Unbe-leaf-able Transformations

Tuesday, September 24 marked the first day of fall, meaning that all of us New Englanders, and anyone in the New England area, can now officially embrace the “it’s fall y’all” attitude. I’ve already seen pumpkins decorating neighborhood steps, fall flavors showing up on menus, and apple picking photos in full swing. One of the many delights of the fall season is the changing of the leaves! It’s something we see every year here in the Northeast, but it never seems to get old. The transition from different hues of green to firey landscapes of oranges, yellows and reds is truly mesmerizing, but what causes this change to happen? Are the trees dying? Why don’t they need their leaves in the Winter and why don’t the leaves stay green?

 
​The first step of answering these questions is understanding why trees even have leaves because not all of them do! Trees with leaves that change color and are shed annually are called deciduous trees. That is what we will be focusing on today.

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In New England, some common types of deciduous trees are Sassafas, Sycamore, Hickory, Birch, Oak and, of course, Maple. Although the leaves on these trees can look very different from each other, they all do the same job. A leaf’s job is to make food for the rest of the tree through photosynthesis. Photosynthesis is the process of creating food by using light energy from the sun to convert carbon dioxide from the air and water from the ground into sugar!

The reason that most of the leaves we see on trees are green in the spring and summer is because there is a very special chemical called chlorophyll that helps the leaves capture the energy from the sun. Without their leaves, trees are not able to complete the process of photosynthesis.

You might be thinking, “Why, then, do deciduous trees shed their leaves in the winter and how do they survive?” Great question! I like to think of trees shedding their leaves like animals going into hibernation for the winter. When animals go into hibernation, their bodies will change the way they are working in order to conserve energy. An animals’ heartrate will slow down, their body temperature will decrease, and they will essentially fall into a deep sleep for months at a time. Trees are also just trying to save energy, but when they do it, it’s called dormancy! If you think of all the leaves on a tree as parts of its body (maybe fingers or toes), it’s a lot of work to keep all those pieces functioning in cold weather (just think of how quickly your own toes and fingers get cold in the winter). Instead of wasting energy to continue to photosynthesize when temperatures are very low and there is a risk of water freezing, trees will suck all of the extra nutrients and energy from the leaves and then shed them.

The draining of all the extra energy and nutrients is what brings us to the changing of the leaf color! In order to get the most out of each leaf, the tree will cut off the supply of water and anything else that goes into the leaves, but it will continue to take nutrients out of the leaves. One of the molecules that gets broken down for its nutrients during the process is the chlorophyll. If you recall, chlorophyll is also responsible for giving leaves their green color. As the chlorophyll is drained from the leaves, other chemicals, many of which that have been present in the leaf for the entire spring and summer, are able to be seen more clearly! Scientists are able to pinpoint exactly what chemicals are responsible for what colors we see in the leaves throughout the fall:

Once a tree has gotten all the nutrients it needs from a leaf, it will drop it to the ground and patch up the spot the leaf used to be with a self-made band aid. After all the leaves have been drained, the tree will go into dormancy for the winter. During this time, growth, energy use, and metabolism will dramatically slow down (just like hibernation) until the weather warms up again in the spring! The leaves on the ground will stay colorful for some time, but without being connected to their main source of nutrients and moisture, they will eventually dry up and turn brown.

Fall Leaf Activity:

Head outside and see how many different colored and shaped leaves you can find! Once you’ve collected a good amount, try to sort them. You can sort them however you want:  by shape, by color, by size, etc. Explore the similarities and differences between the leaves. You can see if similar shaped leaves have similar colors and see if you notice any patterns among your findings.

After you’ve spent time examining and sorting your collection, make some art! You can organize leaves outside in a temporary installation much like artist/sculptor Andy Goldsworthy, or you can tape or glue your leaves to a piece of paper for something you can display inside your home!