Atmospheric deposition may affect northern hardwood forest composition by altering soil nutrient supply

In the northeastern United States, the input of reactive nitrogen (N) via atmospheric deposition has increased rapidly since the onset of the industrial revolution. During the same period of time, acid precipitation and forest harvest have removed substantial quantities of base cations from soil. Because of the dominance of base-poor soils and the low rates of atmospheric base cation deposition, soils throughout the northeastern United States may be increasingly rich in N but poor in calcium (Ca). We studied the consequences of a change in soil N and Ca availability on forest composition by transplanting seedlings of four tree species into replicate plots in the understory and in canopy gaps amended with N and Ca in factorial combination. In this paper, we report on the growth and survivorship of seedlings over a four-year period. Relative to control plots, fertilization with N increased red maple growth by an average of 39% whereas fertilization with Ca decreased survivorship in the understory by 41%. In sugar maple, fertilization with Ca increased growth by 232% and 46% in the forest understory and in canopy gaps, respectively, and significantly increased high light survivorship. Fertilization with N decreased white pine survivorship by 69% in the understory whereas high Ca availability significantly increased survivorship. Fertilization with N or Ca alone reduced red oak growth but had no effect on survivorship. The results of this study suggest that historical losses of soil Ca and the continuing effects of atmospheric-N deposition on N availability are likely to alter the composition of northeastern North American forests because of the positive effects of N enrichment on the growth of red maple and the negative effects of Ca loss on the growth and survivorship of sugar maple and white pine.