Reproductive ecology and dispersal dynamics of the invasive willow, Salix cinerea, in south-eastern Australia

Abstract

Knowledge of the reproductive and dispersal dynamics of invasive species is central to designing control strategies that minimise local population growth, limit range expansion and reduce the likelihood of reinfestation. Molecular ecological methods in plant population studies has allowed a greater ability to quantify ecological processes such as mating patterns and dispersal which are hard to measure with traditional ecological approaches. Such approaches would be beneficial in the management of environmental weeds to provide information on reproductive and dispersal dynamics that can be used in the development of more effective scale-appropriate management strategies. Willows are aggressive exotic components of many river systems in south-eastern Australia. Current control efforts for the most highly invasive willow species, Salix cinerea, are extensive, costly and not always successful due to rapid post-removal reinfestation. A survey of populations in the Ovens River catchment of south-eastern Australia was undertaken to determine the reproductive ecology, catchment scale variance in population fecundity and dispersal dynamics within and between populations of S. cinerea. The species was found to be predominately insect pollinated but can exhibit wind pollination. Mean seed production was found to be higher than 300,000 seeds per tree across sites and years and is driven by tree size and flowering effort. Predictions of seed production at sites across the study catchment based on population and tree size and flowering effort showed that removing the top 20% of seed producing sites would result in a reduction in catchment seed set of over 50%. This high reproductive output combined with high initial germination rates, above 95%, gives the species the potential to reinfest cleared areas and expand its range. Microsatellite-based paternity analysis was used at four sites to ascertain the relative frequency of within versus between population mating events and identify the variation of male reproductive fitness. Observed patterns of within-site mating dynamics have highlighted a highly skewed male fitness distribution with a small number of fathers being responsible for a large proportion of successful fertilisation events. Genetic profiling of populations in surrounding rivers allowed identification of the most likely pollen and seed sources to give an estimate of the scale of dispersal. Results show that 60% of pollen and 50% of seed is moving more than 15 kilometres and a small amount travelling over 30 kilometres. These results suggest control of propagule pressure will be important for preventing future invasion. This may be attained by clearing high density sites that have the greatest reproductive output; subsequently targeting large trees at sparse and remote sites will have the greatest success in reducing propagule pressure. Reproductive dynamics were found to be heavily reliant on inter-site pollen and seed movement. Extensive pollen and seed dispersal was found among sites and among rivers across the catchment. These results suggest that future control efforts should emphasise removal of large high fitness individuals and populations with high reproductive output as well as spatially coordinating control activities across sites so as to account for inter-site reproductive dynamics. Show more