​Many factors play a role in a trees death and even the most educated Arborist may not be able to provide the specific cause, but here are 2 of the main issues we deal with in the PNW
We get plenty of water here but late in the summer we can experience periods of high heat coupled with very low or no rain. This can spell disaster for birch trees especially "Eupopean White Birch" (Betula pendula) and while this may not directly kill the tree it can weaken its defenses allowing a myriad of other present diseases to get the upper hand and deliver the final blow.
1: Don't let your trees dry out! Water your trees with at least one deep watering weekly during these late summer periods.
Bronze Birch Borer (Agrilus anxius) is currently sweeping across the PNW leaving thousands of dead birch trees in its wake. The larvae of these beetles bore tunnels just under the bark within the phloem of mature trees. These tunneling grubs destroy vascular tissue and the adults carry fungal, bacterial, and viral diseases from tree to tree.
2: Learn the warning signs of infection and prevent it with pheromone traps and yearly spraying or injections.