For the last couple of weeks I’ve been admiring and perplexed by interesting looking leaves I find on the ground while mushroom and bug hunting. Most of the ground in the Central California coast is covered by brownish pine needles, tanish oak leaves, other fallen leaves as well as a thick layer of duff. But every once in a while a leaf stops me in my tracks and I MUST take a photo. Can’t resist. Call it an obsession. Fine. Before any new growth starts appearing on our friendly giant plants known as trees, an event which is truly just around the corner I had to do a deep dive into why these funky patterned leaves look they way they do.

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Yes, I realize much of the Northern Hemisphere is well into winter, and perhaps your backyard and beyond is covered in snow, but here in California that ain’t the case. California has a mediterranean climate with hot, dry summers, and mild but wet winters when we get most of the precipitation throughout the year. So the winter is when the growing season begins, and then grown explodes come spring. So I felt like if I don’t share these beautiful leaves soon, it will be too late! Also mushrooms, hello, I see you, your bloggy moment will come soon. 

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Before shedding their leaves, trees drawback energy reserves. They have worked all summer producing sugars created through photosynthesis, which now must be stored under the bark and in the roots. Proteins are broken down as the leaf disintegrates; and during this process nitrogen is transported to storage for growth and photosynthesis in the spring. 

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But there are different strategies within this yearly ritual. Some trees don’t bother to suck all possibly remaining sugars back. Alders will drop perfectly green leaves because they typically grow in very nutrient-rich soils therefore they can “afford” to drop chlorophyll rich leaves. These leaves will then decompose, recycling the necessary materials which will be eventually taken up by the roots.

Uhh, hold on, what’s chlorophyll again? Highschool bio was a while ago Chloe. . .

Essentially Chlorophyll is a pigment, a green pigment. But this pigment is special, it captures the sun’s light energy and converts it into sugar, aka chemical energy usable to plant. 

But most trees, like the leaves seen throughout this post like Pacific Rhododendron (Rhododendron macrophyllum), Tanoak (Notholithocarpus densiflorus), Maple and others. . .  do drop their leaves eventually. The usually green colored leaves we see most of the year is caused by the presence of chlorophyll. When the tree withdraws the energy remaining in the leaves back into its core during fall, the remaining colors include: shades of yellow, brown, red, orange, and the hues in between. Yellow shades are the result of the remaining carotenoids. Whereas browns are created by the leaf producing tannins. But red colored leaves arguably have the most interesting process of appearing, but as you see I don’t have any leaves with red hues, so that’s for another post. In short reds are caused by the breakdown of the chlorophyll corresponding with anthocyanin synthesis, but researchers are still learning about the red pigments in leaves. Love a good unsolved mystery. But if interested, this article from Harvard Forest (so fancy) has a great description and diagrams.

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While these colors to our human eyeballs are beautiful and evoke those cozy warm feelings of fall and wishes of tea and fuzzy slippers, they are a big warning sign to insects! Sound the alarm! Hey bugs, don’t even THINK about moving in! The tree is trying to send a signal to aphids and other insects that “HEY, I’m healthy and I’m going to defend myself, so go find another tree to set up for winter or raise offspring!” Just like exotic amphibians or insects with bright coloration, the bright fall colors are a warning system that harmful toxins could be produced in the spring which are harmful to interested insect invaders. 

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As evergreens, coniferous trees such as pines, firs and spruce trees don’t annually drop their leaves. But as needles get old or damaged they are dropped because they have done their job, and no longer are as effective. Out with the old and in with the new! Apparently you can tell the age of the needles on a tree based on the branch’s growing intervals. I haven’t tried this yet, but definitely want to one day.

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But how do trees know when to drop their leaves? 

Trees can recognize the fall versus the spring by a combination of factors including day length and temperature and their relationship. In the spring, temperatures steadily increase, whereas in the fall the temps fall, umm obviously Chloe. Trees recognize the shortening length of each day, termed the photoperiod, which triggers the trees wintering mechanisms.

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But why drop leaves anyway? Why not keep them like evergreens? 

Ah the beauty of evolution! Deciduous trees evolved around 100 million years ago, so dropping leaves must be an effective strategy. Winters typically mean strong winds, and colder temperatures. Needle-bearing trees evolved to have thick coating of wax on the outside, and inside the needles are pumped with a special antifreeze in order to survive winter. If all the leaves on a deciduous tree remained, strong wind storms could topel them over because all those leaves combine to create a huge surface area. Additionally, while one leaf is quite light, thousands are not! Anyone that has raked leaves knows this too well. In places where winter brings snow, all that additional surface area and weight creates more area for snow to accumulate and causes stress on the tree limbs. No leaves means the snow falls mostly to the ground. No worries! 

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Once leaves are let go the point of connection, now empty, is filled with a layer of special cells. But if a freak system blows through with multiple nights well below freezing trees can prematurely go into a hibernative state, leaves may remain on branches and the holes where leaves once were attached may remain unfilled. 

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So unfortunately I couldn’t really find information regarding my funny patterned leaves, but this week was a short one, and I could search more, but there are beaches to explore and mushrooms to find. Most of my search mostly lead to dead ends involving pests or concerned indoor house plant owners. But as part of the California Naturalist course we are encouraged to continually ask questions, and probe our commonly held naturalist beliefs. So what do I think is going on with some of these leaves? 

• As chlorophyll was breaking down and sugar and nutrients being drawn to the tree, the leaf fell. So we get to see a leaf in mid wintering progress. 
• For the leafs with a green edge but yellow/brown center perhaps the tree started it’s drawback quickly, and only the nearest chlorophyll to the leaf’s stem was broken down.
• Perhaps broken or damaged leaf veins lead to different patterns like the leaves with blotches 
• I would assume pigments are generally equally spread throughout the leaf, but perhaps not, and this causes the different coloration we see when they fall off or are let go and fall to the ground
• I can also believe pest invasion or fungal pathogens attacking the tree or individual leaf cause some of the patterns, especially the black dots seen in a few of the photos.
• Perhaps some type of liquid, sap, or other decomposing juice touched the leaf, and led to some of the circles and blobs.

Got any ideas? Post them in the comments!

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Want to read more about the wonderful world of trees? I highly recommend: 

Read: The Hidden Life of Trees by Peter Wohleben

Listen: Ologies Podcast by Alie Ward, Dendrology Parts 1 & 2 

I hope this post leaves you with joy and inspiration to find interesting and colorful leaves in your area (this year, or next)!

• Resources:
  • The Hidden Life of Trees, What They Feel, How They Communicate Discoveries from a Secret World by Peter Wohlleben
  • What Leaves Change Color in a Matter of Seconds 
  • Scientific American, Reader Question: What causes the leaves on trees to change color in the fall?
  • Chlorophyll, Paul May University of Bristol
  • Mediterranean Climate, Melvin R. George on UC Rangelands Research & Education Archive