Comparative studies have shown that branch infections are almost twice as frequent in aspen (Populus tremuloidesMichx.) in mesic sites as in trees of similar ages in dry sites. This has been attributed to: (1) the proneness to infection of young dead branches, and (2) the greater mortality of young branches in mesic sites. No difference was found between the two sites with respect to the pattern of air temperature and relative humidity. The moisture content of dead branches was influenced to a minor extent by site, but only after considerable rain had fallen did branch moisture reach a level favorable for infection by fungi. Although several species of imperfect fungi and bacteria infected aspen branches from 4 years following mortality, wood-destroying fungi, namelyCorticium polygoniumPers. andPolyporus adustus(Willd.) Fr., did not appear until 7 and 8 years after the death of branches. After 19 years, decay ofFomes igniarius(L. ex Fr.) Kickx. occurred in branches only as lateral extensions of heartwood infection. The feasibility of artificial pruning to reduce the risk of heart rot infection in young aspen stands is discussed.
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