Take the phonetic pronunciation of a letter of the alphabet (i.e. “aitch” or “eye” or “jay”) and follow it up with the names of two of the ten most common trees found in deciduous American forests. The result sounds just like a fourth word: something that you can find thousands and thousands of in American forests, ever since 2001. What is it?

Edited to add: solved by “porpoise spit” within seconds, seemingly, here.