Conifer root discrimination against soil nitrate and the ecology of forest succession

THE high incidence of failure when late-successional conifer species are replanted on disturbed forest sites is a considerable problem. 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. 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 ¹³N 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.