Not sure if this has been mentioned already, apologies if it has..the dry stone wall is an ingenious bit of planning ahead..when the trees grow out and the inevitable happens in 10 years you can simply buy a few more stones, enlarged your enclosure, dig up and move any crowded trees..pure genius!
Not my intention but an interesting point. However, any closer to the poplar tree and the roots of that beast are very superficial and about as thick as logs!