Changes in streamside riparian forest canopy and leaf litter nutrient flux to soils during an emerald ash borer infestation in an agricultural landscape

The invasive forest insect pest, emerald ash borer (EAB), is causing rapid, widespread ash tree mortality across eastern North America. Riparian (streamside) forests in agricultural landscapes often represent residual patches of high quality forest habitat that, owing to their strong land/water ecological linkages, can be critical for maintaining stream water quality and aquatic and terrestrial biodiversity. When riparian forests contain high numbers of ash trees, EAB can pose a risk of disrupting some of those land/water linkages thereby reducing riparian and aquatic habitat quality. We tracked ash tree mortality, streamside forest canopy cover, leaf litter deposition rates, and nutrient flux to soils in 18 riparian forest plots with an average of 24% ash by stem density over a 5 year period from early to late stages of an EAB invasion (2010–2015). Ash tree mortality increased from an average of 10–98% over 5 years in most plots, but when blue ash were dominant, final ash mortality was about 34%. Despite the nearly complete mortality of ash trees in almost all plots by 2015, the average canopy openness measured at 1.5 m above ground only increased to about 9%, which was significantly higher than the 2010 average of 6% open. Final canopy openness was significantly but weakly (r 2 = 0.24) associated with the proportion of ash in riparian plots, indicating highly patchy openness resulting from ash mortality. Ash leaf litter deposition to streamside traps declined by 84% and elm declined by 20%, whereas leaf deposition from the other four most common contributors to riparian leaf litter increased by up to 37% over the same period. The large reductions in ash litter deposition were sufficient to reduce macro-nutrient flux to the streamside forest floor and therefore likely to streams. Amounts of N, C, P, K, and Ca delivered to the streamside soils via leaf litter fall were all consistently, some of them significantly, lower by 2014 than in the baseline year of 2010. Although total leaf litter mass returned to near pre-infestation levels by 2014, the altered composition of leaf litter caused a persistent reduction in nutrient flux to riparian soils. We also measured stem density of regenerating ash saplings (< 5 cm diameter, > 0.5 m height) at the end of the study period and found them to be abundant with an average of 2084 stems/ha. Our results provide an empirical demonstration of EAB-induced disruptions to ecological linkages and processes in riparian forests, at least over the short term.