Phyllode fall and nutrient content in a mulga (Acacia aneura F.Muell. ex Benth.) community in central Australia in response to rainfall

The fall of phyllodes from Acacia aneura F.Muell. ex Benth. (mulga) in central Australia was studied over 22 months from mid-1958 at four locations within a livestock reserve north of Alice Springs in the Northern Territory, in order to identify rainfall or seasonal triggers. Phyllode fall increased by at least an order of magnitude for short periods following rain of 15–20 mm or more on a ‘mature’ mulga site and similar trends were apparent for ‘young’ and ‘desert form’ mulga on the same site, and on a second, independent, ‘mature’ site. When rates of phyllode fall were high after substantial rain, at both ‘mature’ locations, ~30% of nitrogen and ~50% of phosphorus were withdrawn before abscission, suggesting that mulga was markedly conservative of these nutrients. Conversely, under dry conditions, when phyllode fall was relatively low (<200 mg m−2 day−1), concentrations of nitrogen and phosphorus in fallen phyllodes were higher. Concentrations of potassium, calcium, magnesium and sodium did not vary consistently with increasing rate of phyllode fall, although overall levels of calcium and potassium were considerably higher in the second ‘mature’ location. This legacy study is of renewed interest given the potential of mulga communities to contribute to national carbon stocks, and the consequent need for robust field-based data on growth dynamics and carbon fluxes.