Climate stress as a precursor to forest decline: paper birch in northern Michigan, 1985–1990

Widespread paper birch (Betulapapyrifera Marsh.) mortality associated with the activity of the bronze birch borer (Agrilusanxius Gory) was observed across northern Michigan in 1991. This mortality occurred at two study sites on which paper birch growth has been intensively monitored since 1985. Recent warmer than normal growing seasons and lower than normal moisture availability are statistically associated with a reduction in annual diameter growth. On one study site 62% of the paper birch study trees were dead and 13% were visibly declining; on the other study site, although no trees were dead, 25% of the paper birch study trees were visibly declining. Growth reductions since 1985 suggest that the species was under climatic stress, making it more vulnerable to pest–pathogen activity. The evidence of the role of climatic conditions acting as a precursor to decline and mortality on these sites is of serious concern given recent projections of warmer temperatures and lower precipitation for this region by several global-climate models.