Two springs ago I planted about 8 hostas under a Y shaped birch tree. Last
fall we had to cut off one tree of the Y because it was dead. Now the other
half is dead and must go. BRBR

This had nothing whatever to do with the hostas. You do not say where you live
or how old the trees were. In most parts of the country, birch trees only live
about 15 years or so at best, especially in an unfriendly environment, like a
hot, sunny lawn. Your birch tree probably died of old age, bronze birch borer,
or a fungus disease.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)