Changes in nitrogen cycling resulting from simulated emerald ash borer mortality in black ash wetlands of northern Michigan
Emerald ash borer (Agrilus planipennis Fairmaire (EAB)) is an invasive insect which effectively girdles native North American ash trees (genus: Fraxinus) greater than 2.5 cm in diameter, typically resulting in near-complete stand mortality within 3-4 years. Black ash wetlands comprise approximately 270,000 ha in Michigan, within which black ash (F. nigra Marshall) occupies 40 to 90% of the basal area. The loss of a dominant overstory species could result in dramatic changes in above- and belowground nitrogen (N) cycling within these ecosystems. Here we report the results of a three-year study on the impact of a simulated EAB infestation on live foliage and litter N content, litter inputs, and soil nutrient availability within 9 uninfested black ash wetlands located in the Upper Peninsula of Michigan. Within the 9 stands, 3 stands were left untreated (‘Control’), 3 stands had all black ash trees manually girdled (‘Girdled’) and 3 had all black ash trees felled by chainsaw (‘Clearcut’).
Preliminary results indicate higher total inorganic-N and ammonium-N availability within the ‘Girdled’ and ‘Clearcut’ soils, while nitrate-N was higher in the ‘Girdled,’ but lower in the ‘Clearcut,’ compared to the control. Mass-based N content (Nmass) of live foliage and litter of the most common co-dominant species, yellow birch (Betula alleghaniensis Britt.) and red maple (Acer rubra L.), decreased in the treated sites over the same period (foliar Nmass decreased 12% and 15% respectively, litter Nmass decreased 22% and 9%). Leaf mass per area (LMA) of both species was higher in treated sites. However, total N flux to the forest floor via litter from yellow birch and red maple increased by 3% and 75% respectively following treatment. Conversely, black ash crown foliage Nmass increased by 33% following the girdling treatment, while Nmass of foliage sampled from epicormic branches was further elevated (38% greater than crown foliage). LMA of epicormic branches was lower in ‘Girdled’ sites compared to control. Black ash litter Nmass for both treatment types increased 51% following treatment, though total N flux in ash litter decreased. These changes in litter N flux led to an average decrease of 63%, or 23kg ha-1, in N inputs to the forest floor from these dominant species. While leaf-level attributes, such as Nmass and LMA, may respond primarily to changes in the light environment, whole tree nutrient use following disturbance likely depends on other changes in growing space, including soil N availability.