Simulated Nitrogen Deposition Reduces the Abundance of Dominant Forest Understory and Groundcover Plants

Growth in global industrialization is expected to increase the amount of atmospheric N deposition added to terrestrial ecosystems during the next century (Dentener et al. 2006). In North America, northern temperate forests in the eastern half of the U.S. and Canada have received large amounts of N deposition for several decades and there is evidence that these consistent inputs of atmospheric N have increased the availability of N in these forests (Talhelm et al. 2012). Groundcover plants (seedlings, shrubs, and herbaceous plants) are considered to be more sensitive indicators of N deposition impacts than overstory trees (Pardo et al. 2011). Further, these plants are both crucial to forest regeneration following disturbance and, relative to their biomass, disproportionately important within forest ecosystems in terms of species diversity, net primary productivity, nutrient cycling, and litter production (Gillam 2007). In order to understand the effects of chronic N deposition on forests in the north-central United States, we have experimentally added 3 g N m^-2 yr^--1 in the form of NaNO₃ pellets to plots at four northern hardwood forest sites spread across 500 km in Michigan since 1994. From 2005 to 2012, we have made repeated measurements of the abundance of both groundcover plants (< 1.4 m in height) and understory plants (>1.4 m in height, less than 5 cm diameter at 1.4 m). At these sites, sugar maple (Acer saccharum) ) seedlings are highly abundant (up to 200 plants m^-2) and the dominant groundcover plant (79% of all woody stems). Hop-hornbeam (Ostrya virginiana) is the dominant understory plant (42% of all stems). Experiment N additions strongly and consistently reduced the abundance of these two plants in these forest strata at the sites where these plants were most abundant. For sugar maple seedlings, there were significant effects at sites A (-49%) and C (-65%, Site × N: P < 0.001), and for understory hop-hornbeam, there were significant reductions at sites B (-75%) and D (-92%, Site × N: P = 0.008). In 2007, we took advantage of an abundant sugar maple seed crop in the previous year to initiate a survivorship study at all four sites. The N additions did not affect sugar maple seedling abundance upon initial germination (P = 0.296), but did reduce the number of seedlings that survived the first growing season (P = 0.001) and over the next five years (P < 0.001). Although other species were not consistently affected by the N additions, the N additions did alter community composition in both the groundcover (P = 0.001) and understory (P = 0.083). Chronic N additions have not significantly affected canopy leaf area. Instead, we believe that the accumulation of organic matter on the soil surface caused by the N additions is at least partially responsible for these changes (Patterson et al. 2011).