The answer to 'why is my drainage system dead trees' is that a malfunctioning or blocked drainage system can lead to excessive moisture around the roots of nearby trees, causing them to die. Symptoms of this issue include standing water, soggy soil, and the gradual decline and death of the affected trees.
The most common causes are clogged drains, broken pipes, or a drainage system that is simply not adequate for the size and layout of your property. This can be exacerbated by heavy rainfall or poor grading that directs water towards the trees. To fix the problem, you'll need to inspect and clear the drainage system, repair any damage, and potentially upgrade or expand the system to handle the water flow more effectively.