Response of understory vegetation of Adirondack forests to nitrogen additions

The influence of N pulses in the form of experimental additions of HNO3 at two times ambient and (NH4)2SO4 at two and four times ambient on the herbaceous and woody understory plants in three Adirondack Mountain hardwood forests was evaluated. Addition of (NH4)2SO4 decreased cover of dominant herbs and woody seedlings at Woods Lake, a site in the western Adirondacks. Tissue N concentrations of combined herbs and ferns at Woods Lake increased with addition of NH4+ at both levels (9% at two times ambient; 13% at four times ambient) and increased with all three N treatments at Huntington Forest, a central Adirondack site (14% with NO3- and NH4+ at two times ambient; 22% with NH4+ at four times ambient). Seedlings of American beech (Fagus grandifolia Ehrh.) increased foliar N concentration 7-8% with addition of NH4+ treatments at Huntington Forest, but did not respond to N addition at Woods Lake and Pancake Hall Creek, a western Adirondack site. In general, greatest plant nutrient response to N addition occurred at Huntington Forest, where atmospheric inputs of N are lower than at Woods Lake and Pancake Hall Creek.