Tag Archives: trees

“Whirlybirds, helicopters, and Maple seeds”

Photo by Annette Meyer on Pixabay – Samaras of a Japanese Maple, with the seeds inside.

As Maple trees shed their fruits, it is hard not to be captivated by the view and stare in admiration. The free fall of maple seeds is simply graceful. Commonly referred to as helicopters, samaras are the fruit of Maple trees. Inside of each fruit one can find seeds that are used by the parent plant to produce new ones. The nickname helicopter refers to the similarity that exists between its motion as it falls to the ground and that of a helicopter. Indeed, a remarkable aspect of the samaras is the behavior they display as they fall. As the fruit of the Maple seed descends to the ground, it performs a rotating motion that mimics the rotor blade of helicopters in unpowered descent, a behavior that has intrigued scientists and has been the subject of many studies. The auto-gyration motion and flight mechanics of the samaras have been observed in order to explain why and how the fruit rotates on itself as it leaves the tree.

Importance of understanding the whirling motion of Samaras:

The use of seeds to produce new plants is called seed dispersal. Since trees are unable to move, they rely on different means for the dispersal of their seeds, such as the wind, water, animal, or human beings. Samaras being carried away by the wind are an example of wind dispersal. Seed dispersal is important for the survival of species. It allows the plant to spread in its environment. By allowing the seeds to fall at great distances from the parent plant, decreases the chances of interference of the growth of new plants with the development of the parent plant and allows it to colonize new environments. Studying the flight of samaras helps understand how this process can be enhanced. However, not only does it help with seed dispersal, but it may also have applications that can help advance the field of aviation and the conception of flying devices. (Sang Joon Lee et Al. 2014)

Flight Mechanics:

There are two principal categories of seeds that have the ability to use oncoming winds to travel away from the parent plant, pappose seeds, and winged seeds. Maple seeds fall within the second one. As the name indicates, winged seeds tend to have appendages that resemble wings and act as such. While pappose seeds use draft force to maintain their flight, winged seeds rely on lift force. While they share common traits, not all winged seeds rotate. The rotation of samaras is due to the location of its center of gravity which is found near the terminal end of the wings. The wings of the seeds of the Maple tree generate a stable leading-edge vortex (LEV) which accounts for the lift force slowing down the descent. Note that an LEV is a type of airflow that is halfway between a steady and a turbulent flow. Upon being detached from the tree and is released into the air, the very shape of the wing initiates the stable vortex. The mechanism used by samaras has been identified to be similar to the LEVs of insects such as flower flies. 

It’s The Little Things That Make Trees Strong

Plants come in all shapes and sizes, from the smallest blades of grass to trees so big that the tops can’t even be seen from the ground. But all plants are made from the same basic cell structures and components. So why is it that I can easily pick a flower, but could spend hours chopping at a tree and hardly make a dent? Trees are so much stronger than almost every other plant that they have become a staple in the construction industry. The key to the success of the tree is small differences in the structure.

Microscopic view of growth ring structure – Smithsonian Environmental Research Center

Cell walls are made up of cellulose, pectin, and lignin. Cellulose, with a Young’s Modulus of 120-140 GPa, provides the most structure to the cell wall, and the cell walls of trees have high concentrations of cellulose, averaging 45%. The other components in-between the cellulose, also can greatly impact the stiffness of the cell wall. Along with the cellulose, the secondary wall of tree cells contain more lignin that pectin as a binding agent. Lignin is found in all plant cells, but the high concentrations found in tree cells are what set it apart from other plants. Lignin has a Young’s Modulus of 3 GPa, while pectin is a gelatinous component that provides little structure. A study conducted by Donaldson also showed the presence of lignin has increased the size of the microfibrils and cellulose matrix to make the cell wall less porous, further increasing stiffness. The high concentration of lignin in the cell wall is a defining characteristic of tree cells and greatly increases their stiffness and rigidity.

The arrangement of the lignin and cellulose in the cell wall also increases stiffness. As tree cells divide, different layers of the cell are formed. This process is described by Plomion et al. in their article on wood formation.

The layers of a cell wall, from Plomion et al, Wood Formation in Trees.

The first layer is the middle lamella, which is made up mostly of pectin, but lignin is added throughout the differentiation period to eventually increase stiffness. This layer is mostly used to adhere the cell together. The next layer is the primary wall, which can consist up to 70% lignin. The primary wall is very elastic, to allow the cell to continue to grow. In other plants, this layer can have a higher concentration of pectin. The lignin in this layer means the tree cell is less malleable, but also increases stiffness and strength. The final layer is the secondary wall, which is separated into three layers, S1, S2, and S3. Most plants only have only one secondary wall, so having three distinct layers increases the strength and stiffness of the tree. Each layer consists of cellulose microfibrils, which are arranged in parallel, but in a different orientation for each layer. As the cell grows, the secondary layer is packed with more lignin, increasing the stiffness of cell wall. The S2 layer is the thickest, and contains about 45% cellulose and 20% lignin. Again, the higher lignin content, instead of pectin, allows for more rigidity, strength, and stiffness in the layer that provides the most structure to the cell. All of the cellulose and lignin layers, closely packed together, create a very stiff cell wall unlike any other plant.

It’s the small changes to the structure of the cell wall, such as adding lignin and layers into the secondary wall, that make the difference in the structure between a flower cell and that of a tree.

Sources and Further Reading:

What Makes and Breaks the World’s Tallest Trees

Trees have the potential to be the largest organisms on Earth. The world’s tallest tree, dubbed Hyperion, is 380 ft tall and weighs over 1,600,000 lbs. Compared this to the world’s largest animal, a particularly massive blue whale which was 100 ft long and weighed 380,000 lbs, the simply massive size of this tree should be obvious. And unlike a whale, a tree is much less likely collapse and crush itself under its own weight. Trees need to be tall, even if doing so consumes a lot of resources, in order to compete for sunlight. So what lets trees get this big, and what limits their height?

A diagram showing a space shuttle, which when prepared to launch is less than half the height of Hyperion, General Sherman, a wider but slightly shorter tree of a different family sequoia, a blue whale that is much shorter than either tree, and the statue of liberty, whose torch barely comes close to the shorter of the two trees
Comparison diagram of the World’s largest trees – Sequoia Tree Comparison Chart, Sequoias

There are two primary rules that govern tree sizes. The first is mechanics, the way the trunk of the tree is built and how it responds to weight. The wood of the biggest trees has a very high strength to weight ratio, which enables a tree to carry its own massive weight without collapsing. The layout and structure of this wood is analyzed at length in the journal by M. Ramage, but in summary, tall trees have internal cells called tracheids. These tiny circular tubes are 2-4 mm long and around 30 μm wide and provide support to the tree and allow water to flow throughout it, without adding as much weight. 

an image showing the tracheid cell structure of wood, many small cylinders stacked on top of each other.
A section of the annual ring of a conifer- M. Ramage’s The wood from the trees: The use of timber in construction and Dr. Krzysztof Wicher.

The high strength to weight ratio of wood allows trees to support themselves at incredible heights. Using B. Blonder’s research about the scaling of trees, it can be shown that trees are so strong and yet comparatively light weight that a tree would not actually collapse under its own weight until it was almost 15 Empire State Buildings tall. Obviously no tree is this tall, meaning some other factors must limit their height, but the incredible strength of wood should now be clear!

A diagram showing the decreasing size of pine needle branch segments. They decrease dramatically as height increases.
Leaf samples taken from the same type of tree with the height they were taken from listed adjacently in meters, – G. Koch’s The limits of tree height

The second primary rule that governs tree size is hydraulics, and it restricts the height a tree can reach. Hydraulics defines a tree’s ability to move water from its roots to its upper leaves in order to perform photosynthesis. The taller a tree gets, the more difficult this process becomes until the tree becomes incapable of growing any taller. G. Koch’s article, The Limits to Tree Height, explores how this hydraulic system works and how it restricts the heights a tree can reach. Xylem, tiny internal pipes that run from the roots to the tops of the tree, and carry water in a long continuous column this whole length. The longer this column becomes, the more difficult it is to maintain and the greater suction pressure that occurs at the highest leaves. Koch studied how properties in leaves changed the higher up they could be found, determining that the efficiency of Photosynthesis decreased, the pressure at the end of the xylem increased, and the size of leaves decreased. At great heights, the status of leaves seemed remarkably similar to those of a tree undergoing a severe drought.

 

Koch determined that these changes with height would eventually hit a maximum limit which they could not exceed, a limit that was determined to occur between 122-130 meters. So while the efficient properties of wood allow trees to reach incredible heights, their restricted ability to move water limits just how tall they can grow.

Sources and Further Reading:

  • Ramage M., Burridge H., Busse-Wicher M., et al. The wood from trees: The use of timber in construction. Renewable and Sustainable Energy Reviews 68, 333-359 (2017)
  • Blonder B. The size of trees: exploring biological scaling (2010)
  • Koch, G., Sillett, S., Jennings, G. et al. The limits to tree height. Nature 428, 851–854 (2004)