Death or droop by drought

Relentless heat and unremitting drought are taking a toll on the natural world.

Leaves of Cornus drummondii, roughleaf dogwood are wilted to a vertical droop and rolling inward. Box elder leaves Acer negundo are turning green to brown, skipping their normal fall yellow. Entire small branches of red elm, Ulmus rubra are dying from green, straight to brown. Common poison ivy Toxicodendron radicans leaves are browning and dying from the periphery inward.

I wonder if (and hope that) trees can respond to thermal and drought stress with premature formation of leaf abscission layer, so that damage from drought stress can be reduced for the whole tree. I've seen a few yellow mulberry Morus rubra leaves and brown sycamore Platanus occidentalis leaves down. 

I am hoping for enough tree vigor left to form buds, and then form abscission layer and let the leaves go.

My entomology textbook tells me that most insects die at temperatures > 40-45 C (100-113 F). Could that be part of the local collapse of insects? It has been that hot.