Increased nutrient variability promotes dominance of two invasive plants in the field

A fluctuating resource supply makes some of the most problematic invasive plants more successful against native plants. However, the evidence so far comes from simplified pot experiments in the greenhouse or garden, and it remains an open question whether these mechanisms are also relevant in natural conditions. Here, we present an experiment that tested the effects of nutrient fluctuations on plant invasion in the field. In eight sites in South-West Germany invaded by Asian knotweed (Reynoutria x bohemica) or Canadian goldenrod (Solidago canadensis), we manipulated the temporal patterns of nutrient availability by applying liquid fertilizer at weekly intervals over a 10-week period. There were three treatments: (1) constant nutrient supply, with equal amounts at every application, (2) variable nutrients supply, with double amount of nutrient as in the constant treatment but added at every second application, totaling the same total amount for the entire experiment, and (3) control with water only. We found that the invaders did not benefit from the additional nutrients when they were supplied in a constant manner but became more dominant after several months of variable nutrient supply. To our knowledge, this is the first field evidence that resource fluctuations can promote invasive plants in natural communities and shows that fluctuating resources can alter invasion success even within a single growing season. The key questions are now how general these effects are across other invasive plants and possibly also strong native ruderals, and what the longer-term effects of the fluctuating resources are on community composition and diversity.