Impacts of emerald ash borer infestation on forest understory dynamics

Background/Question/Methods

Since its detection in 2002, the exotic emerald ash borer (EAB, Agrilus planipennis) has killed millions of ash trees (Fraxinus spp.) in the North American temperate forest. Ash trees are considered foundation species in many habitats across their range such as riparian zones. Their loss may result in profound community changes across ash species’ extensive range, particularly in systems under high anthropogenic disturbance and exotic propagule pressure. We established a long-term monitoring program in southern Ontario, Canada to (1) investigate the capacity of ash to regenerate and retain its ecological role following initial infestation, and (2) study the role of the surrounding matrix (forested, agricultural, urban) and patch configuration (woodlot, riparian) in post-outbreak understory community assembly dynamics, such as the invasion potential of exotic plant species. The use of a functional approach complementary to taxonomic metrics allows us to delineate the functional syndromes underlying community processes involved in understory response to the opening of the canopy following ash mortality. This method also allows us to outline commonalities among study regions which is particularly relevant in apprehending potential impact of ash mortality throughout its range.

Results/Conclusions

A survey of ash regeneration and survival revealed strong initial ash regeneration after the outbreak. However, all surveyed sites showed signs of active EAB population, with the proportion of recently infested sapling ranging from 7% to 43%, suggesting a low likelihood for successful long-term ash regeneration. Pre-outbreak exotic invasion potential and post-outbreak taxonomic and functional community assembly diverged according to anthropogenic pressure both among matrix types and forest patch configurations. Urban understory communities before outbreak were rich in exotic species exhibiting a set of traits associated with higher competitiveness, driving understory dynamics after the outbreak. While sensitive forest herbs such as spring ephemerals increased in agricultural woodlots after the outbreak, they declined in systems which were more disturbed and more susceptible to exotic propagule pressure such as urban matrix and riparian habitats, where exotic shrubs and grasses were favored, respectively. Understanding facilitative interactions between EAB outbreak and exotic plant responses in connection with understory community assembly is crucial in understanding ecosystem-level effects of the disappearance of a foundation species. In particular, gauging the importance of exotic species according to context and configuration can help inform mitigation strategies in the most sensitive zones and maintain ecosystem function.