In floodplains of the Tanana River in interior Alaska, when floods deposit sediment to totally cover the vegetation, succession begins with the invasion of Salix species, followed by Alnus, Populus, and Picea. The N availability of floodplain soil is very low in the earliest successional stage, but N accumulates rapidly during the establishment of N-fixing Alnus, which is the only stage with significant rates of nitrification. We investigated nitrate use of two dominant woody species from different successional stages: Salix interior from the earliest stage, and Populus balsamifera, the stage following Alnus. Potential and actual NRA (Nitrate Reductase Activities), NO₃^--N and N concentration were measured in leaves and fine roots of these species through the growing season.

Potential and actual NRA, and N concentration in P. balsamifera were higher than in S. interior both in leaves and roots, reflecting higher nitrate availability in these soils. Actual NRA increased through the growing season, while potential NRA showed no common trend in two species. Based on these results and soil N availability, seasonal changes in plant NO₃^--N use and its controlling factors will be discussed.