Predictors of recruitment for willows invading riparian environments in south-east Australia: implications for weed management

1. Predicting geographical areas susceptible to weed invasion is a primary target for land managers. Willows (Salix spp.) invading riparian environments have recently been increasing in number in south-east Australia. In order to prioritize management decisions, a clear understanding of the factors affecting recruitment success for sexually and asexually generated Salix recruits is vital. 2. Multiple variables are hypothesized to determine the spatial distribution of Salix recruits. Therefore an information-theoretic model comparison (ITMC) approach was used to analyse factors determining the abundance and distribution of Salix recruits along four rivers in south-east Australia with differing flow regulation. Generalized linear modelling techniques were applied to multiparameter candidate models, developed from existing knowledge of the system and plausible hypotheses. Predictor variables included parameters describing river disturbance, availability of propagule sources and competition for recruitment space. 3. Models of sexually produced Salix recruits performed well, generating 95% confidence sets of candidate models that were small relative to the potential set of models analysed (18%), whereas models of asexual recruits performed less well, indicated by 95% confidence sets that were large relative to the potential set of models explored (67%). 4. A multimodel inference approach was required: model results indicated that availability of mating partners in the environment plays an important role in determining the abundance of sexually derived Salix recruits, whereas for asexual recruits river disturbance parameters better predicted abundance. 5. For the models of sexual reproduction, model plausibility was strengthened by corroborative results from an independent data set collected in the same geographical region. However, the validation data set was too small to assess the predictive capability of the model-averaged model. 6. Synthesis and applications. Information-theoretic model selection methods helped clarify the relative importance of different model parameters on the abundance of Salix recruits. Knowledge of the link between river regulation and willow recruitment enables a greater degree of management flexibility in response to risk forecasting under future scenarios for different water flows. Targeting control programmes to focus on removal of female and seeding willows in rivers with low levels of disturbance may eliminate Salix recruits altogether, whereas the same treatment applied to high-level disturbance rivers is likely to result in persistent problems with asexual recruits.