@ai6yr If you look at many of the surviving oaks, especially the giants, you'll see large fallen limbs or signs of such. Like many tall trees they have to move water a great height essentially by suction, and if there is too little water, cavitation occurs. They typically don't recover from this, and everything uphill of that spot dies. Limb drops are common during drought years and may even hep the tree by shrinking it during hard times. Sometimes, however, the tree collapses catastrophically.