And since panspermia is necessarily done with microbes, they would get outnumbered and munched by our microbes if they arrived after our tree dominated our plan

And since panspermia is necessarily done with microbes, they would get outnumbered and munched by our microbes if they arrived after our tree dominated our planet. Unless other planets happened to give birth to hardier, more virulent microbes, then welcome Andromeda Strain.