We recently cut many live poplar trees and left 2-4 foot high stumps. Does anyone know if drilling into the stumps and innoculate with mushroom spores (eg. shiitake) would result in successful innoculation and fruiting?
Shiitake might work, though they are said to grow best on Oak. Oyster Mushrooms (Pleurotus Ostreatus) are the most vigorous and easiest mushroom to grow, but you might also have some luck with Lions Mane (Hericium). Since you have Poplar stumps, you might wish to try growing the Black Poplar Mushroom, Agrocybe aegerita, as they are commonly found growing there, (if you can find plugs for that species.)
It is very difficult to tell if a stump is successfully being colonized by the plugs. There is very little to no visual indication that the correct mycelium is growing through the stump, until you actually see the mushrooms fruiting. And it can take a year or more to get mushrooms, so patience is in order.
It's possible to get plug spawn fairly cheaply. Since you have multiple stumps you might wish to try multiple species of mushrooms.