Seed fall, seed predation, twigging and litter fall of Cascabela thevetia (L.) Lippold

Abstract

Cascabela thevetia (L.) Lippold is a garden ornamental in northern Australia and two biotypes, the peach and the yellow, are recognised. In some areas it has naturalised and now has environmental and economic effects. As part of a broader research program into the ecology of C. thevetia, a field study was undertaken in northern Queensland to quantify seed fall and seed predation (by avian wildlife) of the peach biotype. The amount of twigging caused by birds while they fed on the seeds and the level of litter production were also recorded. Seed fall, seed predation, twigging and litter production occurred in all months of the year. Seed fall increased slowly over late spring and summer before peaking in mid- to late autumn (April–May) and then declining until October. Mean (± s.e.m.) estimated total annual seed fall was 19 140 ± 2880 and 17 030 ± 2930 seeds ha–1 in the first and second years respectively. Seed predation by birds was substantial, with 57% of all seeds predated. Birds also chewed an average of 600 twigs ha–1 year–1. Litter production varied from 430 to 950 kg dry weight (DW) ha–1 month–1. In total, 7900 ± 640 and 7390 ± 1420 kg (DW) litter was produced during the first and second years respectively. Although seed production of C. thevetia is less than a lot of other rangeland weeds, seed predation by birds further reduces the number of seeds entering the soil seed bank. The stem damage that occurred in conjunction with seed predation contributed to overall litter production and warrants further investigation in terms of its effect on plant growth.