Parasitic Mushrooms: Blights of the Forest or Agents for Habitat Restoration?

Parasitic Mushrooms: Blights of the Forest or Agents for Habitat Restoration? 



Parasites are predators that endanger the host’s health. In the past, foresters saw all parasitic fungi as hostile to the long-term health of forests. Although they do parasitize trees, they nourish other organisms. Parasitic fungi such as the honey mushroom, which can destroy thousands of acres of forest, are stigmatized as blights. However, more foresters are realizing that a rotting tree in the midst of a canopied forest is, in fact, more supportive of biodiversity than a living tree. Parasitic mushrooms may be nature’s way of selecting the strongest plants and repairing damaged habitats.
Ultimately, parasitic mushrooms set the stage for the revival of weakened habitats that are too stressed to thrive. Of all the parasitic blight mushrooms that are edible by humans, the assorted honey mushrooms such as Armillaria mellea and Armillaria ostoyae are the best known. One mycelial mat from a honey mushroom (Armillaria bulbosa) made national headlines when a specimen was found in a Michigan forest that covered 37 acres, weighed at least 50 tons, and was estimated to be 1,500 years old. In Oregon, a far larger honey mushroom (Armillaria ostoyae) mycelial mat found on a mountaintop covers more than
2,400 acres and is possibly more than 2,200 years old (see figure 60 ). Each time this fungus blight sweeps through, nurse logs are created, soil depth increases, and centimeters of soil accumulate to create ever-richer habitats where once only barren rock stood. (For further discussion of Armillaria blights, see figure 59 .) What makes mushroom mycelia different from the mycelia from mold fungi is that some mushroom species can grow into massive membranes, thousands of acres in size, hundreds of tons in mass, and thousands of years old. 
Many saprophytic fungi can be weakly parasitic, especially if a host tree is dying from other causes, such as environmental stress or parasite infestation. Saprophytes that can take advantage of a dying tree are termed facultative parasites. For example, oyster mushrooms (Pleurotus ostreatus) are classic saprophytes, although they are
frequently found on dying cottonwood, oak, poplar, birch, maple, and alder trees. And although reishi (Ganoderma lucidum) is considered a true saprophyte by most mycologists, the Australian Quarantine Inspection Service has classified this medicinal species as a parasite and has banned its importation. Authorities on other islands including New Zealand and Hawaii also consider this mushroom a threat to their native trees. Some parasitic fungi behave like saprophytes, such as honey mushrooms (Armillaria mellea and Armillaria ostoyae), which may be found thriving on the corpse of their tree host. 
Most parasitic fungi, however, are microfungi, barely visible to the naked eye, but en masse they inflict cankers and lesions on the shoots and leaves of trees. Often their prominence in a middle-aged forest is symptomatic of other imbalances in the ecosystem, such as acid rain, groundwater pollution, and insect damage. After a tree dies, parasitic fungi may inhabit the tree, competing with saprophytes for dominance.
Since the hosts for some parasites can be short-lived, natural selection sometimes favors fast growers. Foresters have observed this with Phytophthora ramorum, the cause of sudden oak disease; this downy mildew pathogen can kill an ancient oak tree in days and an ancestral forest in a few weeks, and remain viable on the dead carcasses of its victims, allowing a new staging platform for infection further into the forest.

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