Deciduous vs Evergreen Trees
Let's talk about trees that lose their leaves - and trees that don't!
Losing Leaves
Deciduous trees lose their leaves in the autumn. That's a big word. Say it with me.
De-ci-du-ous.
The word deciduous is from Latin. Decidere means to fall.
Timing
The more productive the tree is in a given year, the earlier they lose their leaves! So when you have a beautiful, lush growing season you will have an early Autumn. It has less to do with temperature than productivity. Isn't that fascinating?
Here is an image that I put together with autumn leaves. It says Let go and grow.
When a tree loses its leaves and goes dormant for the winter, it's actually doing a lot of work. Why do they do so much work? Why do they drop their leaves each autumn?
They lose their leaves to reduce water loss. Let's talk about how plants lose water.
Transpiration
We respire (sweat). Plants transpire.
Transpiration passes mostly through a plant's leaves, so losing their leaves greatly reduces fluid loss.
We did a simple activity to track plant respiration, from NOAA. Here's a link:
We put a zip-top bag on a leaf and left it there for 2.5 hours. This showed us how much moisture was released through transpiration over that time. Here's a photo of our eggplant leaf in a bag.
And the bag, removed from the leaf after a few hours.
The leaves have adorable little bitty mouths on their undersides that let the water vapor out. Each one is called a stoma. It has two guard cells that can close to stop vapor from being released. Here is a sketch that I made of a leaf, and a zoomed in picture of guard cells and stoma on the epidermis. The plural of stoma is stomata.
This is accomplished with infrared cameras. Here's an example close-up with a pair of house plants.
There's a well planned lesson that they put together on the topic, I pulled the above two images from it. Here's a link:
Comparison
Do plants with different shaped leaves have different transpiration rates? Let's visit our eggplant bush again. 🍆
This time we also put a plastic zip top bag over a rose leaf. 🌹
This rose leaf is relatively young, that is why it is red. Rose leaves develop the color red first then turn green. This rose leaf has several leaflets on each leaf whereas our eggplant leaves were lobed.
Here's what our rose bag looked like after 2.5 hours. We also removed the eggplant leaf at that time.
Here's what it those bags looked like side by side, once we took them off the plants.
Let's measure and compare how much water the eggplant leaves and rose leaves released!
We used a child's medicine dosing syringe to suck up all of the water from the eggplant bag. Here is what it looks like.
Here is what the medicine dosing syringe looked like after we sucked up all of the water transpired by the rose.
Those amounts look very similar. Both are between 2 and 3 milliliters. Which one has more? Looks like the eggplant is just over that half teaspoon line while the rose fell just under it. The eggplant moved more water than the rose in the same amount of time!
Here's a picture we drew explaining what we saw.
Today was a warm sunny day. Our plants were in full sun the whole time their leaves were in bags.
But what happens when the weather cools down? What happens for deciduous trees when they lose their leaves for the winter?
Sugar
Once the leaves have fallen, the tree concentrates sugar in the fluids caught up in their vasculature, and this thickens the liquids throughout every living cell through the entire plant. This high sugar content acts like anti-freeze. So the plant's cells won't burst when temperatures dip. These fluids become so thick that the trees reach a "glass phase", where metabolism slows to a halt and the plant remains in a molecular suspended animation through the coldest months.
Here is a picture of a deciduous tree coated in ice and snow.
Sugar Rush
As temperatures begin to climb again, the trees start to break dormancy and prepare for Spring. Just before Spring is when the sugar starts to Rush. That's when people tap maple 🍁 trees for syrup. The term "Sugar Rush" comes from botanists talking about trees!
Here is an image from the Library of Congress. This is an unnamed Native American woman who is collecting maple sap from a tree during a sugar Rush. The image was taken in 1908 by a photographer named Roland Reed. I wish we knew her name and not just the photographer.
Plant Vascular System
Let's look closer at plant vasculature.
The vasculature is made up of xylem and phloem. The Ph- of phloem is pronounced F. The word comes from a Greek word φλοιός . Phloem is made up of living cells while xylem is actually dead. Xylem is also from Greek: ξύλον. The X of Xylem makes a Z sound. Xylem cell walls have been thickened so much with lignin that they are considered dead. Just the shell remains with no living tissue inside. Xylem makes it possible for trees to get very tall! They move fluid up, against the gravity gradient.
Here is a little sketch that I drew of a cross section of a stem. It shows all the basic parts of plant vasculature.
"Xylem up, Phloem Down!" This is a fun cheer that helps us remember the direction of fluid flow in the vasculature of a plant.
Letting Go of Leaves
Are you curious about how a tree lets go of its leaves? I explained it in another blog post. Here's a link: http://steamma.blogspot.com/2023/09/illustrating-autumn.html
Here is the overall sketch that I made to go with today's lesson. You can sketch, too!
Some trees hold onto their leaves through the winter.
They're called ev-er-greens, Evergreens.
Pines are one common example, but they aren't the only evergreens. There are so many fun trees that hang onto their leaves.
Pine
Spruce
Fir
Cedar
These are some evergreens that have cones. There are trees that hold onto their leaves that don't have cones, like holly.
Let's draw some evergreens today.
Here's a technique to try - from our friends at Kitchen Table Classroom.
They turn markers into watercolors with this fun activity!
Drawing Deciduous Trees
And oh my goodness here's another great tree drawing tutorial from KTC! This one guides you through drawing a Deciduous tree!
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