I think I have seen more effects on our town trees like leaf/needle scorch as well as beetle kill, then I can remember. The effects are cumulative of course, so measures to counter stress needs to take place over the course of several years. Drought stress sets the stage for all kinds of ill opportunities later down the road, and reducing that stress adds a lot of years to any plant’s life.
There is no magic bullet for a tree that is too far gone. How about our current water situation? If your trees relied on TID water, are you doomed? Which trees need water the most, and conversely, which trees need the most water? Is your soil well drained, or the opposite? You should have a soil probe that takes a core sample to a depth of at least 12” or so, the deeper the better. forget about a moisture meter used for houseplants, it is NOT up to the task.
Take several samples from about halfway to the dripline from the stem, out as far as two times the dripline, if possible. These core samples should contain tiny rootlets through the column of the sample. Remove the core sample and squeeze it in your hand. If you can wring water out of it you have more than enough water, and if it crumbles and blows away in the wind like a powder, you know you need to irrigate. If you have never irrigated your tree before and it seems fine, it probably is!
There are formulas that state “about x gallons by caliper of stem” and such and I pretty much ignore all of ‘em, I can find more from my probe about the soil under a specific tree than any formula will suggest.
If your tree is wilting, you need to irrigate, but where? Well, where the absorbing roots are, of course, and you can find these with the probe. All trees are a little different just like soil, so you need to do a little investigation, lest you waste water.
All this work should make you ponder and appreciate a 300-year-old oak on the hill that hasn’t EVER been artificially irrigated, that is just plain miraculous! Watering a tree in a pot is easy peasy, just fill the pot and boom! You are on to the next one, believe it or not, watering a mature tree is pretty much the same, just on a much bigger scale, all you are doing is putting water where it will do the most good, you just don’t have a pot to use as a guide. Make sure you can justify the cost of irrigating the tree vs. what the tree is actually worth as the cost of city water can bite you. Maybe bake the local water truck driver a cake once in a while? Enjoy summer, fall will be here sooner than we think. Plant high and often, just not right now, it’s too damn hot …