Exploring the Population Viability of Green Ash (Fraxinus pennsylvanica) with a Stage-Based Model
The invasive emerald ash borer beetle (Agrilus planipennis, EAB) has caused significant ash tree (Fraxinus spp.) declines and forest changes, which include loss of canopy cover and increased numbers of invasive plant species (Hausman et al. 2010). My focus was to assess ash population dynamics and environmental factors that could play a role in ash survival. My research used a population viability analysis (PVA) approach that combined literature review, targeted field studies, and greenhouse experiments to examine green ash trees (F. pennsylvanica) in the post-EAB peak infestation (aftermath) forest. Aftermath forests dynamics between ash and EAB are likely different from the initial infestation. I developed historic and worst case stochastic stage based ash population models as part of a PVA; these scenarios reflect time periods before and after EAB invaded Northwest Ohio. The ash population growth rates were estimated as 0.76 and 1.03, respectively, in worst case versus historic scenarios. Results indicated that population changes were more sensitive to survival and growth of the smallest stage class in the worst case scenario, where ash populations became locally extinct within 41 years. I examined ash parameters with little known information, such as germination, seedling survival and their environmental conditions, and mature ash tree neighbors. My germination experiments from the lab and the field resulted in a very low germination rate from the local population. Seedling survival was high and they were affected by leaf litter, bare ground and dead coarse woody debris. The number of ash neighbors within 6 m was significantly lower for the healthiest ash canopy class compared to declining health classes (p = 0.02). I modeled changes that could occur to the green ash aftermath forest (2010-2017) by introducing EAB periodically as a catastrophe that lowered ash survival and simulated a slow ash survival recovery. Management scenarios included, 1) reduced EAB catastrophes, 2) increase ash survival and growth, 3) individual ash size classes survival increased, and 4) planted EAB-resistant ash trees. Reduced EAB catastrophes, protecting the largest and smallest ash size class, and increase survival and growth provided an improvement for the population. Planting EAB-resistant trees allowed for a partial recovery, where saplings performed better than seedlings. While conservative, these green ash models indicated that increased tree survival improved population recovery, and EAB population fluxes have a large influence on ash population persistence. PVA, as this research demonstrates, allows us to quickly identify factors that influence the population viability of threatened species, allowing for the development of strategies that promote recovery.