Seed ecology of the invasive tropical tree Parkinsonia aculeata

Abstract

Parkinsonia aculeata is an invasive tree native to tropical America, but introduced to Australia. Propagation and stand regeneration is mainly by seed. To gain baseline knowledge for management decisions, seed bank dynamics were monitored for two months during the fruit dispersal period at a coastal wetland in Costa Rica (native habitat), and at a coastal wetland and two semi-arid rangeland sites in Northern Queensland, Australia (introduced habitats). Seed bank densities underneath dense, uniform Parkinsonia stands were found to be lowest in the Australian wetland but highest in the Costa Rican wetland. Post-dispersal seed losses were highest in the Australian wetland, primarily due to seed germination and/or death. At the other sites, seed losses were minor during the study period, and predation was the most important cause of losses. At the two rangeland sites bruchid beetles accounted for more than 95% of the seed losses by predation. Total predation was lowest in the Costa Rican wetland. In order to test for intrinsic differences of seed characteristics, germination trials were conducted using both canopy seeds and seeds from the soil seed bank. Dormancy release and germination rate were studied under four temperature treatments. In all populations, dormancy release increased with increasing temperature, but averaged responses were significantly different between Costa Rican and Australian seed populations, and between seeds collected from the soil and from trees. Germination rate of scarified seeds was fastest at 35°C in all tested seed populations. While high seed germination levels seem to explain low seed bank densities in the Australian wetland, the large seed banks at the rangeland sites reflect the lower incidence of favourable conditions for germination. In the Australian wetland biocontrol with bruchids is unlikely to be successful, while control by conventional methods, such as killing stands by basal bark spraying, seems feasible, due to a lower long-term risk of re-infestation from the soil seed bank. At the rangeland sites conventional control will be difficult and costly. Parkinsonia stands may be better left to their own, while bruchid populations are monitored and management efforts are concentrated on preventing further invasion.