Have compacted soil around your trees?
There are many things can be stressing on your plants and trees. The first and best place to start is with the soil and its structure. In an urban or suburban setting, the soils are often hard and compacted lacking pore space. A good remedy for this is to aerate the soil. By breaking up the soil and creating pore space in the root zone, you are addressing a very primary issue for the root system of the tree. This can be done mechanically by hand with tools like crowbars or even a piece of rebar. Drive the tool into the ground and move it back and forth creating a hole. The process is very similar to aerating the lawn and has the same effect. However, for the trees, the holes would be deeper and a bit wider. Sometimes using an amendment like mulch, dehydrated manure or even small stones are a good way to keep the space open.
There are also more aggressive ways to break up large areas of soil. A tool such as an airspade or air knife uses compressed air to fracture the soil. These tools are very affective and can also be used to incorporate amendments as well. They are also a great tool to us to blow excess mulch and soil away from the bases of the trees as depth of soil is as critical as structure.