Why Do Trees Drop Their Leafs?

Why Do Trees Drop Their Leafs?

Believe it or not, the closest relative to the whale is the Hippopotamus. What does this have to do with tree dropping leaves? Over time, animals and plants adapt to environmental change or to take advantage of an opportunity to better thrive. A great example of biological advantage is when the Pakicetus, an amphibious mammal that lived 47 million years, evolved into the Ambulocetus. The Ambulocetus (the walking whale) returned to the sea. This transition took about 10 million years. These water creatures evolved into the whales found in oceans today.It is difficult to say why these land animals returned to the sea. Maybe it was predation pressure or a food source opportunity. Regardless, these animals found success in their new water environment and flourished.

About 95 million years ago during the Pleistocene, the Earth cooled and got drier. Hardwoods exploded and spread rapidly about during this period. Deciduous trees coped with cool and dry climates by dropping their leaves at the end of the growing season. The spots where leaves attach to trees seal. Fluids that keep leaves green are cut off and leaves turn color and eventually drop from the tree. Leave dropping reduces surface area and the loss of water. The tree becomes dormant and conserves water and food for the next growing season. This ability to drop leaves allowed hardwood trees to spread across all the continents.

Trees respire through their leafs. Leaves exchange water, carbon dioxide, and oxygen in the summer to fuel the photosynthesis process. Leaves are full of little holes that lose water. For example, a mature oak tree has about 60,000 leaves and moves between 50-100 gallons of water on a hot summer day. Denver is more humid compared to 100 years ago because of its mature urban canopy.

The picture above shows four mature ash trees. The trees were beautiful with their reddish, orange leaves this fall. These four magnificent trees could be in jeopardy. For the latest emerald ash borer update, please visit the DLC Arbor Facebook page.


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