Human activities have dramatically increased nitrogen (N) and sulfur (S) deposition, altering forest ecosystem functioning and structure. Anticipating how changes in deposition and climate may impact forests in the future can inform decisions regarding these environmental stressors. Here, we used the ForSafe-Veg model to simulate the potential response of soil chemistry and understory forest composition to deposition and climate change in the Northeastern U.S. Specifically, we modeled eight deposition and climate change scenarios out to the year 2100 and their interactive effects on soil chemical properties (percent base saturation, acid neutralizing capacity (ANC) and nitrate leaching) and understory composition. We conducted the modeling for 24 Sugar Maple-Beech-Yellow Birch hardwood stands, located from Pennsylvania to Maine. The deposition scenarios consisted of pre-industrial deposition (1850 to 1852) held constant (scenario D1), or historic deposition (1850 to 2012) followed by either: a return to pre-industrial deposition (D2); anticipated deposition reductions under current policies implemented by 2030 and held constant thereafter (D3); or maintaining the average deposition of recent years (2007-2009) out to 2100 (D4). The four climate change scenarios ranged from holding recent climate constant to ensemble medians of representative concentration pathways 2.6, 6.0, and 8.5 (C1, C2, C3, and C4, respectively).
Overall, deposition affected soil chemical properties, whereas climate generally did not. Anticipated deposition reductions (D3) decreased impacts on percent base saturation, ANC, and nitrate leaching; however, they did not lead to recovery of these properties to estimated historical levels. Deposition reductions, for instance, restored percent base saturation to approximately 65%, almost doubling levels currently observed, yet still this was much less than the approximately 90% estimated in 1900. In general, soils in these forests were only able to recover to historical conditions when future deposition was returned to pre-industrial levels (D2). In contrast to soil chemical properties, understory vegetation was comparably impacted by both deposition and climate. Vegetation shifted further away from the estimated, historic and current assemblages with increasing deposition and climate change. Even if deposition was returned to pre-industrial levels (D2) and current climate was held constant (C1), the understory assemblages were not able to fully return to historical conditions. Disclaimer: Authors’ views expressed here do not necessarily reflect views or policies of the U.S. EPA.