Thank you for contacting us, I'm so sorry to hear that you are having this problem.
There are several different possibilities that could cause damage to your trees, so I'll ask you some questions and you can respond on this website, or you can contact us through email at firstname.lastname@example.org. You can also call 477-2181 to talk to a Master Gardener volunteer.
You said this was happening slowly, has it been going on for months or has it been years that you've had the problem? It sounds like the ones on the end are affected, and if this is so, does one die and then the next one starts to decline, or is it a gradual process with multiple trees affected, but the ones on the end die first?
If one dies before the next one starts to decline, it suggests something in the environment that damages only the tree on the end, and when that one is gone, it begins to affect the next one. If several trees are all affected, but the ones on the end are the worst, it suggests maybe something living is causing this damage and slowly spreading from one to another.
This was a bad year for rodents damaging plants, have you seen gopher mounds in the area? They may be feeding on the roots. Sometimes voles (think of field mice) will chew on the bark of a tree over the winter, eventually eating enough to do serious damage. This can even kill a tree in a short period of time.
If the damage happens or gets worse during a certain season, that is also a clue. Spider mites damage these trees during warm and hot weather, where as tip blight diseases are more active in cool moist spring weather. Rodents can be active all year, but winter and early spring are times when we often see the results. Winter damage from low temperatures, snow load, and deice burn happen in the dormant season, but may not be seen until the weather warms.
Are the trees getting enough water? Maybe too much water? It's good to check your system and make sure it's working correctly.
I realize this is a lot of questions to answer, but hopefully your answers will give us some clues to solve the problem. If you are able to cut a sample from your tree and bring it in to our office, that might really help. Ideally a sample will be from an area on the tree where there is a transition from healthy to damaged tissue. Depending on the size of the plant, a sample from 6 inches to a foot long should show us something.
Our clinic is located at 222 North Havana street, just south of the baseball stadium at the Spokane Fairgrounds. Our office is open from 8:30-5pm weekdays and 9am-noon on Saturdays. Our plant clinic is open shorter hours (9-3 Mon-Thurs; 9-1pm on Friday; 9-noon on Sat) but not everyone can visit during those hours. If you can take pictures of the trees and email them to us, that can also be helpful. Our email address is email@example.com. You can also upload them to this site.
I hope this gives you a start and that we can assist you in solving the problem these trees are having.