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Of course this is not the only hypothesis about why trees have lighter bark- others include camouflage so they can attack skiers.
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.
I don't go along with humans having had too little time to impact evolution.
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.
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.
Quote from: another_someone on 11/03/2007 02:58:41I 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?
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).
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)
Quote from: another_someone on 11/03/2007 02:58:41According 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.
As silver birch ages, its bark darkens and becomes rougher and more fissured and prone to attack by the birch polypore fungus Piptoporus betulinus.
Anyway, this is fun, but I feel like it's getting a bit off topic. Colleen