Emerald ash borer invasion of riparian forests alters organic matter and bacterial subsidies to south Michigan headwater streams

Abstract

Emerald ash borer (EAB) has killed millions of ash trees in the United States and Canada, yet impacts on terrestrial-aquatic linkages are largely unknown. Ash tree death along streams creates canopy gaps, increasing light to riparian plants and potentially affecting organic matter subsidies. Six EAB-related canopy gaps along streams across a gradient of timing of EAB invasion in Michigan were characterized for coarse woody material (CWM), terrestrial and aquatic leaf litter and their associated bacterial communities, and macroinvertebrates upstream, downstream, and at the center of the gap. Stream sites downstream of EAB-related canopy gaps had significantly lower dissolved oxygen and macroinvertebrate diversity than sites upstream and at the gaps. Yet there was no difference in CWM or aquatic leaf litter, likely due to downstream movement of organic matter from upstream riparian sources. Low abundance bacterial amplicon sequence variants unique to gap or forest were detected in leaves and leaf litter, suggesting that EAB-related canopy gaps altered leaf-associated bacterial communities. Overall, EAB invasion indirectly impacted some variables, while organic matter dynamics were resistant to change.

Fig. 1.

Map of the six field site locations in three watersheds, across a chronosequence of EAB impacts. EAB-related canopy gaps were apparent in Clinton watershed in 2006 and 2008, Grand River watershed in 2011, and Kalamazoo watershed in 2014. Map created using the Esri light gray canvas basemap (Esri 2011) with ArcGIS desktop (Esri 2020). The coordinate system is WGS84 and the projection is Mercator Auxiliary Sphere.

Fig. 2.

Percent dissolved oxygen (A.), aquatic leaf litter genera richness (B.), macroinvertebrate family richness (C.), and log10 transformed Chironomidae abundance + 1 (D.) over estimated year of gap formation (2006–2014). Dissolved oxygen was significantly lower at sites downstream of gaps compared to upstream of gaps, but not affected by time since gap formation. Aquatic leaf litter richness was not significantly affected by gap location or time since gap formation. Macroinvertebrate family richness was significantly lowered at sites downstream of gaps compared to upstream and in streams with earlier gap formation. Chironomidae abundance was lowered at streams with earlier gap formation.

Fig. 3.

Venn diagram of shared ASVs between the following environments: aquatic leaf litter in Kalamazoo watershed, terrestrial live leaves in Kalamazoo watershed, terrestrial leaf litter in Kalamazoo watershed, and all samples collected in Clinton and Grand watersheds. One ASV, belonging to the family Beijerinckiaceae (Alphaproteobacteria), was in all sample types in Kalamazoo watershed, but not in any samples from the other watersheds.

Fig. 4.

Nonmetric multidimensional scaling based on the zero-adjusted Bray–Curtis matrix for β-diversity of macroinvertebrate community (a) family-level taxonomy (stress = 0.15) and (b) functional feeding groups (stress = 0.09) across the chronosequence of EAB invasion (year of gap formation ranging from 2006 to 2014). Ellipses represent 95% confidence intervals. Points in greater proximity correspond to similar community composition.