THE high incidence of failure when late-successional conifer species are replanted on disturbed forest sites is a considerable problem(1-3). Here we advance a hypothesis that might explain many of these reforestation problems on a physiological basis, within the framework of forest succession. It is known that the chemical speciation of inorganic nitrogen in forest soils changes from predominantly ammonium (NH4+) in late-successional (mature forest) soils to mostly nitrate (NO3-) after disturbances such as clearcut harvesting(2-6). The capacity of plant roots to take up and use these two sources of nitrogen is therefore very important for species establishment on successionally different sites. We have used kinetic and compartmental-analysis techniques with the radiotracer N-13 to compare the efficiency of nitrogen acquisition from NH4+ and NO3- sources in seedlings of white spruce, an important late-successional conifer. We found that uptake of NH4+ was up to 20 times greater than that of NO3- from equimolar solution, cytoplasmic concentration of NH4+ was up to 10 times greater than that of NO3-, and physiological processing of NO3- was much less than that of NH4+. This reduced capacity to use NO3- is thought to present a critical impediment to seedling establishment on disturbed sites, where species better adapted to NO3- would have a significant competitive advantage.