Tree Leaves as a Natural Clock

By Calvin Krogman

The use of the growth cycles of deciduous tree leaves is a useful natural clock that depicts the changes in seasons. In early spring, most deciduous tree branches are barren, with no leaves at all. As spring temperatures rise, the snow cover begins to melt, providing trees with water that is essential to their early season growth. Snowmelt and early rains influence the growth of leaf buds on the tree branches. As temperatures rise and rainfall increases, these buds grow to be leaves.

The young spring leaves are often very small, but continue to grow into the summer. As summer comes, tree leaves reach their maximum size. This is useful to the trees because it allows for an increase in the surface area of the leaf. The leaf contains the photosynthetic elements that the plants use to synthesize carbon dioxide, water, and sunlight to form the sugars that the tree uses as nutrition. During the summer months, the days are longer and the trees are exposed to the sun for longer periods of time. Thus, by increasing the size of their leaves, trees are able to maximize their photosynthetic production and take advantage of these times.

seasonal leaf cyclesAs summer progresses, rainfall amounts typically begin to decrease. For a while, trees are able to continue their growth and photosynthesis based on water reserves from the snow runoff and early spring rains, as well as other standing pools of water above and below ground. Eventually, these water resources are either exhausted, or sunlight exposure decreases to a point where it is no longer profitable for the tree to be attempting to produce sugars via photosynthesis. At this time, usually in fall, the tree leaves begin to change colors, because the tree begins to stop supplying the leaves with water in an effort to conserve through the winter. Thus, the leaves and their photosynthetic elements die, causing them to turn from green to the various shades of yellow, red, and orange which are typical of the fall landscape.

As fall progresses into winter, these leaves have died off, and with the increase of cold winter winds, these leaves soon fall off of the trees. By the time that winter begins, most deciduous trees are again barren of all of their leaf cover. But the cycle will begin anew in the next spring.

Strengths:

The changing of leaves throughout the year is a very regular process. A deciduous tree will continue to go through this cycle of leaf change throughout its life. Thus, it is fairly easy to predict whether it is winter, spring, summer, or fall by looking at a deciduous tree.

Weaknesses:

One major weakness in this system is that it isn't exact. Buds do not form exactly on the first day of spring, or change color the first day off fall. These changes are relative, and do not always occur on the same date. However, the major changes in the leaf's life cycle are usually predictable to within a few days to a couple of weeks.

Another weakness is that some areas do not have many deciduous trees. Extreme latitudes and altitudes are usually occupied by evergreen trees, which as the name suggests, do not lend any information to the changing of seasons based on their leaf cycles. Other areas do not have many trees and are mostly grasslands. But for the most part, deciduous trees are fairly common in most regions of the United States, even if only planted in somebody's yard or in a park.

Factors:

There are many factors that influence the cycle of leaves on deciduous trees. A major influence is temperature. The colder the spring is, the longer it will be before a tree begins to produce buds and leaves. Not only is spring runoff unavailable, but also the young buds and leaves would be susceptible to frost during these early times.

Another factor is the availability of water. In areas that do not receive much rainfall, or during drought conditions, leaves may begin to turn colors earlier than normal. The opposite holds true for those trees that receive excess water throughout the summer and into the fall. Trees like these may not loose their leaves until late into fall.

Time Frame:

The changing of leaves in deciduous trees is a useful natural clock to tell the change in the seasons. By looking at the leaves, you can usually tell what time of year it is to within a couple of weeks. However, because leaf cycles are a continuing process that repeats every year, it is impossible to use the changing of leaf colors to judge time intervals greater than one year. This process can tell the time of the year, but never which year it is.