Has water been a problem for the past few years? Prolonged drought and large trees do not go togethere well. Drops in water tables can casues rapid decline and death.
I've heard that localized deep watering and perc irrigation can help wick water from deep water tables up during times of drought.
Perk waterting a any way you can hget the water to sit and percolate down trough the soil. This could be using a kiddie pool with holes punched in. Many cans layed out, or the Wulkowitz hose dam/flood system.
Then use of broad spectrum organic products in sprin, mid sumer and fall can help reduce stress on trees.
The mid sumer is for when the trees start the prosess of putting on the "late wood" ring growth which is thought to have a major role in compartmentalizatio.
High N fert has been shown to cause more harm then good on stressed plants.
