maybe not the most authoritative reference, but per wikipedia -
Sherry aging produces fungus on the walls"Angels' share" is a term for the portion (share) of a wine or distilled spirit's volume that is lost to evaporation during aging in oak barrels. The barrels are typically French or American oak. In low humidity conditions, the loss to evaporation may be primarily water. However, in higher humidities, more alcohol than water will evaporate, therefore reducing the alcoholic strength of the product. In humid climates, this loss of ethanol is associated with the growth of a darkly colored fungus, Baudoinia compniacensis, on the exterior surfaces of buildings, trees and other vegetation, and anything else that happens to be nearby."
I've read in other places, but can't recall where, that this loss of alcohol is typical with single malt in Scotland (manyof the distilleries have the black fungus around their stillage warehouses) but the reverse is the case with bourbon storage.