You might notice, as you wander around the forest, that many of the ditches are choked with an orangey-brown “gunk”.
The gunk is caused by bacteria that “feed” on iron: it looks like rust, and that is pretty much what it is: oxidised iron.
Although it is “natural”, this gunk is not good for wildlife. As the year goes on and the temperatures rise, the bacteria become more active, the trees take up more water and the ditches become more and more sluggish and suffocating.
Historically, the ditches would have been flushed through from time to time by rainstorms. In those days, the gunk wouldn’t be able to build up so much. But nowadays, the ditches are neglected and choked and they never run freely anymore.