Dry matter and nutrient content of a subtropical semiarid forest of Acacia harpophylla F. Muell. (Brigalow)

Abstract

Measurements on a 1/10-acre sample of brigalow forest indicated a total biomass of 293x 10³ kg/ha, 46% of which consisted of dead materiai, either on the ground or standing. The presence of large amounts of dead material is attributed to the semiarid climate, which results in low decomposition rates, hardness of the wood, absence of termite activity above ground, and lack of running fires. Moisture percentages were low, varying from 4-6% for dead material to 65% for living leaves plus twigs.

Amounts of elements immobilized in the biomass were 1600, 38, 300, 100, 2230, and 180 kg/ha for nitrogen, phosphorus, potassium, sodium, calcium, and magnesium respectively. Brigalow showed high nitrogen/phosphorus and calcium/potassium ratios compared with lucerne and grasses grown on a similar cleared area.

No differences in sodium and calcium concentrations were found in different components of brigalow, but the canopy showed higher levels of nitrogen, phosphorus, potassium, and magnesium than the trunks, with branches and roots intermediate. In addition the canopies of larger trees showed higher concentrations of nitrogen, phosphorus, and potassium than those of smaller trees.

The uneven spatial distribution of living trees associated with the gilgai topography resulted in large sampling errors. The dead material and roots, being more uniformly distributed, showed lower errors. Subsampling techniques used for obtaining samples for chemical analyses were adequate, as standard errors for nutrient concentration determinations were generally lower than those for dry matter determinations.