| 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 standsof Acacia pulchella, A. alata, A. extensa and Bossiaea aquifoliumwere examined using population sampling to assess annual incrementsof N as living biomass, and returns of N as litter, seed anddead plants. Dependence on nitrogen fixation was assessed fromseasonal profiles of acetylene reduction, employing data fromprevious calibrations to convert C2H2 reduced to N2 fixed. After2 years of slow growth and minimal reproduction all speciesgrew rapidly to achieve maximum or near maximum size and seedproduction. Intense self-thinning of stands occurred in thesecond and third years, especially in the highly dense standsof the smallest species, A. alata. Annual turnover in standsranged from 0?3 to 127 kg N ha–1 year–1, dependingon species current age and density of a stand. Returns of Nas litter and shed seed comprised small proportions of the annualbudgets, but returns due to plant death equalled or exceededincrements of living biomass in years when stands were thinningrapidly. Proportional dependencies of the species on fixed N2were 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 wellbelow 1% in the fourth and sixth year stands. Mean annual ratesof N2 fixation over the 6-year post-fire period were 1?6 kgN ha–1 year–1 for A. alata, 0–49 for A. pulchella,0?19 for B. aquifolium and 0-10 for A. extensa Key words: Shrub legumes, post-fire N economy |
