This is a well known fact that берёза (birch tree) is considered as a symbol in Russia but, oddly enough, it is not the one that is most often mentioned in Russian idioms, but another tree липа (the lime tree). For example, you can hear “это липа” (this is fake) “документ липовый” (this document is fake) when talking about a fake document. And if a man happens to pay to much for something then this man says about himself “ободрали как липку” (ripped as a lime tree).
So what is so special about the lime tree? Древесина липы очень мягкая (the wood is soft), which make it easy to work on, поэтому именно её использовали нечестные мастера (and for that reason, it was used by forgers) to make copies of seals. It was simple to reproduce with exactitude the smallest details of any seal and it was very difficult to recognise a fake from a real one.
The bark of the lime tree is also particularly soft, and therefore they would take the bark of this tree to get to the bast fibre inside. It looks like a string and it's easy to split into thin strips and to give it any wanted form. Still, it is quite strong.
In the countryside, people used плели корзины (to make baskets) and изготавливали традиционную русскую крестьянскую обувь – лапти, (traditional Russian peasant shoes, the lapti) out of the bast fibre. Children were taught from an early age how to work these strings because it was considered a very simple job. The expression “лыка не вяжет” (to be too drunk to make sense) is used when talking about a person who doesn't know how to do anything. In modern language, this expression is used in relation to drunk people. If you hear “Он сегодня лыко не вяжет”, when talking about a person, it means that this person is so drunk today, he can't do anything at all.
The expression “не лыком шит” (he is no fool) was understood differently according to the period it was used. In the past, it used to determine the social status of a person. It meant that the person didn't belong to the poor peasant class. In modern Russian, its meaning has become much wider, often characterized a dishonest, very mean person.
This is a well known fact that birch tree is considered as a symbol in Russia but, oddly enough, it is not the one that is most often mentioned in Russian idioms, but another tree the lime tree.