A total of 1754 aspen trees were examined on 47 sample plots located in even-aged stands considered representative of the study area. Decay was found in the merchantable portions of the trunks of 1212 or 69 of these trees. Three types of heart rot were encountered, viz., a white spongy trunk or butt rot and a yellow stringy trunk rot associated respectively withFomes igniariusvar.populinus(Neu.) Campb., andRadulum casearium(Morgan) Lloyd; and stringy butt rots from which several fungi, notablyPholiota spectabilisFr. andArmillaria mellea(Vahl ex Fr.) Queacute;l., were isolated. Two heartwood stains, one brown and the other mottled red, were present in many of the aspen trees examined.Corticium polygoniumPers. and two members of the Fungi Imperfecti,Libertellasp. andPhialophora albavan Beyma, were consistently isolated from these stains and from incipient decays. Fructifications ofF.igniariuswere the most reliable external indication of heart rot in this species. A pronounced relationship existed between age and decay. Older stands as a rule contained higher proportions of decayed heartwood than young stands. A less pronounced relationship was found to exist between soil conditions and decay; trees occurring on drier sites were somewhat more decadent than those growing on moist sites. The possible reasons why the decay process in the heartwood of living aspen is one in which certain fungi, mainly Fungi Imperfecti, colonize and stain the wood prior to its invasion and destruction by the Basidiomycetes associated with advanced decay are discussed.
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