Pine tree in container losing needles
I've had a pine tree in a container on a balcony for a couple of years. It was planted in a mix of john innes no3, ericaceous compost and grit for drainage, and it drains very well. It get's plenty of sun and it's been fine since planted and put on good growth and has healthy new shoots, but as you can see from the photos, it's losing needles.
Anyone any ideas what's wrong ? Could it have outgrown it's container ? Would pruning help and when and how should that be done ?
Thanks in advance,