Nitrogen Economy of Post-fire Stands of Shrub Legumes in Jarrah (Eucalyptus marginataDonn ex Sm.) Forest of S.W. Australia

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Abstract
Hansen, A. P., Pate, J. S., Hansen, A. and Bell, D. T. 1987. Nitrogen economy of post-fire stands of shrub legumes in jarrah (Eucalyptus marginata Donn ex Sm.) forest of S.W. Australia.—J. exp. Bot. 38: 26–41. Growth, demography and N economy of 1–6-year-old stands of Acacia pulchella, A. alata, A. extensa and Bossiaea aquifolium were examined using population sampling to assess annual increments of N as living biomass, and returns of N as litter, seed and dead plants. Dependence on nitrogen fixation was assessed from seasonal profiles of acetylene reduction, employing data from previous calibrations to convert C2H2 reduced to N2 fixed. After 2 years of slow growth and minimal reproduction all species grew rapidly to achieve maximum or near maximum size and seed production. Intense self-thinning of stands occurred in the second and third years, especially in the highly dense stands of the smallest species, A. alata. Annual turnover in stands ranged from 0·3 to 127 kg N ha–1 year–1, depending on species current age and density of a stand. Returns of N as litter and shed seed comprised small proportions of the annual budgets, but returns due to plant death equalled or exceeded increments of living biomass in years when stands were thinning rapidly. Proportional dependencies of the species on fixed N2 were relatively high (13–61%) in first year seedlings, and then declined markedly to 1·1–3·4percnt; in the second, 0·3–1·6% in the third, and, with one exception, to well below 1% in the fourth and sixth year stands. Mean annual rates of N2 fixation over the 6-year post-fire period were 1·6 kg N ha–1 year–1 for A. alata, 0–49 for A. pulchella, 0·19 for B. aquifolium and 0-10 for A. extensa