I have encountered problems with harvesting trees. In the beginning it worked as it should but then it does not work to harvest trees anymore even though I press.
Welcome to the forum. 
What do you mean exactly? Are you using the Harvest (H) or the Clear (C) keys to select an area of trees to be cut down?
Check to make sure you have enough Laborers? Things grind to a halt if those numbers drop too low.
Thanks for reply.The problem is that it is no longer possible to use the trees to get wood. I press the tree as usual and then select the harvest option but nothing happens.