Posted: 20/07/2014 at 16:50
The slugs and woodlice will have moved in rather than causing it.
It'scan be hard to pinpoint a cause of trunk damage, rabbits, strimmers, edge of a mower, anything like that lets the rot in. I doubt if the grasses caused it though they may have hastened the rot. Competition would slow down growth maybe but trees germinate and grow in grass land without rotting.
It's over for that magnolia though sorry