Effects of Fire on the Growth, Nutrient Content and Rate of Nitrogen Fixation of the Cycad Macrozamia riedlei

Abstract

The response of Macrozamia riedlei(Gaud.) C.A. Gardn. to fire, and its contribution to the input of nitrogen to the jarrah (Eucalyptus marginata Donn ex Sm.) forest ecosystem of south-western Australia, were studied by measuring the biomass and nutrient content of plant parts, and the nitrogenase activity of coralloid roots, in relation to time since burning. Leaf growth was rapid during the first year following fire. In forest burnt 1-5 years previously, the ratio of weight of leaves to weight of bole did not differ significantly between sites. On a site burnt 7 years previously a reduced proportion of leaves in the total plant biomass was attributed to a yellowing and senescence of leaves. The ratio of weight of coralloid roots to weight of boles was greatest on the most recently burnt site. Concentrations of nitrogen and phosphorus in leaves, and phosphorus, potassium and zinc in coralloid roots, were significantly higher in plants growing in recently burnt forest. Concentrations of calcium, sodium and chlorine in leaves were higher on sites which had not been burnt recently. The rate of acetylene reduction, expressed per unit of bole weight, was greatest where forest had been burnt 1 year before sampling and decreased to a minimum where burning had occurred 7 years previously. This trend resulted from a decrease in both the weight and nitrogenase activity of coralloid roots with increasing time since burning. Estimated rates of nitrogen fixation were 8.4 and 1.4 kg ha^-1 year^-1 on sites burnt 1 . 5 and 7 years previously. In the period between successive prescribed burns (5 to 7 years), M. riedlei was estimated to fix c. 35 kg nitrogen ha^-1. This appears to be a significant input in relation to the nitrogen balance of the jarrah forest ecosystem.