The global nitrogen cycle has been fundamentally altered by human activities, including large increases in atmospheric nitrogen deposition downwind of urban and agricultural centers. Because of their high turnover rates and diverse composition, groundcover plants can be sensitive indicators of nitrogen deposition impact on forests. Further, although these groundcover plants make up a relatively small proportion of forest plant biomass, they make disproportionately large contributions to ecosystem processes such as litter production and nutrient cycling. Here, we report results from repeated surveys of groundcover (woody plants <1.4 m tall) and understory (woody plants with a diameter <5 cm at 1.4 m in height) vegetation conducted over a 12-year period in four mature northern hardwood forests spread over 500 km in the north-central United States that have received experimental additions of nitrogen (3 g m−2 year−1 as NaNO3) for 23 years.
Sugar maple (Acer saccharum Marsh.) seedlings were by far the largest component of the groundcover vegetation (77% of all stems). Experimental nitrogen deposition decreased overall sugar maple abundance in the groundcover layer by 38% (P < 0.001). However, this effect varied both spatially and temporally. This decrease was only statistically significant at the two sites where sugar maple seedlings were most abundant (site נnitrogen: P < 0.001). Across all four sites, the negative effect of experimental nitrogen deposition on sugar maple disappeared when large cohorts of seedlings were established in 2007 and 2014 following mast events, but subsequently re-emerged due to higher rates of seedling mortality under experimental nitrogen deposition. At the two sites where sugar maple seedling density was low, there was more abundant understory vegetation and this vegetation was dominated by hop-hornbeam (Ostrya virginiana (Miller) K. Koch; 43% of all stems). Nitrogen additions decreased understory hop-hornbeam abundance by 86% at these two sites (site נnitrogen: P < 0.01), but did not have consistent effects at the other two sites. We have not observed significant changes in reproductive litter, initial seedling establishment, or canopy leaf area, and have no evidence of soil acidification. The negative effects of added nitrogen on plant populations may instead be caused by documented increases in forest floor mass or decreases in mycorrhizal abundance.
The views expressed in this work are those of the authors and do not necessarily reflect the views or policies of the US Environmental Protection Agency.