Nitrogen uptake by four tree species of the Catskill Mountains, New York: Implications for forest N dynamics

Watersheds of the Catskill Mountains, New York have marked differences in nitrogen (N) dynamics among dominant tree species stands. Our objectives were to study how tree species vary in N uptake to better understand the basis for the observed variation in these forested watersheds. We conducted a (15)N tracer greenhouse study to determine NH(4)(+) and NO(3)(-) uptake of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga Canadensis L.), red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh.) seedlings. Seedlings and their native soil were collected in November 1997, over-wintered and allowed to break dormancy in spring 1998. Half of the seedlings of each tree species received (15)NH(4)-NO(3) to examine NH(4)(+) uptake and the other half received NH(4)-(15)NO(3) to examine NO(3)(-) uptake. Plants were harvested 4 days following (15)N addition. Tree species varied in their preference for NH(4)(+) and NO(3)(-). Sugar maple and eastern hemlock seedlings took up more NH(4)(+) than NO(3)(-) per unit plant biomass, while beech was the only species to take up more NO(3)(-) than NH(4)(+). Red oak took up more NH(4)(+) than NO(3)(-) into roots, stems and leaves, but the difference between the two forms of N was not statistically significant. These results demonstrate that tree species of the Catskill Mountains vary in their capacity to take up NH(4)(+) and NO(3)(-). Coupled with stand-level studies of N dynamics, this variation can help explain some of the patterns of forested watershed N retention and loss in the Catskill Mountains shown in our field investigations (Templer, 2001; Templer et al., in press Ecosystems).