They tend to be the first to colonize a nutrient rich medium. Harry Hoitnick at Ohio State has published a lot about this phenomenon. His work is on the disease supressiveness of compost. That is essentially a heat-sterilized growing medium. The carbohydrate composition after composting is the key factor determining whether pathogens or beneficial fungi colonize.
Heat treatment tends to make available nitrogen in higher quantities.
That can really shift the miycorrhizal populations. Erik Lilleskov, here at Cornell, did some nice work on this on ectos in Alaska.