Emerald ash borer (Agrilus planipennis, EAB) has killed millions of ash (Fraxinus spp.) trees since its accidental importation from Asia. In Asia, EAB appears to act as a secondary pest, colonizing only stressed trees. However, EAB is killing healthy trees on high quality sites in North America, creating a wood-borer outbreak of unprecedented intensity. Since 2004, we have quantified effects of EAB on ash demography across 38 transects in forests of the Huron River watershed at the epicenter of the invasion in southeast Michigan. To evaluate biogeographical patterns of resistance to EAB, we established a common garden to compare resistance of Asian and North American ash species.
Ash mortality increased 30% per year, exceeding 99% by 2008, with the majority of the few survivors being saplings. Rate of black ash (F. nigra) decline and mortality was advanced about one year relative to that of white (F. americana) and green ash (F. pennsylvanica). There was no relationship between ash mortality and ash density, ash basal area, total stand density or basal area, or any measure of biodiversity. From 2004 to 2006, there was a highly significant negative relationship between percent ash mortality and distance from the putative epicenter of the infestation, with mortality decreasing 2% with each km away from the epicenter. However, this relationship ceased to exist 2007, as ash mortality approached 100% in all plots. Ash species are the most common species in the seedling layer, which could facilitate ash regeneration, or provide continued host material that prolongs the EAB outbreak. Four years of intensive sampling has revealed no ash seed bank, and there was less than one newly germinated seedling per hectare in 2008 and 2009. The ash population is currently dominated by an orphaned cohort of seedlings that established prior to 2008. The common garden experiment revealed that Manchurian ash (F. mandshurica), which shares an evolutionary history with EAB, is much more resistant than are North American species. This suggests that EAB populations in Asia are regulated from below by coevolved host defenses, and that unprecedented wood-borer outbreak occurring in North America is the result of EAB proliferation in defense free space. Comprehensive mortality and the cessation of regeneration in invaded forests suggest a precarious future for ash. As EAB continues to spread, it clearly has the potential to decimate ash throughout North America with pervasive direct and indirect ecological impacts.