Chlorosis is a yellowing of the leaves with dark green veins that can affect birch and maple trees.  This is caused by nutrient deficiency in the soil or a high alkalinity or Ph level. It could also be due to damage or compacted roots or poor drainage. If left untreated, the tree will be unhealthy, it’s growth will be stunted, and it could die.  If the cause is a root issue, core aerification, tiling, or mulching could be possible solutions. If the problem is the soil, there are trunk applications that could last for several years, but they do take a few weeks to see results. There are also soil treatments that could be done, but a soil test should be taken first.