The invasive emerald ash borer (Agrilus planipennis
Fairmaire, EAB) has destroyed ash tree (Fraxinus
spp.) populations across the US, but remnant populations including many small trees and, more rarely, larger trees remain after EAB had its first major impact across the landscape. The future survival of these remnant trees is critical to the viability of the ash species. Several different management options, including biocontrol, EAB-tolerant or resistant ash trees, and silvicultural strategies exist or are under development. Here we model changes that could occur in a preserve’s remnant population of green ash (Fraxinus pennsylvanica
Marsh.) after EAB peak mortality to assess management strategies. We used population viability analysis (PVA) and created a stage based model with baseline conditions and a model with recurrent catastrophes, where a catastrophe was defined as a year with reduced ash survival from an EAB outbreak. The catastrophes model had an increasing probability of a catastrophe occurring over ten years and included a gradual increase in ash survival (decline in EAB impacts) over the 9 years following a catastrophic event. We explored management scenarios for the catastrophes model including, 1) the reduction of future EAB induced mortality events to mimic possible effects of biological control or other environmental constraints; 2) addition of trees with increased survival to mimic restoration by planting ash trees with resistance to EAB, which are currently under development, or mimic clusters of trees with natural resistance to EAB created by natural selection. We also performed a sensitivity analysis to assess which size class impacted the population persistence the most over time and found that new seedlings were the most influential. There was no risk of extinction under the baseline model. The reduced catastrophe scenario was an improvement from the catastrophes model, reducing probability of extinction by 33%. Adding healthy ash improved population abundances over time and reduced the probability of extinction when there were repeated plantings. These scenarios relied on assumptions about how the population would react to management treatments, based on the scientific literature and cautious estimates. This approach provides a starting point for experiments testing individual management treatments to generate testable hypotheses, and the model may be readily updated with new data as it becomes available. Our PVA has revealed potential outcomes of alternative management practices and can increase our understanding of natural ash populations remaining after EAB introduction.
Kappler, R.H.; Knight, K.S.; Bienemann, R.; Root, K.V. 2020. Recovery potential for a green ash floodplain forest using various emerald ash borer management strategies in a population viability analysis. Forest Ecology and Management. 461(4): 117925. 11 p. https://doi.org/10.1016/j.foreco.2020.117925.