context: The Upper Rhine region was changed significantly by a Rhine straightening program in the 19th Century. The rate of flow was increased and the ground water level fell significantly. Dead branches dried up and the amount of forests on the flood plains decreased sharply. On the French side, the Grand Canal d'Alsace was dug, which carries a significant part of the river water, and all of the traffic. In some places, there are large compensation pools, for example the huge Bassin de compensation de Plobsheim in Alsace.
question: In what century did the amount of forests on the flood plain increase?