Seasonal Tree Transition Magic

Photo by Khürt Williams

Do you live in a place where you experience all four seasons in full force? I certainly do. That’s one of the reasons why Minnesota is a cool place to exist. It is also a difficult place to exist, especially if you’re a tree.

Here in the North Star State, and throughout the midwest, we have a mixture of deciduous and evergreen trees. The deciduous ones, those that drop their leaves, are the ones responsible for the characteristic fall reds and oranges, but how do they do it?

All summer long the trees, more specifically their chloroplasts, were working hard to turn sunlight into energy, but when fall rolls around, the party is over. Because of changes in day length and temperature, chlorophyll breaks down red, orange, yellow and brown take over. The brown color comes from tannins, cellular waste products that are also responsible for the brown color and bitter taste of coffee and tea. The red comes from anthocyanin pigments made from stored glucose in the leaf. It’s not entirely clear why trees put energy into synthesizing red pigments just before they drop their leaves, but some say it may have something to do with cold resistance and frost protection. Other fall colors were in the leaves all along. Yellow xanthophyll and orange carotene become visible only after the chlorophyll disappears.

Photo by Sérgio Rola

As fall progresses, the wall of cells that connects a leaf to its branch, the “abscission layer”, begins to harden and block the flow of nutrients to the leaf. Give it enough time and maybe a little wind and the leaf will fall right off.

After the trees drop their leaves and winter arrives, what do they do? Well, like many biological organisms, trees contain a lot of water. When temperatures get really low, they avoid freezing with some pretty sneaky strategies. Some trees convert starch stores into sugars that dissolve in their vascular fluids which lowers the freezing point of the water inside the tree. Some trees acclimate to the cold by allowing their cell membranes to become more pliable which allows water to move out of the cells. The cells shrink to offset the pressure from the extracellular water and it is then less likely that ice crystals will form within cells and rupture them. This might almost seem like magic, but that’s probably just because we can’t see it with our own eyes.

Perhaps a microscope would make us all believers.

Sources

“A Tree Leaf Ages and Drops and Called Senescence and Abscission.” About.com Education. N.p., 23 Sept. 2015. Web. 10 Oct. 2016. <http://forestry.about.com/od/Treebiology/g/Definition-Of-Leaf-Abscission.htm>.

“Autumn Leaves: How Plants Prepare for Winter by Science Made Simple.”Autumn Leaves. N.p., n.d. Web. 10 Oct. 2016. <http://www.sciencemadesimple.com/plants-in-winter.html>.

Communications, SUNY-ESF Office of. “Why Leaves Change Color.” Why Leaves Change Color. N.p., n.d. Web. 10 Oct. 2016. <http://www.esf.edu/pubprog/brochure/leaves/leaves.htm>.

@northwoodlands. “Do Tree Stems Freeze in Winter? | Winter 2009.” Center for Northern Woodlands Education. N.p., 1 Feb. 2010. Web. 10 Oct. 2016. <http://northernwoodlands.org/articles/article/do-tree-stems-freeze-in-winter>.

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