PROBLEM ON THE Y-axis. Check the original PDF.
Jim Zwack, The Davey Institute
from -- 2015 CALCP - Planning your EAB mgmt options - EAB-WKSHP-Jan-2015-Zwack (21)
Even pre-EAB, canopy-thinning of ash trees would NORMALLY range from 0 - 30% due to environmental stressors.
VERY early in an EAB infestation, nearly all trees will be in the 0 - 30% canopy-thinning range, and MOST are not infested. Pest levels are so low that repeated years of attack are required to weaken, then overwhelm tree defenses. EAB is generally not detectable at this stage, and dendrochronology indicates it probably takes 5 - 7+ years for these early victims to die.
As you get out around year 6 on this graph, enough trees have died that someone has noticed and is looking hard for early symptoms. Though many trees have light levels of infestation, they are still healthy enough to manage the internal damage, so exhibit little in the way of drive-by cues. The beetle population is still low enough that it takes an individual tree 3 - 5 years to die from the time it is determined to be infested. If planning is complete, then most management tools will work as expected.
Later in the infestation, when most trees are in the higher ranges of thinning, the good-looking trees in the area are under mass attack from females looking for viable tissue upon which to lay eggs. The damage inflicted by the resulting larvae (vascular-etching grubs) means that trees with "great" canopy scores of 0 - 20% thinning will plummet to scores of 80 - 100% thinning (dead) in less that 2 years, often 1. Few management options are available at this point.
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