Below- and Aboveground N and P Use by Abies amabilis Stands

Annual N and P requirements and internal redistributions were determined for above— and belowground vegetation component of young (23—yr—old) and mature (180—yr—old) subalpine Abies amabilis stands in the Washington Cascades Mountains. The primary objective was to evaluate the importance of fine—root production in determining total vegetation N and P uptakes from soil nutrient pools. In the young stand, N and P requirements for aboveground plant components were, respectively, 42.7 and 5.54 kg°ha—1yr—1, while for belowground plant components, requirements were 75.5 and 10.94 kg°ha—1°yr—1. Aboveground N and P requirements in the mature stand totalled 27.7 and 4.11 kg°ha—1°yr—1, respectively, with belowground components totalling 74.5 and 14.61 kg°ha—1.yr—1. Requirements were met by both nutrient uptake from the soil and internal nutrient redistribution. Total uptakes of N and P for aboveground components were 26.9 and 2.86 kg°ha—1.yr—1 in the young stand; uptake for belowground components were 44.0 and 5.64 kg°ha—1.yr—1. In the mature stand, N and P uptakes for aboveground plant components totalled 11.7 and 1.68 kg°ha—1.yr—1, while those of belowground components were 34.4 and 4.58 kg°ha—1°yr—1. In the young stand 41% of N and 48% of P requirements were provided by internal redistribution. Internal redistribution provided 55% of N and 66% of P requirements in the mature stand. The ratio of below— to aboveground total production was 1.8 in the young stand, and 2.8 in the mature stand. Below— to aboveground nutrient requirement and uptake ratios were from 1.6 to 2.0 in the young stand, while those for the mature stand were from 2.7 to 3.6. Within—stand patterns of nutrient uptake were not fundamentally different from those of nutrient requirements. Ratios of uptake for above— and belowground components were generally within 10% of corresponding requirement ratios.