Legends of the Fall: Colors

“Autumn wins you best by this its mute appeal to sympathy for its decay.” -Robert Browning

English: the forests in new hampshire in autumn (Photo credit: Wikipedia)

When your life depends on visible light, as it does for photosynthetic organisms, life is very colorful. For the majority of the spring and summer, trees are clothed in green. This green color is from chlorophyll, the main pigment of the photosynthetic electron transfer chain. However, in the fall, green turns to brilliant gold and red. What is really going on during this striking show of colors?

What happens to the green chlorophyll?

The diligent deciduous trees have been photosynthesizing all spring and summer and storing that energy within their woody trunks and branches. Once the weather gets colder and the days become shorter, these trees decide to literally cut their losses when it comes to photosynthesis. Continuing to perform photosynthesis throughout the winter would only provide diminishing returns for the plant, so the leaves go through senescence. This programmed cell death coordinates a massive reabsorption of the nutrients contained in the leaves. All of the proteins and membranes found in the leaves comprise a significant percentage of the plant’s nitrogen, and the trees aren’t just going to drop that premium nutrient onto the ground.

Basically, the trees are eating their leaves, but this must be done in a coordinated way. Before the nitrogen from the chloroplast proteins can be reabsorbed by the plant, the proteins must be separated from the chlorophyll pigments. This is more dangerous than it sounds. Plants have elaborate mechanisms for keeping the chlorophyll molecules of the photosynthetic machinery under control so that they don’t become over-excited and induce damage to the surrounding proteins and membranes. Having lots of free chlorophyll around just isn’t an option because it is hazardous to the cells. During senescence, the cells remove the chlorophyll from the proteins and target it for destruction. Upon the loss of this green pigment, other colors appear on the canvas of the forest.

Where do these new colors come from?

The golden carotenoids are always there as pigments in the photosynthetic machinery, but they are usually drowned by the abundance of chlorophyll. Carotenoids are important components of the systems that protect the photosynthetic machinery from damage caused by over-excitation of the chlorophyll. Because of this protective function, the cells are going to keep these pigments around for as long as possible because the photosynthetic machinery is particularly vulnerable to damage as it is disassembled during senescence.

Red anthyocyanins are made especially for the fall. Cells in the leaves synthesize this pigment and store it in their central vacuole causing the leaves to appear red. Anthocyanins are also important for dealing with the consequences of oxidative damage caused by too much energy in the photosynthetic system. They basically act as a sunscreen to absorb the majority of sunlight as the photosynthetic complexes are degraded. This last flash of color helps to ensure that the tree can reabsorb the maximal amount of nitrogen.

Autumn colors aren’t really a death knell. Instead, they advertise that the trees are getting the most out of the last days of their leaves. So enjoy their colors and cheer on the trees. It means they will have plenty of nutrients when they are ready to burst forth again in the spring.

Johnna

References:

http://harvardforest.fas.harvard.edu/leaves/biological

http://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/leaves/Feild_et_al_Plant_Physiology_2001.pdf

http://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/leaves/Hoch_et_al_2003.pdf

http://www.scifun.org/CHEMWEEK/PDF/Fall_Colors.pdf

http://www.scilogs.com/six_incredible_things_before_breakfast/what-autumn-leaves/

By the numbers: U.S. Old growth forests

English: Redwood trees in Muir Woods National Monument, just outside San Francisco, California, United States. (Photo credit: Wikipedia)

In case yesterday’s post about Hyperion left you wondering about the greater state of forests in the U.S., I’ve got some numbers for you.

I mentioned in passing that Hyperion was approximately 700 – 800 years old, which is not particularly ancient as far as trees go. The North American continent was a much different place when Hyperion was a sapling. Before European settlement, about 46% of the land that would become the modern United States was forest. By 1907, this number had decreased to 33% and fortunately has remained stable since that time. However, very little (~26%) of those original forests still exist today. Most of this ‘old-growth’ forest resides on public lands in Nation Forests and National Parks. Interestingly, it was estimated that the tract of land on which Hyperion stands came within probably two weeks of being logged before it was annexed into the protection of the parks system.

The economy of forestry and ecological conservation are in a constant policy struggle to balance short-term human needs with longer-term ecosystem-wide needs. Forest products have a value of ~$230 billion dollars annually, but forests also play vital roles as a habitat for many other species and a mechanism for the large scale natural capture of carbon dioxide from the atmosphere. Nevertheless, these related but conflicting interests must compromise to ensure a sustainable existence of forests.

While these facts all seem so logical, there’s something about ancient forests that gives them inherent value worthy of protection. There is an awesomeness there that demands silence. The immense size of the trees and the diversity of life all around make being human seem insignificant, but at the same time a forest affirms your place in the world and restores your spirit. I’m not the only one that thinks so…

“The clearest way into the Universe is through a forest wilderness.” John Muir

“In my deepest troubles, I frequently would wrench myself from the persons around me and retire to some secluded part of our noble forests.” John James Audubon

“It is not so much for its beauty that the forest makes a claim upon men’s hearts, as for that subtle something, that quality of air that emanation from old trees, that so wonderfully changes and renews a weary spirit.” Robert Louis Stevenson

Being in the forest is refreshing, but I, like all of you reading this, sleep at night in a home made of forestry products. So if you also appreciate the latter, value the former.

Johnna

References:

http://www.fia.fs.fed.us/library/briefings-summaries-overviews/docs/ForestFactsMetric.pdf

http://www.globalchange.umich.edu/globalchange2/current/lectures/deforest/deforest.html

http://understory.ran.org/2008/11/11/how-much-old-growth-forest-remains-in-the-us/

https://www.campbellgroup.com/timberland/primer/economy-importance.aspx

http://www.brainyquote.com/quotes/keywords/forests.html

http://www.brainyquote.com/quotes/keywords/trees.html

Extra reading on forest policy: http://ncseonline.org/sites/default/files/BOG.pdf

Super Photosynthesizer: Hyperion (The World’s Tallest Tree)

In your daily life, I’m sure you take for granted many of the photosynthetic organisms around you. However, there are some so extreme that they deserve attention, accolades, world records and sometimes a name. These ‘Super Photosynthesizers’ will be highlighted and filed under that category on the ‘Basics’ Page.

Today, for this first installment, allow me to introduce Hyperion, a coastal redwood tree in California that holds the record for the world’s tallest tree and the tallest living organism of any kind. It is 379 feet tall; refer to the figure below for a size comparison with the Statue of Liberty*.

Height comparison of Hyperion vs. Statue of Liberty
The picture of the coastal redwood is cropped and resized from an original work: The Simpson-Reed Grove of Coast redwoods (Sequoia sempervirens) by Acroterion, October 1 2009.

Hyperion does not qualify as ‘something new under the sun’ in strict existence terms because its age is estimated to be between 700 – 800 years old. However, it has only been noticed and measured by humans within the last decade. It was discovered in 2006 by naturalists Chris Atkins and Michael Taylor within a section of Redwood National Park. Its official height was confirmed by Humboldt State University professor Steve Sillett** by climbing to the top and dropping down a measuring tape. Check out the video below documenting the climb.

Coastal redwoods (Sequoia sempervirens) are generally regarded as the world’s tallest tree species, although there have been specimens of other species that rival these giants. For example, the tallest recorded tree was a Douglas fir from the Lynn Valley of British Columbia. When it was felled in the late 19^th century, it measured 414 feet. A number of coastal redwoods in old growth sections in protected areas (National Forests and State Parks) have been measured to exceed 350 feet in height. But these giants must all start as seeds. The cones of redwood trees are only about the size of an olive and can contain up to 100 seeds each. Those are humble beginnings indeed. These trees are also able to clone themselves if they are damaged or in response to other environmental conditions. Check out this video from the Redwood National and State Parks describing redwood reproduction.

Great height has other challenges as well. The various branches of these redwoods effectively exist in separate ecological niches even though they are part of the same organism. This video by the National Parks Service features a description by a ranger from Redwood National Park of the differences in needle structure between the upper and lower canopy. The needles from the upper canopy endure more extreme temperature variations and exposure to brighter sunlight, so they have a tighter shape to prevent unnecessary water loss. The needles from the lower canopy experience a more stable environment with respect to temperature, but they must compete for sunlight with the needles on the higher branches of all the surrounding trees. The needles on these branches have a more extended structure to increase their area for harvesting sunlight.

The real limitation on tree height isn’t the light requirement, but that of water. Plants transport water from their roots to the rest of the plant by a specialized tissue called xylem. This is like the plant plumbing system. Xylem tubes function as long-distance straws to pull water and some dissolved nutrients up the plant from the roots. In the case of Hyperion, that distance is more than 379 feet! This is no small feat, and trees as tall as Hyperion are pushing the limits of physics to suck water up this distance. I highly recommend the video below from the 1veritasium channel on youtube for an explanation of just how amazing this feat really is.

Here is another great breakdown of that video if you need extra info. The gist is that the microstructure of the plant cellular system allows the trees to create enormous amounts of negative pressure (-15 atmospheres) at the treetop to suck water up that high, all without allowing it to boil at such low pressures.*** Of course, I’d like to say that plants do this because of the water requirement for the light reactions, but it actually has everything to do with the dark side carbon fixation. The intake of every carbon dioxide molecule involves the release of quite a few water molecules. The uptake of carbon dioxide in the upper canopy is the driving force underlying the tension pulling the water up to the treetop. The molecular structure of the plant xylem system and the carbon dioxide uptake structures (stomata) allow for this amazing feat of physics, but at heights much beyond 400 feet they have problems. This water stress problem ultimately affects photosynthetic ability via the water pressure required for leaf expansion to capture sunlight throughout the entire organism.

The physics and fluid dynamics of water transport in a tree like Hyperion are amazing, but all trees greater than 33 feet (not particularly impressive) must perform this feat because that is the limit for pushing water up a vertical tube by atmospheric pressure. So the next time you are pondering the peacefulness of the forest, appreciate the trees’ exertion for simply pulling water up their heights. Keep it quiet, they need their concentration.

Johnna

*This is not an actual picture of Hyperion, just a cropped picture of a standard coastal redwood originally from Wikipedia. The actual location of Hyperion is quite remote, a difficult trek for even experienced backpackers. However, the exact location is also somewhat secret to protect this great specimen from overzealous fans and mischievous troublemakers.

**I dare you to e-mail him and ask him how boring it is to be a botanist. You did watch the video, right?

*** Before anyone wants to call ‘BS’ on these numbers and this phenomenon in general, let me explain a little more. It’s not exactly that the treetops are inducing such a sucking force on the water coming up from the roots. The negative ‘pressure’ is the result of cohesive tension of the liquid water being stretched up this tube. Also check out this link that further elaborates.

References:

http://en.wikipedia.org/wiki/Hyperion_%28tree%29

http://www.sfgate.com/bayarea/article/HUMBOLDT-COUNTY-World-s-tallest-tree-a-2550557.php#photo-2648829

http://en.wikipedia.org/wiki/Sequoia_sempervirens

http://www.conifers.org/cu/Sequoia.php

http://www.biologyreference.com/Ve-Z/Water-Movement-in-Plants.html

http://www.humboldt.edu/redwoods/sillett/publications/kochEtAl2004.pdf

Additional reading on redwoods and discovering the world’s tallest trees:

http://www.savetheredwoods.org/league/pdf/srl_newyorker.pdf

http://www.backpacker.com/june_2007_above_beyond/articles/12584