Simulated N Deposition Negatively Affects Sugar Maple (Acer saccharum Marsh.) Regeneration in a Lake States Northern Hardwood Ecosystem

During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially leading to a decline in biodiversity of plant assemblages and community structure. We quantified sugar (Acer saccharum Marsh.) maple seedling abundance in replicate northern hardwood forest stands (n = 4) receiving ambient atmospheric N (0.7 – 1.18 g N⋅m-2⋅yr -1) and experimental atmospheric N deposition simulating future amounts in eastern North America (ambient plus 3 g NO3 --N⋅m-2⋅yr -1). First-year seedling abundance did not differ under ambient and simulated N deposition. (P = 0.961); however, there were greater abundances of secondand third-fifth-year seedlings under ambient N deposition ( P < 0.001). In this experiment, simulated atmospheric N deposition has slowed litter decay, resulting in an accumulation of forest floor. We reasoned that a greater forest floor mass would impose a physical barrier to sugar maple seedling establishment, thereby reducing populations of seedlings. To test this idea, we experimentally manipulated forest floor mass over sugar maple seeds under ambient and simulated N deposition. In all cases, a greater forest floor, equivalent to that under simulated N deposition, resulted in significantly (P = 0.001) fewer established individuals, regardless of whether the greater forest floor mass occurred under ambient or simulated N deposition. Finally, to assess the effect of simulated N deposition on established seedlings, we transplanted first-year established seedlings into areas receiving ambient and simulated N deposition and quantified their mortality after one year. Fewer seedlings survived when grown under simulated N, albeit that result was not significant (P = 0.059). Our results indicate that levels of atmospheric N deposition similar to levels found in many terrestrial ecosystems around the Earth, have the potential to negatively affect stand dynamics in sugar maple-dominated forests, which further has the potential to elicit ecosystem change in regards to overstory carbon storage.