The leaves have already begun to change here. It seems a little early and I wondered if that was a bad sign, but it’s been such an odd year weather-wise.
The wondering led to: We’re always told that leaves change because the trees stop making chlorophyll and they stop making chlorophyll because winter is coming.
But how does the one connect to the other, and what are those other colors doing hiding in there in the first place?
It seems to be another of those more interesting and more complex than it sounds subjects!
Leaf cells contain organelles (diminutive for the word organ-a cell organ-it’s so much fun to say and I probably haven’t been able to use it in writing since college, so sorry for the gratuitous vocabulary) called chloroplasts.
Chloroplasts are the part of the cell that uses photosynthesis to make food for the tree. Photosynthesis is a process that uses the energy from light to turn carbon dioxide and water into oxygen and sugar. These sugars are what gives the tree the energy to go about living. (And incidentally us oxygen so we can do the same…)
Chlorophyll is the pigment that absorbs the sunlight, so it is critical for photosynthesis. In good weather this green pigment masks any other pigments.
Chlorophyll is not chemically stable, it requires warmth and sunlight. As the days get shorter and trees start blocking water to their leaves as they prepare to shed them, the pigment breaks down. This exposes the orange and yellow pigments-carotenoids and xanthophylls. (The pigments found in corn, carrots, bananas, buttercups and daffodils amongst others…noticing the color scheme?) Their spring and summer jobs are to protect the chlorophyll from breaking down and are only seen in the fall as the days grow shorter.
The reds and purple hues come from pigments called anthocyanins. (They’re what make the reds in cranberries and apples, the purple in plums and the blues in blueberries.) These pigments are manufactured by the trees in the fall. What’s interesting is that scientists aren’t sure why trees go through the effort of creating a new pigment so late in the season. It has something to do with the trees trying to save as much sugar as possible. They’ve observed that there seems to be a correlation between soil quality and color intensity. The less nutrients in the soil the richer the crimson and purple leaf color as the trees try to salvage as much from the leaves as they can before they fall.
Temperature is a huge influence on the crop of reds and purples. Let’s hope for warm sunny days and cool but not freezing nights!
(Sounds good to me anyway…)
On sunny days leaves are still producing sugars but the cool nights and closing veins as the leaves shut down prevent the glucose from leaving as easily and combine to produce the strongest anthocyanin concentrations.
Moisture is also has a large role to play in the colors of the season. Drought can delay the color change and dampen the colors when they do emerge. Every fall is a bit different, since you’re not going to have the same moisture and temperature variations from year to year. (Or pretty much ever.)
The browns are from wastes left in the leaves after food production has stopped.
Studies done in Europe and Japan have shown that trees are flowering earlier and holding onto their leaves longer. This seems to be linked to long term climate changes. While it sounds like a nice growth period for trees, there is a danger that some species could be killed off in their native habitats is the summer temperatures get too warm.
Sources and Further reading:
This science project just looks like fun, studying the pigments in leaves.
PBS article from last fall.
Nature article on the question of why leaves turn red.
A nice USDA Forest Service article.
Today I Found Out article.
(The title quote is from the 1995 movie of Sense and Sensibility, I just always liked it and this seemed like an excuse.)