In weed management, using native parasites to control exotic weeds is considered a better alternative than classical biological control. But the risk must be assessed because of the potential damage caused by these agents. We conducted this project to investigate the mechanism driving the choice of a native obligate parasite, Cuscuta australis, between the exotic, Humulus scandens, and native plants as its host through field and pot experiments. The results showed that C. australis preferred the exotic weed over native (naturalized) hosts and caused a notable reduction in the biomass of H. scandens in the field. In contrast, the results of the pot experimentindicated that C. australis preferred a mix of native (naturalized) hosts over the exotic weed. Both texperiments indicated that the parasitic preference of C. australis was induced more by light irradiance than plant water, carbon (C), nitrogen (N) and phosphorus (P) contents, indicating that the native parasite can only be used to control H. scandens when the exotic weed forms mono-cultures or dominates the community. Accordingly, induction and release of C. australis to control H. scandens should be conducted with great caution.

The roles that adventitious roots play in flooding tolerance have been well-studied in willows, while which of the maximum height of adventitious roots and the abundance of adventitious roots in willows is more important for their flooding tolerance is not well known. In this study, we analyzed the effects of adventitious roots on the flooding tolerance of Salix cavaleriei by comparing the maximum height of adventitious roots and the abundance of adventitious roots in dead and live willows along a flooding gradient from 0 to 180 cm in Lake Erhai, China. The results showed that willow mortality increased drastically when the water depth suffered by willows exceeded 100 cm. Live willows developed more adventitious roots and produced them higher on the trees compared with dead willows, however, the pest infestation percentage of the dead willows was larger. Additionally, both the maximum height and the abundance of adventitious roots in live willows were significantly correlated with water depth, whereas in dead willows, these variables were not significantly correlated or only weakly correlated with water depth. The results indicate that producing adventitious roots higher on the trees may be more important than developing abundant adventitious roots lower on the trees when S. cavaleriei is subjected to high flooding levels. Our data highlight that a faster adventitious root response promotes this species' survival under flood stress, although pest infestation accounts for a small percentage of willow mortality. Accordingly, we should choose those willows that can develop more abundant adventitious roots and higher on the stems to plant in regions with abrupt water-level fluctuations.

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