Dead Trees and the Great Curve

Dead ash trees
Dead white ash in a suburban neighborhood. These trees were alive in 2014.

This is the year of death in central Kentucky.  The emerald ash borer has been slowly expanding its population, and has now reached the point of mass destruction. You can see dead and dying trees all over town, most of which have died this spring.  From here on, barring some miraculous change in ash borer populations, the fate of the remaining ash trees is sealed.

Ash tree mortality
Simulation of the death of ash trees in Lexington, KY. First mortality was seen in 2009. Assuming the beetles were already present for 10 years, the curve shows that in 2015 (red arrow), we have entered the grand phase – the time of steep increase in tree death. At this rate, we would expect most untreated trees to be dead by 2025.

This is a predictable outcome of a biological phenomenon called the Great Curve of Growth. Mathematically, the Great Curve follows a logistic equation.  It works like this:

  1. At first, ash borer numbers are very small and few trees die even as the borer population increases.  This is called the lag period. Typically, the presence of the beetle is not observed for about 10 years after they arrive in an area.
  2. As beetle numbers increase, an explosion of death takes place, increasing steeply. This is the logarithmic phase. We are early in the logarithmic phase, and each subsequent year will bring approximately a doubling of the number of dead trees.
  3. Eventually, the beetles have killed most of the trees, and only a few live trees remain to be attacked. This is the stationary phase and ends with the elimination of white and green ash.
Dead white ash
A white ash that died in the Spring of 2015 from emerald ash borer

We can conclude from this analysis that the next few years will see an explosion of dead trees. Is there anything that can stop this explosion of tree death.  Yes.

  1. Weather changes, especially extremely cold winters, could slow the beetle. However this beetle has thrived in the cold of Michigan, so this is unlikely to happen;
  2. Predators and pests of emerald ash borer could increase in populations and lower the population of beetles. This would slow the rate of death of trees.  There are programs to releasepredatory wasps, but they are probably too little, too late.
  3. Treatment of trees is a known method to preserve important trees.  The cost of treatment has gone down and treatment is affordable for more landowners. Contact a certified arborist for more information.  In spite of the success of treatment, it is not possible to treat the majority of trees even in urban areas.  Treatment should be focused on large or important trees.
  4. Blue ash trees appear to be resistant. Our large population of blue ash trees are probably, but not certainly, safe.
Dead white ash trees
A dead white ash stand in suburban Lexington. These trees were alive in 2014.