We got called to a customer's house with a common cry - "my beautiful tree all of a sudden started to die and was gone before I knew it!" In this case, an 8 foot spruce tree that had been in the ground at least 4 years.
We hear this all too often, and I almost always find the problem immediately - incorrect planting! Some landscapers slap a landscape in and sure, to you, it looks good and was maybe cheaper than someone else. But what is lurking under the surface is death and disappointment. The installer here did not plant this tree properly, did not even remove the nylon tie around the trunk, which doomed the tree at time of planting to die. What's the most important and valuable part of your landscape? Your trees! Mature trees are not replaceable, so be sure they were planted correctly.