Why Leaves Really Fall Off Trees | Hacker News Reader

Why Leaves Really Fall Off Trees

npr.org

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There is interesting science behind deciduous trees losing their leaves.

http://treephys.oxfordjournals.org/content/18/8-9/537.full.p...

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...

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.

> 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.

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

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.

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.

Conifers still exist in places other than mountain slopes; boreal forests and taigas being the perfect example. It's more a question of climate than terrain as far as I know.

Losing needles is not a disadvantage as much as an advantage in the most northern regions of the world where the old needles can still do photosynthesis when there's good light but it's still too cold outside to grow new leaves and get nutrients from a still frozen earth.

Note that a few coniferous trees like the larch are in fact deciduous.

Leaves were a 'major invention' and a 'big step up' in climates where they are the most appropriate thing. In some climates, coniferous trees still dominate because they are the most adapted species to that specific environment. Evolution works through what's the most apt at surviving in given conditions; dominance is conditional to the environment you frame it into.

>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.

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

Thanks, that does make sense.

> Leaves were a major invention millions of years ago (not that long ago really). They are a big step up from needles

Leaves were indeed a major event in the the evolutionary history of plants, BUT, leaves are actually older than needles. Many primitive gymnosperms had leaves, as did primitive ferns. Needles are basically leaves that have rolled in on themselves.

Conifers are dominant over very large areas, they have not been pushed out of almost every niche in the world. They are the dominant group of plants in northern and southern boreal forests and cover a significant portion of the earth's surface.

Hm. A quick Wikipedia lookup show that 3 out of 4 conifer orders went extinct since the Jurassic; species diversity is centered in mountains of China, Japan, Mexico and California; many families exist only in the southern hemisphere tho fossils show them once worldwide.

And about leaves: we were talking trees - of course herbs existed hundreds of millions of years earlier.

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.

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.

Better than waste: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1945073/

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?

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.

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

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

> 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.

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?

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.

There are plenty of non-native deciduous trees in Australian cities and suburbs too.

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!

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