Pairs

I'd noticed before about the forest that quite often trees appear to have been planted in twos, no doubt to hedge the foresters bets on forming a regular wood.  Here in Camel Green I've noticed how many of these paired trees follow a similar pattern; one thriving tree paired with one dead upstanding tree. I grow plants and veg, and you regularly have plants that don't take or wither when still little more than a shoot. These trees though appear to have thrived together for some considerable time, before one just gave up the ghost. Why is that? What's effected one that hasn't effected the other? They share the same environment, the same ground, the same source of water, everything! It's not just one or two either; pairs of one living, one dead are all over this wood. Another woodland mystery.