Evaluating the population viability of green ash trees (Fraxinus pennsylvanica) before and after the emerald ash borer beetle (Agrilus planipennis) invasion

Abstract

The invasive emerald ash borer beetle (Agrilus planipennis, EAB) has caused significant ash tree (Fraxinus spp.) mortality and cascading forest changes in the United States. We quantitatively estimated the viability of a local green ash tree (F. pennsylvanica) population to evaluate the magnitude of change caused by EAB. We developed historic and worst case stochastic stage based population scenarios to model changes in viability for a natural stand of green ash trees. Historic parameters were based on the literature, and worst case parameters were based on field data and the literature during dates that reflect time periods before and after EAB impacted the focal population in Northwest Ohio. The worst case scenario assumed that parameters remained the same as the initial EAB attack, when many of the adult trees died after a few years. The ash annual population growth rates were estimated as 0.99 and0.76, respectively, in historic versus worst case scenarios. The historic scenario had a population trajectory that dipped slightly at first and then reduced slowly over time. This was not unexpected since we included stochasticity in plant survival and reproduction, but it showed that the model may not account for ecological variation that stabilizes ash tree populations in forests under favorable conditions. In the worst case scenario ash populations became locally extinct within 34 years, and changes in the population growth rate were more sensitive to changes in the survival of the < 1 cm diameter at breast height stage class. Caution is warranted given the high variability and stochasticity within the system, and the trajectory can change if post-EAB population dynamics differ from initial EAB invasion dynamics or if human intervention occurs. The use of PVA allows us to quickly identify factors that influence the population viability of species, allowing for the development of strategies that prevent further species endangerment.