Why Leaves Drop and What Their Job Is

Why Leaves Drop and What Their Job Is

Why Leaves Drop and What Their Job Is

As autumn deepens, more and more leaves are turning colour, and falling to the ground, every day. For most Calgarians, the turn of the season is pretty, and the last display of our amazing plant life before winter takes hold. Mirage Landscaping is a full service commercial and residential landscaping design, construction, and maintenance firm. Fall clean up time in Calgary is upon us! 

Leaves, Leaves, Everywhere! 

The problem with the visual beauty of oaks, maples, birches and other deciduous trees with their red, orange, and yellow leaves, is that all of that fall beauty is going to hit the ground. Mathematicians have algorithmically modeled mature trees and have found that the typical large deciduous tree, depending upon its species, as well as current age and access to life supporting water and minerals, will be covered with approximately 90,000 to 200,000 leaves. Each leaf weighs between 0.5 to roughly 5 grams. This means that a 50-foot tall red oak could be sporting a metric tonne of leaves in its canopy, all of which will eventually end up on the ground.

The Story Behind Turning and Falling

Most have heard of photosynthesis. This is the conversion of solar energy into chemical energy as food for the tree, made possible by chlorophyll, which provides leaves with their green colour. What many people do not know is that chlorophyll is but one of up to three other compounds that assist in converting sunlight into starches and carbohydrates for the tree or are a waste byproduct of the process, all of which are responsible for the colours of fall.

Throughout the spring and summer, leaves are making the food that the tree will partially store to get through the winter. Chlorophyll uses the red and blue part of the visual light spectrum for energy and reflects away the green colour you see. What you cannot yet see is two other pigments that assist many trees in the photosynthesis process, because chlorophyll is so dominant. Carotene, which reflects away yellow, absorbs the blue and green light to make it useable by the tree. Anthocyanins are present in the sap of many trees and reflects away red light to reduce the heat load on the tree. If you ever see red and green in the same apple, the red side was in direct sunlight.

Chlorophyll production requires abundant light and water. As the days grow shorter and colder, the production of chlorophyll ceases to conserve water for the tree. As the chlorophyll dissolves, the leaves may turn yellow, orange, or red, as the balance between the remaining yellow carotene and red anthocyanins in the leaves becomes apparent.

Ever since the leaves formed in the spring, the source of their falling off the tree has been there too. At the base of every leaf are specialized cells that in the growing season are the “pipes” that exchange water to the leaf with carbohydrates from the leaf, all transported by sap. As the leaf shuts down the photosynthesis and the transpiration of carbon dioxide and oxygen, these cells swell to form a cork like matrix that dries out and becomes a weak point. Then, wind, or gravity take care of the rest and the leaves fall out of the tree in their thousands.

Don’t Let Nature Take Its Course

Then you are left with a large mess that can become quite hazardous if not removed, or at least mulched. First, there is one last color change, to brown. As the leaf decays, its cellular breakdown results in the production of tannins. There is nothing wrong with that, until the leaves become saturated by rain or snow melt. Then the tannins react with water to become tannic acid. This acid, though weak, is notorious both for staining concrete and etching the clear coat layer of car paint. It also can alter the pH level of the soil surrounding the tree, eventually retarding the growth of any plant life near it, including grasses.

Leaves are much more than a chemical threat, they are slippery. When green, leaves are waxy and hydrophobic, they shed water so that it falls to the ground for use by the root system. The leaves remain slick on the ground too. Not only do they present a slip and fall hazard, even when covered by snow, they can make even the heaviest vehicle lose traction when they are left in a driveway or the road.

Contact Mirage Landscaping for a free quote for our comprehensive fall cleanup.


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