Release and suppression: forest layer responses to emerald ash borer (Agrilus planipennis)-caused ash death

Abstract

Emerald ash borer-caused canopy gaps alter resource availability which affects woody plant community dynamics in eastern US forests. Our findings indicate that both canopy trees and seedlings are impacted by the disturbance. Seedlings of sugar maple and introduced woody plants benefited most from ash death, perhaps because they are adapted to ephemeral resource fluctuations. Ash, Fraxinus spp., mortality caused by emerald ash borer (EAB), Agrilus planipennis, results in canopy gaps which could increase resource availability and profoundly affect eastern US forests. We tested two mechanisms: (1) EAB-caused ash decline releases growth of upper forest layers (non-ash canopy and subcanopy trees), constraining any release of lower forest strata; (2) EAB-caused ash decline increases canopy openness, releasing lower strata (shrub and seedling layer). Sites representing a gradient of ash mortality were sampled throughout western Ohio to investigate how forest strata relative growth rates (RGRs) relate to EAB-caused ash mortality. Models including individual and additive effects of ash mortality and upper strata effects on woody seedlings were also investigated. Greater RGR of non-ash canopy trees, particularly maple (Acer spp.), was found in sites with more poor condition ash. Abundance of introduced seedlings was correlated with greater ash mortality and shrub cover. Sugar maple seedling height growth improved with ash loss and more subcanopy basal area (BA). Sites with greater subcanopy BA had greater introduced seedling recruitment and native seedling survival. We found evidence to support both mechanisms. Our findings indicate species best adapted to ephemeral resource fluctuations, specifically sugar maple and introduced seedlings, benefited most from ash death.