My locust tree  has leaves that are turning yellow and dropping.  Also, there are some new leaves that are turning brown and curling.  The ends of new limbs are turning black.  We have been experience a heat wave (100+) for two weeks straight.  There is black rock on top of black weed barrier.  We use a drip system for 1" of water a week. Is it the rocks or the black weed barrier?  Is it from underwatering?  Thank-you for your help.