The answer to 'why is my tree yellowing' is that it is likely due to a nutrient deficiency, environmental stress, or disease. Yellowing leaves can be a sign that the tree is not getting enough nitrogen, iron, or other essential nutrients. It can also be caused by factors like drought, excessive sun exposure, or pest infestations.
If the yellowing is isolated to the older leaves at the bottom of the tree, it may just be natural leaf drop. But if the yellowing is more widespread or progressing up the tree, it could indicate a more serious issue that requires attention. Try to identify the underlying cause and address it through proper watering, fertilizing, or pest control. If the problem persists, it's best to consult a certified arborist or tree care professional.