Chronic nitrogen deposition reduces the abundance of dominant forest understory and groundcover species

Humans have altered the global nitrogen (N) cycle, greatly increasing atmospheric nitrogen deposition in industrialized regions of the world. Groundcover plants can be sensitive indicators of nitrogen deposition impacts. Here, we report results from repeated measurements over a 7year period of groundcover (plants<1.4m tall) and understory (plants with a diameter<5cm at 1.4m in height) vegetation in four mature northern hardwood forests in the north-central United States receiving experimental additions of N (3gm−2year−1 as NaNO3 for 18years). Experimental N deposition reduced the average abundance of sugar maple (Acer saccharum Marsh.) seedlings in the groundcover by>50% (P<0.001). This reduction occurred at all four sites, but was only statistically significant at the two sites where these seedlings were most abundant (site×nitrogen: P<0.001). Our observations of mortality within a large cohort of sugar maple seedlings across three sites provide further evidence of this effect. For these seedlings, experimental N deposition significantly (P<0.05) increased mortality in the weeks following germination, as well as over the longer term, reducing overall survival after 5years by almost 90%. Although groundcover plants accounted for<0.5% of aboveground plant biomass, they contributed up to 10% of ecosystem leaf area and 5% of aboveground litter. At the two sites where sugar maple seedlings were infrequent, understory vegetation was more abundant and dominated by hop-hornbeam (Ostrya virginiana (Miller) K. Koch; 42% of all stems). At these two sites, experimental N deposition significantly reduced the abundance of understory hop-hornbeam by more than 75% (site×nitrogen: P=0.008). The effects of experimental N deposition on the understory and groundcover vegetation occurred without significant decreases in reproductive litter or increases in canopy leaf area. Instead, the negative effects are more likely a by-product of other documented changes caused by the experimental N deposition: increased forest floor mass, decreased mycorrhizal abundance, and increased production of potentially alleopathic phenolic compounds. Because the late-successional species in these forests rely upon groundcover and understory plants for regeneration, the effects of added N on this vegetation have potential implications for future forest composition, particularly given the likely loss of some species in these forests due to exotic pests and pathogens.