Nutrient limitation of juvenile trees in a northern hardwood forest: Calcium and nitrate are preeminent

Nitrogen (N) has historically been considered the most important mineral nutrient affecting performance of saplings in transitional northern hardwood forest of eastern North America, but recent attention has focused on the role of exchangeable soil calcium. Relative limitation by these factors may be changing because of enrichment of soil N from atmospheric deposition, and concomitant depletion of soil calcium due to leaching by acidic precipitation. We conducted a fertilization experiment to determine the relative importance of calcium, other base cations, several forms of nitrogen, and aluminum toxicity for the growth of saplings in a forest in northeastern Connecticut, USA. Five broadleaved deciduous and one needle-leaved evergreen species were examined; two of the broadleaved species are most abundant on fertile soil patches, and the other four are most abundant on the more prevalent low-fertility soils. Fertilizer was applied over 3 years to naturally established plants growing across the fertility gradient, and likelihood methods were used to determine whether light-dependent growth rate was affected. Of the two species that occur most frequently on fertile soils, one species responded most strongly to calcium application, and the other to nitrate. Of the four species of poorer soils, one responded to both calcium and ammonium, and the other three showed little response to any soil factor. The results suggest that calcium and nitrogen are of equal importance in determining juvenile growth across the suite of canopy tree species in transitional northern hardwood forest. The strong response of species occurring most commonly on fertile soils is suggestive of fundamental niche differentiation mode of community organization. Ongoing processes of atmospheric nitrogen deposition and soil calcium depletion from acidic precipitation may change competitive relationships of the canopy tree community of northern hardwood forests.