Wisconsin, as many other states, has an epidemic of a destructive non-native weed, Garlic Mustard, taking over our woods in the spring. It invades gardens, too--and is amazingly opportunistic. Garlic Mustard rises high in spring, sets zillions of seeds (for next spring) and then dies back.
Scientists report that its roots emit a toxic substance very damaging to tree roots. It literally changes the nature of the soil, affecting wildflowers as well.
I have been gathering wild mushrooms for many years, and have noticed a definite decrease in wild mushroom populations that grow in association with trees--boletes, chantarelles and others.
Since these and other mushrooms, including morels, are associated with certain trees, I'm wondering if morel gatherers have noticed a decrease.