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Several species of the tree genres Betula have a white bark,especially when they are young. This is quite peculiar to trees - at least in central Europe; so I'm wondering if there is a ecological reason behind it.

The first idea I have is that they protection against the sun. But normally protection of nature against the sun is to make the opposite: put more pigments in the skin, so that the sun does not damage the underlying DNA. If this is the explanation, how do other tree species protect against this?

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It is protection against rapid warming of the cambium layer. A lot of far northern timber has light colored bark which reflects sunlight. The rapid heating from very cold after sunrise can actually damage or even split the bark of dark colored species. This is called sunscalding.

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    $\begingroup$ Note that aspens, which grow at high elevations in the mountains, also have very light-colored bark. But oddly enough, the conifers growing in dryer areas at the same elevations generally have dark brown to greyish bark. $\endgroup$
    – jamesqf
    Commented Dec 24, 2017 at 20:01
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    $\begingroup$ conifers of course provide their own year long shade. $\endgroup$
    – John
    Commented Dec 24, 2017 at 22:17
  • $\begingroup$ Not usually around sunrise, unless located in a dense stand. $\endgroup$
    – jamesqf
    Commented Dec 25, 2017 at 3:43
  • $\begingroup$ @jamesqf thick, rough or rugose bark also aids in resisting it, conifers also experience less dramatic dormancy which helps resist the effects. Different solutions to the same problem. $\endgroup$
    – John
    Commented Dec 26, 2017 at 1:21
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    $\begingroup$ The definitive study seems to be Karels, TJ & Boonstra, R "Reducing solar heat gain during winter: The role of white bark in northern deciduous trees" Artic vol 53 no 2 pg 168 (2003). $\endgroup$
    – Bryan Hanson
    Commented Dec 26, 2017 at 21:47

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