Abstract
Emerald ash borer (EAB, Agrilus planipennis Fairemaire) is a phloem-feeding beetle that was introduced into North America in the late 20th century and is causing widespread mortality of native ash (Fraxinus) species. The loss of an entire genus from the forest flora is a substantial disturbance, but effects vary because of differences in Fraxinus dominance and remaining vegetation. At three sites near the center of the North American EAB range, we investigated the impacts of Fraxinus mortality on recruitment of woody and non-native vegetation in 14 permanent plots from 2012 to 2017. We used the change in relative Fraxinus basal area to determine the impact of EAB on density of woody species and non-native vegetation less than 2.5 cm diameter at breast height (dbh). Changes in canopy cover were not correlated with loss of Fraxinus from the overstory, and only the density of shade-tolerant shrubs and saplings increased with Fraxinus mortality. Both native and non-native shrub species increased in density at sites where they were present before EAB, but no new invasions were detected following Fraxinus mortality. These shifts in understory vegetation indicate that Fraxinus mortality enhances the rate of succession to shade-tolerant species.
Highlights
In the nearly two decades since emerald ash borer (EAB, Agrilus planipennis Fairemaire) was first detected in North America [1], the response has evolved from preventing or mitigating damage to Fraxinus [2,3,4] to understanding how the introduced insect might impact other species and ecosystem function in forest communities [5,6,7]
Our evidence shows that the resources released through Fraxinus mortality are insufficient to modify the process of mesophication in mesophytic forests, but instead generate changes that resemble those observed with small gaps
The primary beneficiaries of released resources include individuals already existing in the understory canopy, among them small shade-tolerant trees, saplings, and shrubs typical of mature mesophytic forests, conforming to what is observed in forest gap dynamics [8,49,50,51,52]
Summary
In the nearly two decades since emerald ash borer (EAB, Agrilus planipennis Fairemaire) was first detected in North America [1], the response has evolved from preventing or mitigating damage to Fraxinus [2,3,4] to understanding how the introduced insect might impact other species and ecosystem function in forest communities [5,6,7]. Forest plant communities of eastern North America, like any system, are dynamic and subject to change, and EAB’s impact on Fraxinus is not exceptional. These species have a wide distribution, and despite their relatively low abundance in this forest type [12], the loss of Fraxinus by EAB has the potential to drive changes in plant communities throughout the region, similar in respect to previous introductions that changed forest composition on a broad scale. Chestnut blight (Cryphonectria parasitica (Murrill) Barr) led to the widespread loss of Castanea dentata (Marshall) Borkh. throughout its native North American range, in the Appalachian region [13], and Dutch elm disease
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