There are several things I do when I know the unseasonal temperatures are on the way.
* water early in the day or even the day before so the trees are well-hydrated ahead of the heatwave
* water often: after watering in the morning I'll overhead water the trees, benches, ground and nearby fences throughout the day
* add a layer of white sphagnum moss (or mountain moss/yamagoke for azaleas) to the surface of the soil
* move sensitive trees into the shade: if the temps are high, I'll move trees under benches or to corners of the garden where there's less sun
* I also use shade cloth and can move more trees under the shade cloth during hot spells
* for accents and smaller trees, I set them on a tray with pumice
Of course, it gets much warmer in other parts of the country and world. Am curious to hear from others about successful strategies for dealing with the heat.