Finally, I understood the data set and its limitations, while working on the next tree exploration.
My former explorations tried to show the evolution of planting trees. While trying to achieve that, I didn’t realized that it is not possible with that data set. The data shows only the existing trees and their age. All the trees which don’t exist any more are not in there. I guess a huge amount of trees burned down during the war. All these missing trees falsify the image. There is no evolution possible with als those missing trees.