Litterfall and nitrogen cycling following invasion by Chrysanthemoides monilifera ssp. rotundata in coastal Australia.
On the south-east coast of Australia there are extensive infestations of the environmental weed bitou bush Chrysanthemoides monilifera ssp. rotundata. This exotic weed is highly invasive and persistent, yet little is known about its impacts and how it dominates coastal vegetation. A field study was carried out with sites selected for having an area heavily invaded by C. monilifera and an uninvaded native shrubland area. The mass and components of the litterfall and ground litter layer were monitored and compared over 12 months. Nutrient analysis was performed on the abscised leaves and mature green leaves. Soil properties and nitrogen content were also examined. There was a greater input of litter to the native areas (4800±450 kg ha-1), which was highly seasonal, compared with the C. monilifera areas (1690±150 kg ha-1). There was also a greater mass of litter on the ground in the native areas (9600±560 kg ha-1) than C. monilifera areas (3750±170 kg ha-1), probably as a result of the higher leaf fall rate and slow decomposition rate. A large proportion of nutrients was withdrawn from the native (48-80%) and C. monilifera leaves (38-60%) before abscission. The litterfall was estimated to contain more than twice the amount of nitrogen and phosphorous within the native areas, because of the high leaf fall rate rather than a high nutrient content. The total soil nitrogen was two to five times higher in the invaded areas (average 2570±620 kg ha-1) than within the native areas (730±90 kg ha-1). This was accompanied by an increase in ammonia content. The bulk density was lower in invaded areas, but there was no clear trend in the change in soil pH. Synthesis and applications. The higher soil nitrogen beneath the weed infestations could increase the competitive superiority of C. monilifera directly by increasing growth rate, or indirectly by impairing the establishment of native species that are adapted to growing in nutrient-poor soils. The control technique of spraying C. monilifera with herbicide, then burning and spraying seedlings again, could lead to greater regeneration success for natives species than spraying C. monilifera with herbicide only.