Emerald ash borer (EAB) (Agrilus planipennis), an invasive beetle, has killed more than 25 million ash (Fraxinus spp.) trees since its introduction to North America in the 1990s.  We recorded ash tree health and other characteristics in 22 EAB-infested forest stands in Michigan and Ohio in 2005 and 2007.  Ash condition was rated on a scale of 1 to 5, where 1 was a healthy tree, 5 was a dead tree, and 2-4 were stages of decline.  A general linear multivariate mixed model was used to test the effect of ash condition 2005 (ordinal), tree diameter, ash species, stand condition 2005 (average ash condition), habitat, ash density, stand average ash diameter, and ash composition on ash condition 2007 (ordinal) with individual ash trees as the unit of replication. 

Results/Conclusions

Ash condition 2005 was correlated with ash condition 2007, i.e., trees that were in poor condition in 2005 were likely to be in poor condition or dead in 2007.  Smaller diameter trees underwent more rapid changes in ash condition within the two-year period than did larger diameter trees.  Stand condition in 2005, the average condition for all ash trees in the stand, was a strong predictor of ash condition in 2007.  As the average condition of the stand declined, individual ash trees declined more rapidly.  Stands were separated into four groups based on stand condition 2005, and these data were used to create four transition matrix models of ash decline, which show the probability of a tree transitioning from each ash condition in 2005 to each ash condition in 2007.  In newly infested stands, most of the healthy trees remained healthy over the two-year period.  In slightly stressed stands and declining stands, mortality and decline increased, but some healthy trees remained healthy.  In the final stages of infestation, almost all trees died within the two-year period.  A test of the transition matrix model, using stands had not been used to create the model, showed that the model accurately predicts the future conditions of stands.  The model can be used to forecast the future conditions of forest stands in newly infested areas.  For a hypothetical stand that begins with mostly healthy trees in 2007, the model predicts a 5% increase in mortality by 2009, 50% mortality by 2011, and 98% mortality by 2013.  Therefore, a healthy stand of ash trees can be nearly completely killed within six years once infested by EAB.