The Times They Are A-Changing

27 09 2009
Beautiful fall colors! But where do they come from, and why are they coating the ground?

Beautiful fall colors! But where do they come from, and why are they coating the ground?

Hello! I haven’t done one of these long posts in a little while, so I thought I would remedy that. If you’re one of those people who seem to be suddenly coming to visit this site in great numbers, welcome! I’m glad to have you. Anyway, let’s get on with it.

I wanted to discuss the changes going on all around for those of us living in temperate deciduous forests like the kind in the northeastern United States. Every year, billions and billions of trees shed their leaves to prepare for winter. The precursor to this amputation is the emergence of beautiful and vivid fall leaf color.  But what accounts for this color, and why do trees shed their leaves in the first place? In this exciting two-part series, we’ll answer both questions! First, why do some trees lose their leaves in the fall?
CLICK HERE TO READ MORE!

Clouds form over the Amazon rain forest, condensing out of the trillions of gallons of water being transpired by the plants below.

Clouds form over the Amazon rain forest, condensing out of the trillions of gallons of water being transpired by the plants below.

In order to understand why trees lose their leaves, it’s important to understand what has been called the transpiration-photosynthesis compromise. The transpiration-photosynthesis compromise refers to the balance between photosynthesis and water loss that every plant must deal with. If you’ve read the article here on water and hydrogen bonding, you know that plants like trees transport water to their upper parts because the evaporation of water from the leaves creates a suction that pulls water up through the trunk from the roots. This process of evaporation from the plant’s surfaces is called transpiration, and a good sized tree can transpire hundreds of gallons of water a day. Transpiration is such a significant mover of water out of the soil and into the air that it actually affects weather patterns and forms clouds above actively transpiring forests. So anyway, what we’re talking about is the transpiration-photosynthesis compromise – where does photosynthesis come in?

The eye-shaped openings visible in this scanning electron micrograph of a leaf epidermis are the stomata. The puzzle-piece shaped cells are the epidermal cells, fitted tightly together to keep the leaf safe from external threats. Click for bigger.

The eye-shaped openings visible in this scanning electron micrograph of a leaf epidermis are the stomata. The larger puzzle-piece shaped cells are the epidermal cells, fitted tightly together to keep the leaf safe from external threats. Click for bigger.

The processes of transpiration and photosynthesis are linked by the leaf structures known as stomata. Stomata are small channels in the leaf surface through which gasses like carbon dioxide and water vapor can pass. It is through these stomata that 90% of the water lost in transpiration passes. The rest of the leaf surface is covered in a thick waxy cuticle which helps prevent evaporation.

Is it likewise through these stomata that the carbon dioxide which the tree requires for photosynthesis enters the plant.* Now, plants can control whether their stomata are open or closed. When they are open, plants lose water at a rapid rate, but also can access the carbon dioxide they need to produce food for themselves.

When they are closed, the plants lose water at a much reduced rate, but also create food at a much reduced rate since access to the ingredient carbon dioxide is now blocked off. Plants open and shut their stomata based on a number of environmental conditions, such as humidity, wind speed, temperature, drought, and presence or absence of sunlight, all of which affect transpiration rates and photosynthesis rates. If it’s very hot and dry, transpiration rates increase drastically, and plants close their stomata to prevent deadly water loss. However, this defense mechanism also means that the plants shut down their sugar factories, since carbon dioxide can no longer make it into the cells. In order to make sugar, plants must lose water, and in order to save water, plants must slow down their sugar production. This is why we call it the transpiration-photosynthesis compromise – there is a constant tension between the need to conserve water and the need to produce food.

As I said before, factors like humidity affect the rate of transpiration. If the air is more dry, more water is lost to the air. As summer turns to fall, humidity drops. This is a blessed reprieve for those of us who suffer through hot, humid summers each year, but it’s a death sentence for a tree with big leaves, unless something is done fast. With all those big, flat leaves and their large, transpiration-accelerating stomata, water is lost too quickly to replace. Now if you’re dying of water loss and you have thousands of little leaves on you that account for almost all of your water loss, what is the solution? Cut them off! Fundamentally, leaf loss in the fall is a defensive response to the lack of water in the air and in the soil which accompanies the seasonal shift.

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This crude drawing shows the two layers that form when a tree needs to sever its leaves.

The shedding of each individual leaf is accomplished through a process known as leaf abscission. The tree severs the leaf by creating a layer of weak, thin-walled cells between the leaf and the trunk or branch. It also produces enzymes in the area which break down the walls of the cells in the area, further weakening them. The result is a weak fault line called an abscission layer that makes it much easier for mechanical stresses like the wind to strip the leaf off the tree. This is similar to how we perforate things like ketchup packets or sheets of notebook paper to make them easier to tear. In addition to this layer of weak cells is a protective barrier of cork cells, like those in bark, which forms behind the abscission layer to protect the tree from threats like bacteria and insects once the leaf has fallen. It also disconnects the leaf from its supply of water and soil nutrients. Without this layer of protective, waterproof cork cells sealing off the area, leaf loss in autumn would result in thousands of wounds that invaders could take advantage of.

So now you know how and why trees lose their leaves! I hope you found the discussion illuminating. Next time, we will discuss why fall is also a time for brilliant colors. As always, don’t hesitate to comment or ask any questions that are on your mind! We love to hear from you.

-Neil

*The formula for photosynthesis is 6CO2 + 6H2O + Light à C6H12O6 + 6O2. In other words it takes 6 molecules of carbon dioxide, 6 molecules of water, and some light in order to produce the sugar that the plant and the herbivores that feed on it use for food. The byproduct is oxygen, a happy fact for those of us who enjoy breathing. In fact almost all the oxygen you breathe was put together by photosynthetic organisms like trees. Be thankful for them!

**This dry air is also why you may suffer from dry skin in the winter months.

P.S. Why don’t evergreens lose their needles? Don’t they have the same problems with water loss as broadleaf trees? Yes and no. Trees with needles have a number of advantages over broadleaf trees in terms of water conservation. First is the needle instead of the broad leaf. The needle shape means less surface area, and less surface area means less evaporation. Second are its unique stomata, which are in sunken pits on the leaf surface, protecting them from the cold wind that strips water from broad leaf trees. Note, however, that these advantages in stemming water loss are part of a trade off. Though they conserve water and allow leaves to be kept year round, photosynthesis proceeds much more slowly due to less leaf area, and the plant grows much more slowly as a result. But in the colder and drier parts of the world, this tradeoff is well worth it.

The protected position of these pine needle stomata helps prevent water loss due to wind. Some plants also possess tiny one-cell-wide “hairs” called trichomes which further insulate the stomata from the wind.

The protected position of these pine needle stomata helps prevent water loss due to wind. Some plants also possess tiny one-cell-wide “hairs” called trichomes which further insulate the stomata from the wind.

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