Emerald ash borer (EAB), Agrilus planipennis Fairmaire, has killed millions of ash trees (Fraxinus spp.) since its introduction, propelling this invader to the forefront of natural resources concerns. Despite the prominence and deleterious impacts from this invasive species, information about the impact of widespread ash mortality on terrestrial-aquatic linkages is scarce. Mortality of ash trees along stream corridors can result in canopy light gaps, which potentially alters riparian plant community structure and subsequent organic matter subsidies into streams. We postulated that the composition of leaf packs and wood debris in-stream would shift in response to the changing riparian forest. Specifically, we anticipated an increase of ash woody debris at EAB-related canopy gaps and an increase in leaf litter taxon richness both at and downstream of these gaps. We hypothesized the microbial communities colonizing leaf litter would also be influenced by the altered allochthonous riparian subsidies. In 2016, we identified canopy gaps in six headwater streams of Michigan associated with EAB-related dead ash trees and characterized the riparian plant communities. Leaf litter (terrestrial and aquatic) and their associated microbial communities were sampled upstream, downstream, and at the center of one light gap per stream before, during, and after autumnal leaf senescence.
Results/Conclusions
Overall, although we found significant changes in the riparian zone after EAB invasion, the only significant aquatic response occurred within submerged leaf litter packs. EAB-related ash mortality facilitates shifts in plant communities towards increased taxa richness and woody debris biomass in riparian forests (paired t-test, p<0.01). Despite these riparian community shifts, the in-stream woody debris (ash and total) was not significantly altered compared to upstream and downstream sites (ANOVA, p>0.05). Aquatic leaf litter taxon richness significantly increased at and downstream of the gap (ANOVA, p<0.05), and gap colonizing species, such as American Elm Ulmus americana, were the additional leaf taxa found in litter to increase this richness. We further observed microbial community compositional changes towards increased Cyanobacteria abundance in the litter located at and downstream of canopy gaps. Future work will investigate how these organic matter alterations influence aquatic consumers. These results documented EAB impacts on riparian and stream habitats, and provide an important case study on how riparian invaders influence terrestrial-aquatic linkages. The response of ecological communities to disturbance, such as EAB invasion, serves as an important area of research, because extreme disturbance events have become increasingly common and may be mediated by ecosystem resilience within the residing communities.