Question: My fir tree has died after a couple of years in the ground. What do you think happened?
Answer: (this required a home visit)
After investigating how the plant sat in the ground it revealed that it was planted about 6 inches below the natural root zone. Also there were several plastic cords still around the trunk.
Both of these conditions could have caused severe damage to the tree. It did not seem that the cords had dug in tight enough to cause harm and it did seem that the entire root system was completely dead. Planting trees and shrubs deeper that their natural depth will cause the roots to suffocate. While the leaves use carbon dioxide to make food, the roots respire and need oxygen to do their work.
The extra soil on top of the root system limits the penetration of oxygen to the roots and will cause their slow death. I think this was the likely cause of the death of this specimen.
This can explain why excessive application of mulch in gardens and around trees can cause problems. Mulch depth should be limited to 2 inches with the mulch tapering to zero at the trunk.