The answer to 'tree yellowing' is that it is a common problem caused by a variety of factors, including nutrient deficiencies, disease, or environmental stress. The leaves of the tree may start to turn yellow, sometimes with green veins remaining. This can be a sign of issues like nitrogen or iron deficiency, root rot, or drought stress.
In many cases, the cause can be diagnosed and treated with proper care and maintenance. However, if the problem persists or the tree appears to be in serious decline, it's best to call in a professional arborist or landscaper to assess the situation and provide the appropriate treatment.