CONIFER ROOT DISCRIMINATION AGAINST SOIL NITRATE AND THE ECOLOGY OF FOREST SUCCESSION

THE high incidence of failure when late-successional conifer species a re replanted on disturbed forest sites is a considerable problem(1-3). Here we advance a hypothesis that might explain many of these refores tation problems on a physiological basis, within the framework of fore st succession. It is known that the chemical speciation of inorganic n itrogen 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 ve ry important for species establishment on successionally different sit es. We have used kinetic and compartmental-analysis techniques with th e radiotracer N-13 to compare the efficiency of nitrogen acquisition f rom NH4+ and NO3- sources in seedlings of white spruce, an important l ate-successional conifer. We found that uptake of NH4+ was up to 20 ti mes greater than that of NO3- from equimolar solution, cytoplasmic con centration of NH4+ was up to 10 times greater than that of NO3-, and p hysiological processing of NO3- was much less than that of NH4+. This reduced capacity to use NO3- is thought to present a critical impedime nt to seedling establishment on disturbed sites, where species better adapted to NO3- would have a significant competitive advantage.