exactly alike.