The Science of Autumn

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

User avatar
mathmannix
Posts: 1445
Joined: Fri Jul 06, 2012 2:12 pm UTC
Location: Washington, DC

The Science of Autumn

Postby mathmannix » Mon Mar 30, 2015 6:04 pm UTC

OK. I have a two-part question. There is a debate around my office about this! I won't say which side I argue on, to avoid biasing and/or to avoid ridicule. My Googling is not helping me, as the internet seems divided on this issue. The answers are multiple-choice, although "None of the Above" is also acceptable. But I want real answers, with proof, experiments, whatever is available. I thought about making this a poll, but I want the real answers, not the most popular answer.

Question 1: What causes leaves to change colors in the autumn? Is it
(A) Less daylight
(A-1) - specifically, the length of the day
(A-2) - specifically, the lower angle of the sunlight due to the Sun moving southward
(B) Colder temperatures (which is rather vague; is it that the daily high fails to reach 50 degrees for several consecutive days? is it that the nightly low reaches 20 degrees for at least 12 hours?)
(C) Both (A) and (B) are necessary
(D) Either (A) or (B) is sufficient

Question 2: What causes leaves to drop off trees in the autumn?
(Same answers as above. I have also heard "The wind blows them off", which is somewhat plausible - not because it is windier, but because the leaves are not as well-attached, for reasons A-D.)

I suspect the answers may be different, unless "the wind" is necessary as well. My suspection is due to the fact that it appears that lately many trees haven't been dropping their brown leaves (even though they should), until the new ones push the old ones off in the spring. Also, you sometimes see dead trees with leaves that haven't blown off, but this could just be because they died before the leaves got the memo to start dropping off.

Thank you in advance, science dudes and dudettes.
I hear velociraptor tastes like chicken.

vbkid
Posts: 41
Joined: Wed Oct 31, 2012 5:53 pm UTC

Re: The Science of Autumn

Postby vbkid » Mon Mar 30, 2015 6:41 pm UTC

Fully answers your seconded, and skirts for first question.

http://www.npr.org/templates/story/stor ... =114288700

User avatar
mathmannix
Posts: 1445
Joined: Fri Jul 06, 2012 2:12 pm UTC
Location: Washington, DC

Re: The Science of Autumn

Postby mathmannix » Mon Mar 30, 2015 7:59 pm UTC

vbkid wrote:Fully answers your seconded, and skirts for first question.

http://www.npr.org/templates/story/stor ... =114288700


Yeah, sorry but I had found that website before I posted the question. It doesn't even answer the second question:

Around this time of year in the Northern Hemisphere, as the days grow shorter and colder, those changes trigger a hormone in leaf-dropping trees that sends a chemical message to every leaf that says, in essence, "Time to go! Let's part company!"

When the days get short and cold, food production slows down, giving the tree an option: It can keep the kitchen staff or it can let it go.

(emphasis mine)
I hear velociraptor tastes like chicken.

User avatar
SDK
Posts: 701
Joined: Thu May 22, 2014 7:40 pm UTC
Location: Canada

Re: The Science of Autumn

Postby SDK » Mon Mar 30, 2015 8:43 pm UTC

Just based on my own observations over the years, I think it's both.

First off, the cold alone doesn't seem sufficient. Several times I've seen snow dump on green leaves in early October, and those leaves just keep right on trucking even if it stays cold (like, near freezing) for the next couple of weeks. They recover and get another couple weeks of growth before falling off. I can remember two winters where the leaves were still green come December, presumably because they just froze that way without getting the chance to go through the proper fall process.

On the other side of things, it doesn't seem like shorter days are sufficient alone either. If we have a really mild autumn, the leaves will still turn and fall, but it's delayed. Or, delayed to my recollection. One thing you could do to prove this out is to find data on what time of year the leaves fall. That's gotta be recorded somewhere. One of those winters where we still had green leaves come December was because the leaves stayed green so long (I think it was well into November), then winter hit and that was that.

That's just my memory though, so might be useless to you. We all know humans love finding patterns.
The biggest number (63 quintillion googols in debt)

User avatar
Whizbang
The Best Reporter
Posts: 2238
Joined: Fri Apr 06, 2012 7:50 pm UTC
Location: New Hampshire, USA

Re: The Science of Autumn

Postby Whizbang » Mon Mar 30, 2015 8:53 pm UTC

The United States National Arboretum says it is the amount of daylight.

The USDA Forest Service says:
The timing of color change and leaf fall are primarily regulated by the calendar, that is, the increasing length of night. None of the other environmental influences-temperature, rainfall, food supply, and so on-are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature's autumn palette.


And this research article says that recently color changes have been more delayed than in years past due to increased CO2 in the atmosphere.

So, there you go. Mostly it is due to sunlight, but other factors may affect the exact date, but only by a few days.

User avatar
Mokele
Posts: 775
Joined: Fri Aug 21, 2009 8:18 pm UTC
Location: Atlanta, GA

Re: The Science of Autumn

Postby Mokele » Mon Mar 30, 2015 8:58 pm UTC

This suggests it's very complex, involving the integration of multiple independent environmental factors including light, CO2, water, photosynthesis, and temperature. So the answer is basically "everything".

The leaves apparently have an "abscission zone" at the base, with weak cell walls. As the leaf dies and nutrients recovered from it, the abscission zone weakens, and eventually the structure isn't strong enough to withstand environmental forces (wind) or even just the weight of the leaf itself.
"With malleus aforethought, mammals got an earful of their ancestor's jaw" - J. Burns, Biograffiti

cphite
Posts: 1360
Joined: Wed Mar 30, 2011 5:27 pm UTC

Re: The Science of Autumn

Postby cphite » Tue Mar 31, 2015 6:19 pm UTC

mathmannix wrote:OK. I have a two-part question. There is a debate around my office about this! I won't say which side I argue on, to avoid biasing and/or to avoid ridicule. My Googling is not helping me, as the internet seems divided on this issue. The answers are multiple-choice, although "None of the Above" is also acceptable. But I want real answers, with proof, experiments, whatever is available. I thought about making this a poll, but I want the real answers, not the most popular answer.

Question 1: What causes leaves to change colors in the autumn? Is it
(A) Less daylight
(A-1) - specifically, the length of the day
(A-2) - specifically, the lower angle of the sunlight due to the Sun moving southward
(B) Colder temperatures (which is rather vague; is it that the daily high fails to reach 50 degrees for several consecutive days? is it that the nightly low reaches 20 degrees for at least 12 hours?)
(C) Both (A) and (B) are necessary
(D) Either (A) or (B) is sufficient


The answer is E - it's the length of day, colder temperatures, CO2 levels, and various other factors. An over-simplified explanation is that when the tree determines that having leaves will be more of a detriment than an advantage, it lets them die and lives off what energy it has stored.

Question 2: What causes leaves to drop off trees in the autumn?
(Same answers as above. I have also heard "The wind blows them off", which is somewhat plausible - not because it is windier, but because the leaves are not as well-attached, for reasons A-D.)


When the leaves change color they're essentially dying. Their color depends on the chemical composition; when they finally turn brown they're completely dead. Once the process starts, the stem begins to dry out and gets weaker, and so the leaf can be blown off.

I suspect the answers may be different, unless "the wind" is necessary as well. My suspection is due to the fact that it appears that lately many trees haven't been dropping their brown leaves (even though they should), until the new ones push the old ones off in the spring. Also, you sometimes see dead trees with leaves that haven't blown off, but this could just be because they died before the leaves got the memo to start dropping off.


They start to weaken as soon as they start to die; but "weaken" doesn't necessarily mean something will fall. It also depends on how much wind hits them, where they're positioned, and even on the species of tree - some have stronger stems than others. You might see some trees start dropping leaves that are still partially green; others hold on to their leaves long after they're brown.

Thank you in advance, science dudes and dudettes.


\m/ 8-) \m/


Return to “Science”

Who is online

Users browsing this forum: No registered users and 17 guests