[chris (OP)]

Whilst bored on a long train journey recently I looked out of the window and saw hundreds of silver birch trees. This got me thinking why are they "silver"? There must be an evolutionary purpose? Does anyone have any suggestions?

Chris

[WylieE]

One theory is that it reduces the heat attracted in the winter and prevents sunscald injury.  Most white trees are found in temperate regions.  And the trees with the most reflective white bark grow the furthest north.  So one would expect it might have to do with survival in the winter.  Sunscald injury occurs when the living layer of cells thaws out during the day and refreezes at night.  (It is the thawing and refreezing that damages the cells).  Having a white trunk is thought to help reflect the sunlight and keep the layer of cells below the surface from heating up in the day and thawing. 
Tim Karels (now at California State University, Northridge) showed that when painted brown, white trees had a large increase in sunscald injury.

Colleen

[another_someone]

Sounds like the distribution of 'fair skinned' trees has a similar broad pattern to the distribution of fair skinned people - could the reasons for the two also be similar (i.e. not that the white bark reflects more light, but that it lets more light through to the inner core) - or has that been ruled out as an explanation?

[WylieE]

Hmm, Interesting- trees do seem to follow the same distribution of bark color as human skin color.     

One advantage of darker skin color in places with a lot of UV exposure is that the melanin protects cells from damaging UV.  Maybe dark tree bark in tropical regions is doing the same thing.  However, in tropical areas the plants don't really ever lose their leaves so the bark really wouldn't have much exposure to the sun.  In fact in many plants, when the stem does have exposure to the sun it is often green (and thus taking advantage of the sun for photosynthesis).  So I think in the case of tropical regions the skin color and bark color being the same isn't for the same reason, at least not the major reason.  Although plants do produce melanin and it is important in UV protection, so this maybe some component.   

  So the main issue is probably that in temperate regions in the winter the trees lose their leaves and the bark gets sunlight.  In humans living in regions where there is less sunlight the skin is lighter to let enough light through to synthesis vitamin D, "letting light through to the inner core".  In this case isn't it just a lack of pigment that causes light skin? I think this might be different in plants where they are more truly "white" and actually reflecting all wavelengths of light. 

  Plants in these same regions collect sunlight all summer through their leaves (and at this time the bark is shaded), so they should have plenty of opportunity to get any energy they need through the leaves.  In the winter they pretty much shut down.  They lose their leaves and they stop almost all metabolism so extra sunlight is damaging - they can't harvest it through photosynthesis so it can damage DNA / protein / cell walls etc. 
   
  In the Karel's' paper they measured the temperature of the cambium (inner core) of birch trees that were either painted brown, white, or natural.  Those painted brown had a much higher core level than the natural or white trees.  They also showed that how reflective a tree species is (its "whiteness index") correlates inversely with the temperature increase- so the whiter trees had less temperature increase when illuminated for 30 min. 

  Of course this is not the only hypothesis about why trees have lighter bark- others include - a role in insect / pathogen / pest defense, regulation of vascular flow, and camouflage so they can attack skiers.
 
Colleen

[another_someone]

  Of course this is not the only hypothesis about why trees have lighter bark- others include
camouflage so they can attack skiers.

Sounds absolutely the most likely explanation to me []

[another_someone]

  So the main issue is probably that in temperate regions in the winter the trees lose their leaves and the bark gets sunlight.  In humans living in regions where there is less sunlight the skin is lighter to let enough light through to synthesis vitamin D, "letting light through to the inner core".  In this case isn't it just a lack of pigment that causes light skin? I think this might be different in plants where they are more truly "white" and actually reflecting all wavelengths of light. 

  Plants in these same regions collect sunlight all summer through their leaves (and at this time the bark is shaded), so they should have plenty of opportunity to get any energy they need through the leaves.  In the winter they pretty much shut down.  They lose their leaves and they stop almost all metabolism so extra sunlight is damaging - they can't harvest it through photosynthesis so it can damage DNA / protein / cell walls etc. 

If white bark were simply to compensate for the loss of winter foliage, would that not simply mean that all trees that lose their foliage in winter should have white barks, and if anything, those trees that lose their leaves in winter in more southerly latitudes should be whiter than those trees that lose their leaves in more northerly latitudes.  Yet it is plainly apparent that not all trees that lose winter foliage are white, and you have implied that in the more extreme northerly regions, where one would expect less winter sun, the prevalence of white bark actually increases.

I suppose one could argue a hypothesis that there might be a correlation between the number of months that a tree is denuded of its foliage, and the whiteness of its bark, but that is a hypothesis that should be fairly easy to check against reality.

The corollary question to this must be: why are not all trees of a white bark?  With human skin, one reasonably understands the tradeoffs between light and dark skin, and why having a light skin under strong sun is bad, just as why having a dark skin in the absence of sun is bad.  What is the trade-off for trees.  Why are trees in the tropics not white?  What would they have to lose by also being white?

[WylieE]

Oops- I guess I didn't explain that very well, I didn't mean that the bark is white to compensate for the loss of winter foliage.  I meant the opposite, that there isn't a need for the plant to collect sunlight in the winter- it would be collecting plenty in the summer. 

  The level of whiteness does correlate with the extremes to which the tree grows.  Those that grow up near the limits of where a tree can grow are the whitest.  Good point about the daylight, I didn't think about that.  One answer to that would be that real problem would not be in the middle of winter when these places would be so cold that even direct sun wouldn't bring them above freezing (it's been like that here for the last month).  It is probably more of an issue in the early winter and early spring when the refreezing at night could be a problem.

 So why just this tendency for whiteness in the extreme north?  Maybe, as you point out, there is some selective pressure against whiteness- another major function of the bark is to protect against pests- maybe there are some compounds in darker bark that help with this (totally speculating here).  Perhaps in the extreme north, the pest season is also shorter so the advantage of having white bark to protect against the freeze /thaw cycles outweighs the loss of pest protection. 

So for the corollary question- I should first point out that while the majority of the trees that live in the coldest regions are white, I'm not sure if the opposite is true.  There are plenty of tropical trees with white bark- banyan trees, ghost gums- a strikingly WHITE barked tree from the outback (a favorite haunt of drop bears- so don't stand too close).  Tropical tree barks can have a wide range of shades from very light grey to bright red. So perhaps white bark offers an evolutionary advantage in the coldest regions, but in the warmer regions there is really no selective pressure for white bark due to temperature.  Therefore without the temperature pressure, selective pressure from other functions of the bark, like protection from pests can take over.   

Personally, I'm going with the camouflage to attack skiers hypothesis.  Where it is snowy or icy there are trees with more white bark- coincidence? Perhaps not.

[another_someone]

The reference to Banyan trees, and trees in the outback, is consistent with another hypothesis I was dwelling upon.

Although you have mentioned cold in the northern latitudes as a factor, but what strikes me as being a more common factor amongst these trees (maybe you will now come forward with examples that will contradict this pattern) is what we are looking at is tree spacing.  Trees in northern latitudes, as with trees in the outback, tend to be more widely spaced (as I believe the banyan tree might be) than many trees in the tropics.

This would not rule out some of your other arguments.  Isolated trees are likely to be less susceptible to attack because many parasites that move from tree to tree would find greater difficulty reaching a more isolated tree.  On the other hand, a tree standing on its own will have more sunlight reaching its bark, and thus might more easily find itself subject to sunscald injury.

Also, trees out in the open, and subject to more sunlight, may actually be protected from parasites by the sunlight, as many parasites will themselves find the sunlight too much (and the fact that the tree is reflecting back that sunlight possibly might even increase the amount of sunlight falling on the parasite - depending on how the bark channels the sunlight along its surface).

[WylieE]

Interesting point about the spacing.  That could make some sense for the ghost gum.  Maybe there is no pressure for these to maintain the darker bark (if it has to do with pest).  Maybe you're on to something here- I'll have to look up other white barked tropical trees.

Cool idea about the sunlight reflection also serving as an insect repellant- this could be a means of defense against fungus as well.  Plants use sunlight reflection to signal in pollinators, why not use it as a defense mechanism too?  The old myth about moss only growing on the north side of the tree comes to mind now.

 One thing to keep in mind though, on the spacing, is the effect humans have had.  Have these trees really (in the timespan of tree evolution - aka a LONG time) been in areas of sparse growth or is that an effect of human selection?  "Clear out the brush, but keep a few trees."
 
Silver Birch trees (yay we made it back to the start of this post) would normally grow in a dense forest (dense forest- a bunch of stupid trees [] )- but now we mostly see them on fence rows or just along the edge of a river.  So we might think of these as sparse, but they weren't for most of the time they were selecting white bark.  For the other temperate trees with white bark, they usually grow in forests (or at least used to)- here I am thinking of white birch, aspen, some species of poplar, and Alaskan white birch. 

More evidence for the anti-skier hypothesis- the more dense they grow the better their chance of  getting a skier.   

[another_someone]

I don't go along with humans having had too little time to impact evolution.

True that trees are longer lived than insects, and so evolutionary time is slower for them, but certainly with insects, there is good experimental evidence that human activity simply since the Industrial Revolution has been sufficient to impact the colour of moth populations.

It is not a case of having to wait for DNA mutations to appear - all it takes is for a small fraction of the population to naturally have a different colour bark, and if these trees suddenly find that because of human activity they have a competitive advantage, then they will start to dominate their species.

The question must be as to what degree of variability naturally occurs in the bark colour of a single species, and so do the genes already exist that would allow a rapid change in the species.

The other thing to bear in mind is that trees such as Birch are not the longest lived of trees (yes, they are long lived in comparison to typical animal lifespans, so we could not have affected their evolution as quickly as we could for moths, but they still do not live for millennia, so human pressures that have been consistent for a thousand years or more might still have been able to have an impact upon them).

Is it possible even, that at least with some trees, the colour of the bark is not genetically determined but environmentally determined, so the same tree, if grown in shade, might develop a dark bark, but if grown in a sunlit environment, will develop a white bark?

Although I cannot find comparitive data for the silver birch, the paper birch (according to Wikipedia), which also has very white bark, requires lots of sunshine, and so clearly would not be in the midst of a dense forest (this is without any human intervention).

According to http://www.plantpress.com/wildlife/o467-silverbirch.php the silver birch, like the paper birch, is a pioneer species, and so one would expect it to grow in isolated locations rather than in densly wooded areas.

[WylieE]

I don't go along with humans having had too little time to impact evolution.

I certainly did not mean this.  [] I would totally agree humans have an impact on the selection of species, even of trees.   We probably will see more and more of this as time goes on. That's the scary thing- with issues like trees and their long lifespan- will they be able to adapt to the changes occurring?

 But in this case, I don't think that has anything to do with why the bark is white (at least of the trees I am talking about).    My point was simply that it probably isn't due to spacing, because the change in spacing has really only happened in the last generation or two (and when these trees are grown in thick forests they are also white).

 For these trees, the colour of the bark is not environmentally determined.  For some non-woody plants the color of the stem is affected by stresses such as drought, UV, and cold.  There are some examples of trees changing their bark . . .but these might not be exactly what we are talking about- but just to point out that what you are suggesting probably is possible:  Sycamore trees (except I think y'all call them something else, but I can't remember what now) start out brown, but lose their bark revealing a white underneath as they mature (this would also correlate with when they are more likely to be exposed to the sun above the canopy); many woody stems start out green when the plant is growing and contribute (how much is debatable) to the photosynthesis done by the plant.

 

It is not a case of having to wait for DNA mutations to appear - all it takes is for a small fraction of the population to naturally have a different colour bark, and if these trees suddenly find that because of human activity they have a competitive advantage, then they will start to dominate their species.

But . . for the fraction of the population to have a different colour bark it is most likely (I think always- is this safe to say?) due to some DNA mutation.  The rest would be exactly as you say- if this population had a change that was advantageous based on some human change to the environment they will dominate.  We can see this in action- to take an extreme example- just look at an apple orchard- years ago that was probably a forest, now apple trees that taste good have a competitive advantage and are the only species of tree in the whole area. 

 I agree it is not an issue of "waiting" for DNA mutations to appear, they are happening all the time.  One really cool thing about plants (and now there's some evidence in animals too) when in stress they can increase their rates of mutation.  That's a pretty cool adaptation, isn't it?

Cheers,
Colleen

[WylieE]

According to http://www.plantpress.com/wildlife/o467-silverbirch.php the silver birch, like the paper birch, is a pioneer species, and so one would expect it to grow in isolated locations rather than in densly wooded areas.

Pioneer species are usually the first ones in a tough area- they are able to establish quickly with little nutrients- but this doesn't mean that they don't grow in dense groups (and wouldn't be exempt from pests due to isolation- isn't that what started this conversation?) -  I've walked through some beautiful birch groves (don't know what particular species- I think paper birch) that were fairly thick. 

Anyway, this is fun, but I feel like it's getting a bit off topic.  []

Colleen

[another_someone]

I don't go along with humans having had too little time to impact evolution.

I certainly did not mean this.  [] I would totally agree humans have an impact on the selection of species, even of trees.   We probably will see more and more of this as time goes on. That's the scary thing- with issues like trees and their long lifespan- will they be able to adapt to the changes occurring?

As you say, way off topic, but certainly this is one of the problem with hardwood trees, which have long lifespans, and particularly yew trees which are but a small fraction of the population they would have once been.

But generally, even in animal populations, in times of stress, the short lived ones (e.g. rodents) will have an advantage over long lived ones (e.g. turtles).

A corollary to this is ofcourse that as the human species has reduced the stress upon itself, so its life expectancy has risen, but this factor alone will make it more vulnerable should future stress levels upon the species increase.

But, as you say, this is diverging off topic []

My point was simply that it probably isn't due to spacing, because the change in spacing has really only happened in the last generation or two (and when these trees are grown in thick forests they are also white).

I would place greater weight on the second statement than the first (not because I disagree with the first, only to ask whether, for a tree with maybe a 60 year lifespan, we are really talking about two generations, or are we talking about 20 generations or more that humans may have had an influence over the trees?  This situation may differ in Europe and the Middle East than it would on the American continents).

Sycamore trees (except I think y'all call them something else, but I can't remember what now) start out brown, but lose their bark revealing a white underneath as they mature (this would also correlate with when they are more likely to be exposed to the sun above the canopy)

Are the sycamores changing colour only in those parts of the bark that is above the canopy, or otherwise exposed to open sunlight, or does exposure of the upper reaches of the tree cause the entire tree cause a change in colour of the entire tree?

[another_someone]

According to http://www.plantpress.com/wildlife/o467-silverbirch.php the silver birch, like the paper birch, is a pioneer species, and so one would expect it to grow in isolated locations rather than in densly wooded areas.

Pioneer species are usually the first ones in a tough area- they are able to establish quickly with little nutrients- but this doesn't mean that they don't grow in dense groups (and wouldn't be exempt from pests due to isolation- isn't that what started this conversation?) -  I've walked through some beautiful birch groves (don't know what particular species- I think paper birch) that were fairly thick.

 

Wikipedia suggested that paper birch did require sunlit areas to grow, although how this correlates with your observation of dense groves (if dense they were) of paper birch (or even if wikipedia have got it wrong - not unheard of), I don't know.

For the silver birch, what you say about pioneers makes sense, since from I have read, silver birch are whiter when they are young (and thus before they have had time to form a dense grove), and become darker as they mature (with underlying dark bark showing through the outer white bark - even more so near the base, where less sunlight will reach as the tree grows) as the trees mature and begin to provide mutual shade.

http://www.arkive.org/species/ARK/plants_and_algae/Betula_pendula/

As silver birch ages, its bark darkens and becomes rougher and more fissured and prone to attack by the birch polypore fungus Piptoporus betulinus.

It is not clear from this if it is simply the fissures in the bark, of whatever colour, are exposing the tree to fungal attack, or whether this is consistent with my earlier speculation that the whiteness of the bark may contribute to some protection against some parasites.

Anyway, this is fun, but I feel like it's getting a bit off topic.  []

Colleen

I certainly don't wish to push you beyond what you think reasonable in terms of your time, etc., but although we have digressed a bit insofar as to ask a more general question, why are some trees white and other not so (rather than limiting ourselves to Silver Birches), I don't think it is totally off topic.

As far as I can see from our discussion, it does seem that sunlight is the determining factor, but latitude and foliage cover are merely co-incidentals (i.e. we can find white bark tress at all latitudes, and some trees will change the lightness of the bark colour in accordance with changes in the amount of shade they expect through their life, but not with respect to changes in latitude).  Since we are also talking about tropical trees that are white barked, clearly this is not a trait limited to deciduous trees.