
Why Leaves Really Fall Off Trees - sajid
http://www.npr.org/templates/story/story.php?storyId=114288700
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tokenadult
There is interesting science behind deciduous trees losing their leaves.

[http://treephys.oxfordjournals.org/content/18/8-9/537.full.p...](http://treephys.oxfordjournals.org/content/18/8-9/537.full.pdf)

Deciduous trees have an important role in urban environments.

<http://www.sustland.umn.edu/maint/trees.html>

[http://extension.usu.edu/forestry/HomeTown/Energy_TreesandEn...](http://extension.usu.edu/forestry/HomeTown/Energy_TreesandEnergy.htm)

This link caught my eye because the source was National Public Radio in the
United States, in the northern temperate zone (Northern Hemisphere). As I
expected, the original date of the article was October (2009). At this time of
year (April 2011), deciduous tree leaves are mostly falling only in the
Southern Hemisphere.

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cosmicray
> deciduous tree leaves are mostly falling only in the Southern Hemisphere

Here in Florida, and due to the mild climate, certain species of Oaks will
hold their leaves right up to the time they put out fresh buds in the spring.
I have a large quantity that need to be raked up and mulched/burned/etc.

~~~
Vivtek
Here in Indiana, the same is true of oaks - so I'm pretty sure it's the oaks,
not the climate.

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guygurari
_If trees kept their leaves permanently they wouldn't have to grow new ones_

If leaves were kept indefinitely they'd also have to deal with damage, wear
and tear and so on. Biological systems often seem to prefer replacing over
repairing damaged parts, which is perhaps another reason why leaves are
replaced. For instance our skin is constantly being replaced, so injured
pieces are discarded instead of repaired.

~~~
JoeAltmaier
Conifers (pines etc) have antifreeze in their needles. Some Do freeze anyway
and fall off (they turn red). I imagine that antifreeze is expensive (to the
tree) and interferes with normal biology. Leaves were a major invention
millions of years ago (not that long ago really). They are a big step up from
needles, and deciduous trees pretty much pushed conifers out of every niche in
the world except mountain slopes, where the cold plus conifer height advantage
gave conifers a stronghold.

~~~
metageek
> _pretty much pushed conifers out of every niche in the world except mountain
> slopes_

Definitely not true; there are plenty of pine trees in Georgia (the one in the
US), and not just in the (rather mild) mountains.

I will admit that there seem to be more pine trees the further north you go.

~~~
JoeAltmaier
I see (wikipedia) that fire-resistance was the factor that favored pine over
other trees in the US South.

~~~
metageek
Thanks, that does make sense.

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flipbrad
tl;dr: deciduous trees send 'scissor cells' to lop off the leaves as winter
arrives. This disconnects the plant's "water pumps" (since evaporation through
leaf stoma draws water through the plant xylem up from the roots). If the
pumps were to run relatively heavily on a relatively windy and warm winter's
day, the fresh water in the plant's veins would freeze overnight and present a
serious risk of death or inconvenience to the tree. The tree thus sheds its
leaves to avoid over-reacting to nice weather in the midst of winter.

~~~
pavel_lishin
I'm still a fan of the hypothesis that trees send their waste to the leaves,
and then cut them off.

tl;dr as a child, you jumped into big piles of tree poop.

~~~
bioh42_2
Better than waste: <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1945073/>

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vacri
I'm a bit puzzled by the assertion that we used to believe the wind pulled the
leaves from the trees. Does the wind not blow in spring and summer?

~~~
jamesbkel
As I understand, they are referring to the belief that wind was the mechanism
by which the "dead" leaves fall off of the branches, not that wind in and off
itself would blow leaves off of trees. According to the article, it appears
that the trees actively dispose of the old leaves, rather than passive removal
via wind.

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cesarsalazar12
Can't avoid reading "_why leaves, really falls off trees"

~~~
Vivtek
This should be upvoted a lot more. I'm still giggling about it.

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kijinbear
> all around us right now (if you live near leaf-dropping trees in a temporal
> zone), leaves are turning yellow and looking a little dry and crusty.

At first, I was like "Huh? Do they have NPR in Australia too?" and then I
remembered to check the publication date.

Very clearly written, and easy to understand. I love articles like this.

~~~
hugh3
I was about to write that there are no native deciduous trees in Australia but
I decided to double-check that fact first and it turns out I'm wrong:

<http://anpsa.org.au/faq-18.html>

there are a couple of varieties of deciduous trees in Australia, but rather
than being cold-hardened they live only in the tropics (!) and lose their
leaves shortly before the wet season starts and grow them almost immediately
afterwards.

I'm really not sure why these guys are deciduous though. Is it just random
evolutionary happenstance (as in, they evolved from cold-weather deciduous
trees and haven't been able to shake off this particular habit) or is there
some tropics-related reason to shed your leaves?

~~~
whyenot
Many tropical trees are "drought deciduous." Trees are always losing moisture
through their leaves. By dropping their leaves when the soil becomes dry, they
can better survive the dry season.

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zopticity
NPR, I hate to break it to you, but leaves falling off trees has nothing to do
with evolution. Fall and Winter come because the goddess Demeter is sad that
her daughter Persephone has to spend 6 months in Hades! I'm so disappointed,
usually your stories are so accurate!

