50 years old is old for birch so you want to take good care of this tree! Birch tend to release sap or "bleed" when pruned in spring, and can be more susceptible to pests. The best time to prune is late summer / early autumn. 

You want to keep your birch as stress-free as possible. Birch grow well in cool, organic, slightly acidic soils. Think about the birch you see growing up on the North Shore. 

Mulch your tree's root area to moderate soil temperatures and to hold moisture. Water your tree when conditions are dry Watering established trees

Birch tree roots are very shallow, so avoid trying to grow grass under the tree if possible as the grass will compete with the thin feeder roots of the tree that take up water from the soil. Shade-loving perennials like Hosta do well under birch because they require little water and their large leaves shade the birch roots.