AVAILABILITY OF NITROGEN AND PHOSPHORUS IN THE FOREST FLOORS OF ROCKY-MOUNTAIN CONIFEROUS FORESTS

Nutrient supply rate and limitation were measured in forest floors of lodgepole pine, white spruce - lodgepole pine, and Engelmann spruce - subalpine fir (pine, spruce, and fir forests, respectively) forests in the Kananaskis Valley of southwestern Alberta. Earlier analyses of the nutrient content of foliage and litter indicated low N and P supply in the pine forest, high P supply in the spruce forest, and high N - low P supply in the fir forest. Measurements of nutrient supply (in situ rates of net mineralization, extractable P, and uptake of N and P from the forest floor in pot trials) confirmed the differences in N and P supply among the forests and indicated that nutrient concentrations in needle litter were useful as an index of nutrient supply rate. Subtractive tests were useful in identifying the most limiting nutrients in each forest: lodgepole pine seedlings grown in forest floor material from the pine and spruce stands responded with increased growth to the addition of N; those in fir forest floor material responded to P addition. Vector analysis of N and P concentrations and contents in needles from trees fertilized with ammonium phosphate sulphate showed responses to both N and P in the pine site, no response at the spruce site, and response to P at the fir site.